How to Choose a Telescope
Purchasing Amateur Telescopes FAQ
Slc.Dennis Bishop
102063.112@compuserve.com
Last Updated: 9707.15
Copyright (c) 1997 Slc.Dennis Bishop All rights reserved.
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|This FAQ is under construction There may be some sections |
| that are not totaly done yet. |
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Questions in this FAQ:
1. What is the single most important thing I should know before
buying a telescope?
2. Recommendations for Beginners.
3. What Does All the Jargon Mean?
4. What Are Some Good Introductions To Amateur Astronomy?
5. What Will I Be Able To See?
6. Buying A Telescope
6.1 What Company Makes the Best Telescopes?
6.2 What is the Best Telescope to Buy?
7. Where Do I Buy My Telescope?
7.1 What About Buying Used?
7.2 What About Building A Telescope?
8. What Accessories Will I Need?
9. What are Digital Setting Circles?
10. Why Should I Start With Binoculars?
10.1 How Do I Hold Binoculars
11. What Books and Star Charts Are Recommended?
11.1 What About Computer Programs?
12. About this FAQ
Contributors to this posting include:
Pierre Asselin Dana Bunner Doug Caprette
Mike Collins Kevin Deane Jay Freeman
Chuck Grant Dyer Lytle Christopher Gunn
Doug McDonald Andy Michael Dave Nash
Jim Van Nuland Bill Nelson Leigh Palmer
Alan Peterman Tom Randolph David Smith
Geoff Steer Mario Wolczko C. Taylor Sutherland
Paul Zander
1. What is the single most important thing I should know before
buying a telescope?
This is the single most important thing you should get out of this
FAQ: DO NOT BUY YOUR TELESCOPE FROM A DEPARTMENT STORE.Ignore
everything any literature tells you about magnification and such.
Buy from a telescope store, where you will get a telescope that
makes smaller claims, but will give you FAR better performance.
The reason is that as far as telescopes go, how much you can magnify
is a function of the amount of light the telescope receives, which
is almost entirely determined by the telescope's aperture (the size
of the lens or mirror that points at the sky). As far as
magnification goes, you can expect 50x per inch of aperture on a
normal night.
Department stores always show little 2 1/4 inch refractors from
125+ dollars and say that the refractor can get up to a whopping
600x or so. Strictly speaking, this is true. However,applying the
50x rule, it is easy to see that 125x would be pushing the optics,
and that is assuming that they were high quality ones. With the
quality of the parts they usually give you are lucky to get 100x
with reasonable resolution.
Recommendations for Beginning Amateur Astronomers
=================================================
Occasionally, amateur astronomers ask for recommendations about
telescope buying, learning the sky, and so on. Here are some thoughts.
(Let me state credentials. I am primarily a visual observer: Over
40 years I have logged about 6000 observations of nearly 3000 objects,
and used perhaps thirty telescopes and binoculars enough to know them
well. I have made roughly ten optical surfaces to 16-inch diameter (a
sphere -- my biggest paraboloid was 8 inches). My forte is deep-sky
work; observations I am proud to include the Sculptor Dwarf Galaxy
(10x70 binocular), Maffei I and Leo II (Celestron 14), and S147 (6-inch
Maksutov). My interests led to a physics PhD, studying the interstellar
medium from a spacecraft: By training I am an astrophysicist, but I
maintain amateur status in visual wavelengths -- my thesis work was in
extreme ultraviolet.)
What to do First.
================
First, some meta-advice. Written words do not substitute for
experience. Join an astronomy club, go to observing sessions, and try
other peoples' telescopes. You will learn a lot, and will find people
who like to discuss equipment and observing.
To find clubs, ask at science stores, museums, planetariums, and the
like. Physics and astronomy departments of colleges may know, though
clubs aren't strictly their line. Two popular astronomy magazines,
_Astronomy_ and _Sky_&_Telescope_, publish annual directories of clubs,
stores, observatories, and such. Look for them on newsstands, or go to
a library and read back issues, or try their web pages.
Been to a club already? Honest? Okay, you can keep reading...
Some Basic Questions.
====================
In buying a telescope, you face bewildering, expensive choices. To
help deal with the confusion, here are some questions to ask yourself.
(1) How much effort are you willing to put into learning the sky?
If you know the constellations, and have practiced finding things by
"star-hopping" -- using charts instead of dial-in or punch-in
coordinates -- you will be able to use a telescope cheaper, smaller,
lighter, and easier to set up than one using precise alignment or
computer control to locate objects.
(2) How much effort are you willing to spend on your observing
skills? Seeing fine detail in celestial objects, or just seeing faint
ones at all, requires practice and special knowledge. Yet the rewards
are enormous: An experienced observer may see things with a small
telescope that a beginner will miss with one five times larger, even
with objects and sky conditions that favor both telescopes equally.
(3) How far will you have to lug your telescope to get it from where
you keep it to where you use it, by what means, and how much effort will
you put up with to do so? Differences in size and optical design create
vast differences in telescope portability, and any telescope that you
take out and use will be far better than one that sits in the closet
because it is too heavy or too cumbersome.
(4) Some people are into technology for its own sake, without regard
to whether it is useful or cost effective. Are you willing to pay extra
for sophisticated features, even if you don't need them? If so, fine --
lots of us like neat equipment. But if not, take care technology
enthusiasts don't sell you things you don't need.
(5) Do you want to take photographs or CCD images of celestial
objects? "Astrophotography" is an expensive word. I am not into this
side of the hobby, but friends who are typically take several telescopes
and several years before they are satisfied, and spend lots more money
than visual observers do.
Some Realities.
==============
With these thoughts in mind, I can make some general comments.
(A) The most important thing in determining the optical performance
of a telescope is the diameter of the beam of light that goes into it --
its "clear aperture". Obviously, the more light, the fainter the things
you can see, but less obviously, image detail is limited by clear
aperture, via physical optics. Bigger telescopes produce sharper
images, just because they are bigger.
There are important qualifiers. First, bad craftsmanship can make
any telescope perform poorly. Cheesy optics won't work. Fortunately,
it is not too hard to make optics of the sizes and types common in
amateur telescopes: most manufacturers routinely turn out units that are
okay. Bad ones turn up, but major manufacturers will often fix or
replace a real lemon, if you have wit to recognize that you have one,
and will to complain. (Most of us have neither; that's how some
manufacturers make money!)
Second, different optical designs perform differently. Schmidt-
Cassegrains, Newtonian reflectors, and refractors all have good and bad
points. People who love telescopes, or sell them, will be eager to
debate the matter. However, variations are relatively minor. It is
usually adequate to assume all telescopes of given clear aperture and
given quality of optical craftsmanship have the same optical
performance: Real differences will correspond to changes in aperture of
usually no more than 10 to 20 percent. Shabby optical work will
increase that percentage enormously.
Third, atmospheric turbulence ("seeing") limits the ability of a
telescope to show detail, and sky brightness limits its ability to show
faint objects. Poor seeing usually hits large telescopes harder than
small ones. When seeing is poor, there may be no reason to take out and
set up a big telescope. If you always observe from such conditions, you
may have no reason to buy a big telescope. Yet, even in bright sky, a
large-aperture telescope will show fainter stuff than a small one. And
many of us have found dark-sky stable-seeing sites within a reasonable
drive of home -- from sites near San Francisco Bay, sometimes I have to
stare through the eyepiece of my Celestron 14 for several minutes before
I can tell that there is any air between me and what I am looking at.
Notwithstanding these caveats, APERTURE WINS, and wins big. If you
buy the finest 90 mm fluorite refractor in the world, do not be
chagrined if a junior high school student shows up with a home-made
6-inch Newtonian that blows it clean out of the water: The 6-inch I made
at 13 puts my world-class 90 mm fluorite to shame. There is no contest,
and it's not because I was a master optician at 13, it is because six
inches is bigger than 90 mm, hence intrinsically better.
(B) Hundreds of deep-sky objects are big and bright enough to show
well through apertures of two inches or so, at low magnifications.
Thus, medium sized binoculars -- 7x50 or 10x50, say ("7x50" means "7
power, 50-mm aperture") make inexpensive, highly portable, easily
operated beginner instruments. Perhaps you have one already. To use
them well, you must be willing to learn the sky enough to find things
with a hand-held instrument. And don't get one that gets too heavy to
hold steady before you are done observing.
(C) Speaking broadly:
(C.1) The most optical performance per unit of clear aperture comes
from modern, high-quality refractors -- but they are outrageously
expensive compared to other designs of the same aperture. Also,
in sizes much above four-inch aperture, the tubes are generally
long enough to make the whole instrument cumbersome and heavy.
(C.2) The most optical performance per unit of portability comes
from Schmidt-Cassegrain and Maksutov designs -- but they are
still pretty expensive.
There's a qualifier here: What makes them portable are
short, stubby tubes, but for small apertures -- say, four inches
or less -- portability of all types is dominated by clumsiness
of the tripod, so the portability advantage of Schmidt-
Cassegrains and Maksutovs diminishes.
(C.3) The most optical performance per unit of cost comes from
Newtonians -- particularly those with Dobson mountings.
Compared to other telescopes of the same aperture, they are
clumsier than Schmidt-Cassegrains and Maksutovs, but not
nearly as clumsy as refractors.
Let me regroup that information into three questions telescope
buyers often ask:
(C.1') What gives most optical performance for a given aperture?
Usually, a high-quality refractor.
(C.2') What gives most optical performance for a given car to carry it?
Usually, a Schmidt-Cassegrain.
(C.3') What gives most optical performance for a given budget?
Usually, a big Dobson.
(D) Though costly and cumbersome, small refractors are durable and
difficult to get out of whack. Good ones make respectable beginner
instruments, particularly for beginners with extra thumbs. And a good
small refractor provides a wonderful way for an experienced observer to
embarrass folks with humungeous Newtonians who lack observing skills to
exploit them. But BEWARE of mass-marketed junk refractors, advertised
as high-power and sold in department stores.
(E) Altazimuth mountings tend to be cheaper, lighter, less clumsy,
and more quickly set up than equatorial ones, but to use one you must be
willing to learn the sky well enough to find things without dialing in
celestial coordinates. (Computer-controlled altazimuth mounts allow use
of celestial coordinates to find things, or perhaps will look up the
coordinates for you, in an internal data base, but they are not cheap.)
(F) There's another way to look at this material. There are variety
of ecological niches for telescopes, corresponding to different uses and
requirements. I know of seven:
(F.1) Big Iron: This is the giant Dobson-mounted Newtonian, or
humungeous Schmidt-Cassegrain, that fills your garage. To
transport it requires a small trailer, pickup truck, or panel
van, and setting it up calls for the concerted efforts of three
used fullbacks and a circus elephant. The ladder to climb to
the eyepiece is so tall you need supplemental oxygen to deter
altitude sickness. This telescope is your galaxy-gazer and
cluster-buster supreme, and if it is well made, then when the
seeing is good it will show detail that those condescending
high-tech dweebs with their confounded itty-bitty seven-inch
apochromatic refractors can only dream about.
My "Big Iron" is a Celestron 14, with a little tiny
single-axle cargo trailer to haul it.
(F.2) Largest Conveniently Portable Telescope: This is the most
telescope that will fit easily in your regular vehicle without
hiring a bulldozer to clean it out. What it is, depends on what
your vehicle is -- with a ten-speed, or a subway train, you have
a problem. An eight- to eleven-inch Schmidt-Cassegrain is the
right size for many people; that is one reason these telescopes
are popular.
I have had several Largest Conveniently Portable Telescopes,
over the last few cars. Once I built an eight-inch Dobson
whose key design parameter was that the tube just barely fit
crosswise across my back seat. I used it a lot till I bought a
smaller car. For a while, my Largest Conveniently Portable
Telescope was a Vixen 90 mm f/9 fluorite refractor on an
altazimuth fork or a Great Polaris German equatorial (I have
hardware to fit both), but at present I use a six-inch f/10
Intes Maksutov on the Great Polaris. A somewhat faster Dobson
than my 8-inch f/5 would work equally well, and would have
more performance for most purposes.
(F.3) Public Star Party 'Scope: You'll want something pretty
portable, with the added provisos that it's nice to have a
sidereal drive so you won't have to keep re-pointing it between
viewers, and that it shouldn't be so expensive you worry about
kids and idiots. Your SCT will do nicely.
I put the Intes or the Vixen fluorite on the Great Polaris,
but I set the tripod legs to maximum length, so the expensive
optics are out of reach. So far, no one has slam-dunked a rock.
(F.4) Quick Look Scope: The idea here is to leave something all
set up in your entrance hall, or hidden under a stack of old
_Sky_&_Telescopes_ in the back of your car, so you will have a
telescope on two minutes notice if a truly close comet comes
whizzing by, or if you are too lazy to assemble one of your real
telescopes. Such an instrument can also double for nature
watching or spying on the neighbors, which may be the same thing
-- just don't tell your fellow amateur astronomers, or you will
lose observer points. Many people have a spotting 'scope on a
light tripod, or perhaps a 90 mm Maksutov on one a bit heavier.
Lately, my Quick Look 'Scope has been a 102 mm f/9.8 Vixen
refractor with a conventional achromat, on a Vixen bent-fork
altazimuth mount that has clutches and slow motions on both
axes. I have a couple of smaller refractors that I sometimes
use similarly, but since I have room to leave the 102 mm set up
in my living room, I benefit from the extra aperture.
(F.5) Binocular: A good binocular is very useful, and can do
much of the work of a 'Quick Look Scope. I have too many; ones
I use for astronomy include the 7x35 Tasco ($29.95 at Sears)
that I keep in my car for bird-watching (oops, lost observer
points), an old Swift Commodore Mark II 7x50 (long out of
production), which was one of the first binoculars I saw with
BAK-4 prisms, and an Orion 10x50 and 10x70 with BAK-4 prisms
and fully multicoated everything, up to but not including the
case. At star parties I tend to wander around with one
dangling from my neck. I tried two, but lacked sufficient eyes.
(F.6) High-Tech Conversation-Stopper: This is how you put to
shame those grass-chewing hillbilly clodstompers who have giant
cardboard Dobsons with tubes so big that they echo. Odds are
the seeing will never get good enough for them to demonstrate
that a half-meter shaving mirror will blow eighteen centimeters
of optical perfection clean out of the water, and if they start
talking about faint galaxies you can always change the subject
to diffraction rings and modulation transfer functions, and ask
them to compare internal baffles and background sky brightness.
Besides, your telescope has more knobs than all theirs put
together, and it cost more than all theirs put together, too.
The default choice for the High-Tech Conversation-Stopper
these days is typically an apochromatic refractor, or some
close approximation ("apochromat" is a precise technical term;
not all superb refractors are apochromats, and vice-versa),
which if well made and well baffled will deliver outstanding
performance for its size. The apertures available suffice for
many amateurs who have either recovered from aperture fever
or have not yet succumbed, or who have exhausted their supply
of fullbacks and circus elephants to set up the Big Iron.
Few other kinds of telescopes qualify -- you're not allowed
to have a Schiefspiegler unless you can remember how to spell
it, and nobody wants a Yolo because people expect you to walk
the doggie. Some folks like Questars, but not me.
My present High-Tech Conversation-Stopper is the 90 mm
Vixen fluorite refractor I mentioned earlier. It is not big
enough to be as impressive as I might want, and is rather short
on knobs, but I can talk fast enough to make up the difference.
(F.7) CyberScope (Suggested by Bill Arnett): With as much
central processor power as an average microwave oven and
servomechanisms of sufficient accuracy to bring an object within
the field of a medium-power eyepiece, this computer-controlled
telescope declares to astronomers and computer types alike the
owner's level of technical sophistication in both disciplines.
Advanced versions log observations in your very own digitally
simulated handwriting, brew coffee to keep themselves awake, and
buy off the local raccoons with Oreos, all while you sit inside
at your real computer, writing space-combat video games in
graphically-enhanced modularized compiled Tiny BASIC for
Windows 95 NT. And oh, yes, the battery truck is humungeous.
CyberScopes do a decent job of locating large numbers of
objects from an internal database, and permit motorized
tracking with a telescope that is lighter, less bulky, simpler
to set up and align, and vastly more expensive than if it were
equatorially mounted. Low-quality mechanisms and sloppy
construction often impede realization of their potential. Even
so, those fond of technology may like them a lot, and folks with
skill and equipment to program the control interface have a
field day doing things most of us have never dreamed of.
I do not presently own a CyberScope. That's because I write
programs for a living, and too many of them. In my hobbies,
I avoid anything even suspected of containing electrons.
What about accessories?
======================
I have already said most of what you need to know about accessories,
which is that (A) aperture wins. If you are planning a telescope
budget, and eyepieces, finders, and such account for the lion's share of
your funds, sit back and think carefully about what you are about to do
-- it might be better to get a bigger telescope instead of fancy
accessories. A 10-inch telescope with a hand magnifier as an eyepiece
will give a better view of most objects than an 8-inch telescope with
the finest eyepieces in the world. Why? Because (A) aperture wins.
Yet if you are up against limits of telescope portability, or have
lots of money, or like technology, go ahead and buy fancy accessories.
I won't tell, provided you remember that (A) aperture wins.
In any case, I will mention some plain-vanilla accessories that you
might want to have, and maybe a few chocolate ones, too:
(a) Eyepieces. A small number of good ones is better than a large
number of bad ones. You will need a low-power, wide-field eyepiece,
both for finding things and for low-power views of big, diffuse objects.
It might give a magnification equal to five or six times the telescope
clear aperture, in inches. On my f/11 Celestron 14, the low-power
eyepiece has a 55 mm focal length, and is mounted in a two-inch barrel,
so that the front lens -- which sets the field diameter -- can be as
large as possible. (In little f/10 or f/11 telescopes, internal baffles
may mean that no light gets to the edges of a two-inch wide eyepiece; if
so, don't bother with the extra cost of one.) On my f/5 8-inch Dobson,
I use a 20 mm eyepiece, which doesn't need a two-inch barrel.
The next power you will likely reach for is medium to medium high,
for a good look at detail in the object in view. Such an eyepiece might
give a magnification of 20 to 30 times the telescope clear aperture, in
inches. On my C-14 I use a 12.4 mm eyepiece, and on my 8-inch Dobson, a
4 mm. The objects you look at with high power probably won't be very
wide (though they might be), so for economy, you might not want a
super-wide-field type.
Your next choices will depend on what you like to look at. If you
are not sure, hold off buying more eyepieces till you find out.
"Fast" f-numbers, typical in Dobson-mounted Newtonians, require
fancy, expensive eyepieces to give good views, because the steeply
converging light cones of these instruments are difficult for an
eyepiece to cope with, particularly away from the center of the field.
Slower instruments can use simpler eyepiece designs. It is a "Catch-22"
of amateur astronomy, that cheap telescopes (fast Dobsons) need
expensive eyepieces, but expensive telescopes (most refractors and
Schmidt-Cassegrains, with slow f numbers) can use cheap eyepieces.
"Zoom" eyepieces, which change focal length at the twist of a
knurled ring, tend not to be very good. Barlow lenses, also called
telextenders, multiply the focal length of the telescope with which they
are used: It used to be that they generally worked well only with
telescopes with large f-numbers, where they were not needed -- another
"Catch-22". Yet I have heard that there are now Barlow lenses that work
with fast telescopes, where they are indeed needed, but I urge a
try-before-you-buy approach to selecting one.
For over fifteen years I used an eyepiece set bought in roughly
1980. It featured no fancy designs, just a 55 mm Plossl, 32, 20, and
12.4 mm Erfles, and 7 and 4 mm Orthoscopics. The 55 and 32 mm eyepieces
were in 2-inch barrels, the others in 1.25 inch barrels. All were very
good quality -- the 55 and 32 mm were from University Optics, and the
others were Meade Research-Grade. All worked reasonably well even at
f/5, and the 68-degree apparent field of the Erfles was enough that I
was untempted to buy wider-field types. Besides, a big Erfle is already
so heavy that I must rebalance the telescope to use one. I did use the
4 mm eyepiece on the C-14 now and then, but occasions where I want that
much power are rare.
In mid 1996 I bought more eyepieces, mostly out of curiosity. I
found that decent Plossls are comparable to decent Orthoscopics. I
bought several Vixen "Lanthanum" eyepieces, which have built-in matched
Barlow lenses to give 20 mm eye relief, even at such short focal lengths
as 2.5 mm. I don't need glasses to observe, but even so, long eye
relief makes viewing more relaxed -- I'm not worrying about bumping the
eyepiece. It also facilitates public viewing -- I focus with my glasses
on, and tell everyone to leave theirs on and not refocus.
Note what high-tech eyepieces can and cannot do. The best give
wider fields of view, with fewer eyepiece aberrations near the edges,
than older types. The improvement is most noticeable at fast f numbers.
If that's important to you, you might want some. But eyepieces are not
aperture stretchers. They can neither increase image detail beyond the
theoretical limit for the aperture, nor increase the number of photons
that make it to the focal plane. If you think otherwise, you are making
the same mistake as the clueless beginner who buys a drug-store
refractor because it says "Magnifies 400 Times!!" on the box. The best
an eyepiece can do is not make things worse. A simple eyepiece, with
good coatings and well-polished lenses, will show all the on-axis detail
a telescope has, and absorb almost no light. That's what counts most
for astronomical work.
In 1980, I bought 6, 12 and 25 mm Ramsden eyepieces -- an old,
simple, design -- for about ten dollars each. I use them at star
parties without telling what they are. They have only four surfaces, so
simple coatings give good throughput, and there are few chances for bad
polish to scatter light and ruin contrast. The field of view is narrow,
but on axis, at slow f numbers -- f/10 or longer -- they give up nothing
to new designs; images are superb.
(b) Finders. What kind of finder you get depends on how you use it.
If you plan on looking mostly at fine details in bright objects, then
you might buy a big finder, in the hope that most of what you look at in
the main telescope will be visible in it, too. But that won't work if
you push your telescope to its faint-object limits -- you would need a
finder as big as the main telescope. You might then consider a finder
that will show stars exactly as faint as on your charts. It helps a lot
in identifying what you are looking at through the finder, if every star
you see is charted, and vice-versa. Once the right pattern of stars is
in the finder, you can put the crosshair where the object lies, even if
it is too faint to see.
In dark sky, the 10x40 finder on my C-14 shows stars to about
magnitude 9.5, which matches my big charts. The 7x35 on my 6-inch
Maksutov does almost as well. In suburbia, the 5x24 finder on my 8-inch
Dobson goes to about magnitude 6.5 (which would be the naked-eye limit
in darker conditions), thus matches many naked-eye star atlases.
Unit-power finders, like the Telrad, let you to stare at the sky
with both eyes open and see a dot, circle or crosshair of light where
your telescope is pointing. A peep sight, made by taping bits of
cardboard to your telescope tube, may work as well, and will be much
cheaper, and any magnifying "straight-through" finder (in which you look
in the direction the finder is pointing) can be used with both eyes open
-- let your brain fuse the images, so you can use the finder's crosshair
with the other eye. I tried a unit-power finder (Orion's) on my 90 mm
refractor, but found it always inferior to the original 6x30 finder.
My opinion about unit-power finders is in the minority. Many prefer
them to those which magnify. Some folks use the Telrad's circles of
known diameter to measure angular distances when finding things.
(c) Charts. Preferences vary greatly. What I find useful, in order
from simple to complicated, is more or less the following:
(c.1) A simple planisphere, preferably a plastic one that won't
sog out with dew and that may survive being sat upon. It's
a fast way to find out whether a particular object is up
before I go observing, or to determine how long I have to
wait before it is well-placed.
(c.2) A "pocket atlas". I am particularly fond of Ridpath and
Tirion's _The_Night_Sky_, from Running Press in Philadelphia,
PA. It is about three by five inches and half an inch thick,
and it is out of print. Write Running Press and complain.
(c.3) A "table atlas", bound as a book that will lie reasonably
flat, showing stars to the naked-eye limit and lots of
deep-sky objects to boot. I happen to use an old Norton's
_Star_Atlas_; there are lots of others.
(c.4) A "deep atlas", such as _Uranometria_2000_ or the AAVSO
atlas, with a stellar magnitude limit of 9 or 9.5 and a vast
number of objects. What's important here is to have enough
stars charted that there are plenty in every finder field.
(c.5) A planetarium computer program (Bill Arnett reminded me).
If you are a beginning astronomer, I do *not* suggest you
rush out and buy a computer, but if you already own one, you
might bear in mind that there are programs that will turn
your console into a window onto the simulated heavens, with
features for finding, displaying, and identifying things.
I happen to have the rather old Voyager 1.2 for my even
older Macintosh II; there are plenty more, both for Macs
and for the world of MS-DOS and its descendents.
Some folks run such a program on a laptop, at the
telescope. Please put red cellophane over your console,
if you do.
I have had limited use for the popular oversize-format charts with
lesser magnitude limits, like 7.5 to 8.5; they don't show enough stars
to be useful with most of my finders, and are too cumbersome. The
plastic-laminated versions make good place mats, though. Everyone
should use the box of a Dobson as a picnic table at least once.
(d) A red flashlight, so you can read your charts and notes without
ruining your night vision, or that of people near you. The kinds that
have a red light-emitting diode (LED) instead of a flashlight bulb are
particularly good. If other observers scream and throw things, your
light is probably too bright.
(e) A logbook. This item is not for everyone, but I find it useful
to record my observations, even if I don't do anything other than note
that I saw a certain object with a certain telescope and magnification.
Logbooks make fun reading when it is cold or cloudy, and often there
will be reason to look up something long after the fact. Besides, if
you quote frequently from your logbook, you can make your friends think
you are an active observer when you really gave it up years ago.
What about observing skills?
===========================
Even some experienced amateur astronomers think that seeing things
comes free and easy, with no more effort than opening your eyes: But as
currently popular slang so evocatively articulates,
** NOT **.
Vision is an acquired skill. You must learn it, you must practice, and
you must keep learning new things, and practicing them, too.
Buying a bigger telescope to see more is like buying a bigger kettle
to be a better cook, or buying a bigger computer to be a better
programmer. Not that it won't help -- it might -- but cooking and
programming depend far more on knowledge and experience than on
artifacts. So does visual astronomy. People with garages full of
telescopes (pardon me while I try to close the door to mine) are in
great part victims of materialism, marketeering, and hyperbole.
Practice is cheaper, and works better. As I said near the beginning of
this article, an experienced observer may see things with a small
telescope that a beginner will miss with an instrument five times
larger, even with objects and sky conditions that favor both equally.
What skills may you hope to cultivate? What techniques should you
practice? Not all have names, but here are a few, in what I think is
order of importance; what matters most comes first.
(a) Patience. It can take a long time to see everything in a field,
even if you know exactly what you are looking for.
(b) Persistence. Eyes, telescope, and sky vary from night to night.
(c) Dark adaptation. Avoid bright lights before observing: It takes
your eyes hours to reach their full power of seeing faint objects.
(d) Averted vision. The part of your retina that sees detail best,
sees low light worst. Look "off to the side" to find lumps in the dark.
Many observers use averted vision on faint objects, but not for
faint detail in bright ones. Detecting something doesn't mean you've
seen all you can. Don't let the dazzle of a galaxy's lens keep you from
tracing spiral arms out beyond the width of the field. How about
increasing magnification, and using averted vision to see if you can see
more detail in the paler, but larger, image?
Averted vision helps with double stars, when one star is much
fainter than the other, even if the faint star is bright enough not to
need averted vision if it were by itself. That is, averted vision seems
to facilitate the detection of low contrasts as well as faint objects.
(e) Stray light avoidance. Even when it's dark, background glow
interferes with detecting faint objects. Keep it out of your telescope
and out of your eyes. Try eye patches and eye cups for eyepieces.
My first view of the Sculptor Dwarf Galaxy was with my jacket collar
pulled up over my binocular eyepieces. I looked like a cross between
the Headless Horseman and the Guns of Navaronne, but I saw the galaxy.
(f) Moving the telescope. The eye sometimes detects motion, or
changing levels of brightness, more easily than static images. Jiggle
the telescope, or move it back and forth, to make an object "pop out".
Try it while using averted vision.
(g) Not moving the telescope. The eye sometimes adds up photons
over many seconds; if you can hold your eye still for a long time,
faint things may appear. Try it with averted vision.
(h) Respiratory and circulatory health. If you smoke, try taking a
break before and during observing -- carbon monoxide from incomplete
combustion interferes with the ability of the blood to transport oxygen.
Clear sky, and enjoy your telescope.
-- Jay Freeman
freeman@netcom.com
3. What Does All the Jargon Mean?
OK, by popular request, here is a glossary of common astronomy
terms encountered in amateur astronomy.
altazimuth mount
This is what you think of when you think of a tripod mount. It
allows movement in two directions: parallel to the ground (azimuth),
and at right angles to the ground (altitude). It is very useful for
terrestrial observations, as it is a very natural way of observing.
(Note: Dobsonian Telescopes are mounted this way)
aperture
The diameter of the objective.
Barlow
A Barlow lens is a device which has the effect of increasing the
magnification. It does this by lengthening the effective focal
length of the telescope you are using. Thus a 2x Barlow will double
the magnification, a 3x will triple it. Barlows used to have a bad
reputation, stemming largely from rather poor quality ones being
sold. Modern Barlows are high quality and a good choice for
expanding your collection of eyepieces. You should keep the Barlow
in mind when buying eyepieces- buying a 3mm,6mm, 12mm, and a 24mm
and a 2x Barlow is a very dumb idea. The only use you get from the
Barlow is changing the 3mm to a 1.5mm (which is probably going to
give you higher than usable magnification anyway). On the other
hand, a 6mm, 9mm, 15mm and 24mm would be complemented very well by a
2x Barlow.
catadioptric
Any of a number of compromise telescope designs, using both a
lens and mirrors. Examples are the Schmidt-Cassegrain and
Maksutov-Cassegrain. Because the light path is folded twice, the
telescope is very compact. These are pretty expensive. Pictures
can be seen in the ads in any issue of a popular astronomy magazine:
the Meade 2080 and the Celestron C-8 are examples of Schmidt-
Cassegrain; the Celestron C-90 and Questar are examples of
Maksutov-Cassegrain.
chromatic aberration
In refractor telescopes, which use lenses to bend the light,
different wavelengths of light bend at different angles. This
means that the stars you see will usually have a blue/violet ring
around them, as this light is bent more than the rest of the
spectrum. It is not present at all in reflectors, nor to any
significant degree in catadioptrics. Different glasses and crystals
(notably fluorite) are sometimes used to compensate for the
aberration. Such telescopes are termed "achromat," or "apochromat"
if the correction is is nearly perfect.
collimation
This refers to how correctly the optics are pointing towards each
other. If a telescope is out of collimation, you will not get as
clear an image as you should. Refractors generally haved fixed
optics, so you don't have to collimate them. Reflectors and
catadioptrics usually have screws that you turn to collimate.
(This only takes a few minutes to do- it is dead easy).
coma: This refers to the blurring of objects at the edge of the
field of view, most common in short focal ratio Newtonian telescopes
(at f/10 and longer, Newtonians are very well corrected for coma).
Dobsonian
Named for John Dobson of The San Francisco Sidewalk Astronomers
(who prefers to call these "Sidewalk Telescopes"),this is a design
which allows for very large apertures at very affordable prices. The
trade-off is that they are mounted on altazimuth mounts, instead of
equatorial ones, which makes them essentially useless for
astrophotography, but an inexpensive alternative if you only plan to
do visual work. These are light buckets. If you are planning to
build your own telescope, you might want to consider a Dobsonian.
Note: That this design is now the #1 Design seen at many Star partys.
equatorial
An equatorial mount is set to the current latitude, and is polar
aligned (pointed at the North Pole in the Northern Hemisphere, the
South Pole in the Southern Hemisphere) and then moves only in Right
Ascension and in Declination. This take a while to get used to, but
offers the wonderful side effect of being able to track the
astronomical objects you are looking at as they move across the sky
(which is very visible motion at telescopic magnifications) by
moving in only one direction (Right Ascension). Most equatorial
mounts come with motor drives that take care of this for you.
exit pupil
This refers to how wide the beam of light exiting the eyepiece is,
and is equal to the aperture divided by the magnification. If it is
bigger than the size of your pupil in the dark (7mm when you are
young, 5 or 6mm when you are over 40, as a general rule) you will
not be taking in all the light available- effectively, you will be
using a smaller aperture telescope than you have.
eyepiece
This is the thing you actually look into. Almost all telescopes
separate the Optical Tube (the telescope proper) from the eye piece.
Essentially, the telescope makes a really tiny image of what it's
pointed at. The eyepiece acts as a magnifying glass to allow you to
see the image bigger than it would otherwise be. The magnification
is the focal length of the telescope divided by the focal length of
the eyepiece. Eyepieces are described by the diameter of the barrel,
always expressed in inches (.965", 1.25" and 2" are the sizes in
common use) and the focal length always expressed in millimeters
(4mm - 40mm is the usual range). Short focal length eyepieces are
also termed high power, long focal length are low power.
Also significant with eyepieces is the apparent field of view
(expressed in degrees) and eye relief (expressed in millimeters).
The apparent field refers to how big the circle of space you see
in an eyepiece appears. Bigger is better. Eye relief is a measure of
how far from the eyepiece you can have your eye and still see. If
you wear glasses to correct astigmatism, you will need fairly long
eye relief (the focus knob will correct for almost all vision
problems except astigmatism).
There are several types of eyepiece designs. The most popular are
Kellner (inexpensive, most popular for cheap telescopes, short eye
relief and narrow fields of view. Good to avoid if you can afford
better); Orthoscopic (good price/performance compromise); Erfle
(wide field of view, expensive); Plossl (perhaps the best all-around
eyepiece. Some moderately expensive versions available); and Ultra
Wide (very expensive, almost double the number of lenses as other
designs makes for more light loss in the eyepiece, large exit
pupils. Can cost more than a small telescope. Not a good place to
spend your money when you are just starting out).
You really don't want to buy many .965" eyepieces- they are
generally not as well made as the 1.25" ones, and if you get a
bigger telescope it will probably not accept your .965" eye pieces.
You can buy an adapter to let you use 1.25" in your .965" focuser.
This is probably worth the money.
f/10, f/6.3
See Focal Ratio
finder scope
The finder scope is a low power telescope attached to the telescope
you are using. Because most telescopes show such a small portion of
the sky, it is virtually impossible to locate anything just by
looking through them. So you look through the finder scope to center
the object you want (the finder has crosshairs) and then you can use
your real telescope on it. Note that you can ignore all the claims
about big finder scopes. You almost certainly don't care. All you
need is to be able to point your main telescope at something in the
sky. Finder scope size only matters when you are starhopping through
fairly dim stars (where the larger aperture allows you to see dimmer
stars). This will not be an issue for you for quite a while (if
ever). Many people use a Telrad sight, which is simply a red LED you
can sight on- you get absolutely no more aperture than your naked
eye. The finder scopes are usually advertised as 8x50 (or such). The
eight refers to the magnification, the 50 to the aperture in
millimeters-just like binoculars.
focal length
This is the length of the light path, from the objective to the
focal plane. The magnification is the focal length of the telescope
divided by the focal length of the eyepiece. See also focal ratio.
focal plane
The plane that the telescope (or eyepiece) focuses on. When you
turn the focus knob on the telescope, you are moving the eyepiece
back and forth until you make the two focal planes coincide.
focal ratio
Also referred to as the "speed" of the telescope, is the ratio of
focal length to aperture, and is always expressed as an f/number.
Thus an 8" telescope with a 2000mm focal length is f/10 (because 8"
is 200mm, and 2000 / 200 = 10). An f/10 telescope is "slower" than
an f/4.
Fast telescopes give wider, brighter images with a given eyepiece
than slower ones (but note that at a given magnification, the images
are-assuming identical optics-exactly the same: what you see
through a f/6.3 telescope with a 12mm eyepiece is identical in width
and brightness to what you would see through a f/10 telescope with a
19mm eyepiece).
In general, the slower the telescope the more forgiving it is of
optical errors in the objective and eyepiece. A telescope of f/10 is
fairly forgiving, f/6.3 much less so.
focuser
This is the thing that holds the eyepiece. It moves in and out so
you can focus the telescope. It is always included with the
telescope when you buy one. The size, almost always .965", 1.25" or
2" refers to the barrel diameter of the eyepieces it accepts.
fork mount
A fork mount is a type of mount where the telescope is held by two
arms, and swings between them. A fork mount can be either
alt-azimuth or equatorial (through the use of a wedge). Fork mounts
are most commonly used with Schmidt-Cassegrain telescopes, and are
almost always equatorial.
German Equatorial Mount
The first equatorial mount devised and still the most common for
small to moderate sized reflectors and refractors. Unlike the
equatorial fork, the german equatorial is suitable for telescopes
with either short or long tubes (although, if poorly designed, a
long tube may strike the tripod, preventing viewing at the zenith).
They usually are designed with movable counterweights, which make
them easy to balance, but heavy and bulky.
The tube of the telescope is joined to a shaft (the Declination
shaft or axis) which rotates in a housing that in turn is joined at
right angles to another shaft (The polar axis). The polar axis is
pointed at the celestial pole (just like any other equatorial
mount). A counterweight, which is required for balance, is placed on
the other end of the decination shaft.
Tracking an object past the zenith requires that the telescope be
turned (both Right Ascension and Declination rotated through 180
degrees), which reverses the field of view. Not so much a problem
for visual astronomy, but a limitation on astrophotography.
light bucket
A common slang term for a large aperture. The cure for "Aperture
Fever."
Maksutov-Cassegrain
See catadioptric.
Meridian
An imaginary north/south line passing through the zenith.
Newtonian
See reflector.
objective
This is the thing that gathers light from the sky and folds the
light into a cone. In a refractor it is the big lens that points at
the sky, in a reflector it is the big mirror at the bottom of the
tube. The job of the objective is to create a light cone which comes
into tight focus at a single focal point.
optical tube
This is the telescope proper. It is the tube which holds the
objective. The rest of the stuff are accessories, such as the mount,
tripod, and eyepieces. When reading ads, note that some times
optical tubes are sold by themselves. You will need to go out and
buy (or build) a mount for them before you can use them.
reflector
A reflector is any telescope which uses a mirror as its objective.
The most common type is the Newtonian reflector, which has a mirror
at the bottom of a tube, which focuses the light into a cone which
is deflected by a flat "secondary" mirror (which is mounted near the
top of the tube in something called a "spider") out a hole in the
side. This is where you put the eyepiece. The advantages of the
Newtonian design are numerous: there is only one optical surface on
a mirror, as opposed to two on a lens, so it is cheaper to make;
part of the light path is at right angles to the length of the tube,
so it can be somewhat shorter than a similar refractor; you can get
it in much larger apertures than a refractor, and there is no
chromatic aberration .
refractor
This is what you usually think of as a telescope- it has a lens at
one end, and you look straight through the other. This is sometimes
referred to as a "Galilean" telescope, as it is of the same design
that Galileo used (although strictly speaking, a Galilean telescope
is a specific kind of refractor- one with a simple double-convex
objective lens and a simple double-concave eye lens.
right ascension
See declination.
Schmidt-Cassegrain
See catadioptric.
spherical aberration
A problem where a lens or mirror in a telescope is not shaped
correctly, so the light from the center is focused at a different
location than the light from the edges. You should never have to
worry about this. This only shows up in really cheap telescopes.
spotting scope
A small telescope, always a refractor or catadioptric, generally
used for terrestrial viewing. Of limited utility for astronomy,
though many are marketed as such. Probably the wrong choice unless
you want to use it also for birdwatching, or as a powerful telephoto
lens on a SLR camera.
wedge
This is the thing that a fork-mounted Schmidt-Cassegrain tele-
scope will attach to, to connect it to the tripod. You want it to be
sturdy.
worm drive
This is the sort of drive most telescopes come with, if they come
with a drive. It is a very accurate and smooth drive. However, due
to imperfections in the manufacturing process, there will be
periodic errors that occur at the same point in every worm cycle
(usually about 8 minutes). To deal with this, higher end telescopes
come with drives which compensate for the mechanical defects.
zenith
The sky directly overhead. An object "transits" when its line of
right ascension crosses the zenith.
4. What Are Some Good Introductions To Amateur Astronomy?
In the United States, there are two popular astronomy magazines: Sky
and Telescope (S&T), and Astronomy. Of the two, S&T is more
technical, while Astronomy has more things like "artist's conception
of Jupiter-rise on Ganymede" which are very pretty. I consider S&T a
necessity, but getting both is not a bad idea.
In the U.K., there is a magazine called Astronomy Now which you
probably want to subscribe too. It is rather like Astronomy in
style, but slightly less bulky. Also, fewer pretty pictures.
There is also a magazine called Popular Astronomy (which is not
sold on newstands- you have to join the Junior Astronomical
Society).
There is also a U.K. monthly periodical The Astronomer
(ISSN 0950-138X). This is stapled A4 format with glossy front
and back covers.It is the journal of a group of observers of
the same name and is devoted to rapid publication of observations.
Not for the absolute beginner. Contact John Colls, 177 Thunder Lane,
Norwich, NR7 0JF,United Kingdom.(+44 603-36695). Subscriptions are
21 pounds (UK),25 pounds (rest of Europe).
P. Clay Sherrod's A Complete Guide to Amateur Astronomy,available
through Sky Publishing Company, is a more technical introduction.
Sidgewick's books are absolutely excellent books,probably the very
best ever written on amateur astronomy.
Nightwatch by Terence Dickinson is a good introductory book on
Astronomy. Great section on purchasing a telescope. Star charts are
so-so.
The Backyard Astronomer's Guide by Terence Dickinson and Alan Dyer.
A comprehensive introduction to astronomy and the equipment amateurs
like to use. Written by and for amateur astronomers.
Also see below, the section on Books and Starcharts.
5. What Will I Be Able To See?
The best way to find out is to go observing with someone. Look for a
local astronomy club (S&T lists them periodically). This is also a
very good way to get a good price on a used telescope of proven
quality.
In general, you will be able to see all planets except Pluto as
disks.You will be able to see the bands and Red Spot on Jupiter and
the rings around Saturn. You may be able to see the ice caps on Mars
(although Mars is probably the most disappointing object in the
Solar System). Venus and Mercury will show phases but not much else.
You will be able to see four of Jupiter's moons as points. Ditto
Saturn's moon Titan. You will be able to see comets.
Do not expect your images to be anywhere as nice as the ones you see
from the Voyager spacecraft. If a $2000 telescope could get these,
nobody would have spent billions of dollars to send a spacecraft out
there.
As far as "deep sky" objects, you will be able to see all the
Messier objects in most any modern telescope. Galaxies will tend to
look like bright blobs. Look a while longer and you may find some
spiral arms or dust lanes (assuming it has them). Galaxies look
nothing like their pictures - you will not see the arms anywhere near
as clearly.
You will also find that the colors you see are considerably more
muted than the pictures you see. This is because our retinas work by
having two different types of light sensitive organs, rods and
cones. Rods are very sensitive to dim light, but relatively useless
for color vision. Cones are the opposite. Thus when looking through
a telescope you are using your rods,and you aren't seeing a lot of
color.
6. Buying A Telescope
6.1. What Company Makes the Best Telescopes?
This is a very unfair question at the best. There are many companys
which make good telescopes. A lot will depend on just how much you
want to spend for a telescope. The Major companys that make and/or
sell telescopes are as follows:Orion Telescopes, Meade, and
Celestron, but you have to be carefull with what you buy from even
these companys,as they ALL are selling telescopes which are coming
from prison factorys in 'Red China' and are the same as the Junk
department store telescopes. There are other smaller companys that
make good scopes too.
Televue has a very good reputation, at a somewhat higher price.
Tasco is sold at Toys R Us, K-Mart, & Wal-Mart. Waste of Money.
Simmons: Another waste of money worse than Tasco.
Bushnell: I have looked at this companys telescopes 1st hand and
I do not belive that they would withstand one full night of
useage viewing the sky. They are even WORSE than Simmons! They
are so bad they make Tasco junk look good!
There are now a lot of smaller companys poping up that are selling
the same 'Made in Red China' telescopes under names never seen before
it would be a good idea to stay away from them too.
There are some companys importing telescopes from Russia, I have not
seen these scopes first hand, but have read some good reports of
them.
6.2. What Is The Best Telescope To Buy?
Once more this will depend on the answers of questions you need to
ask yourself. Are you going to use the telescope for just viewing?
or are you going to into the field of Astrophotography? Also it will
depend on how much you want to spend too. In the end,only YOU can
answer this question.
You will also find useful articles in the November 1991 issue of
Astronomy (specs on a wide range of telescopes, and answers to a lot
of the questions about technical jargon surrounding advertisers
and equipment. There is also an article in the November 1991 issue of
"Popular Astronomy." Both Astronomy and S&T (especially the former)
do review articles on telescopes, accessories, etc. on a fairly
regular basis. Also, no FAQ list is going to be truly definitive - we
all have our own opinions and interests, and one person's
"piece-of-junk optics" might be another person's dream telecope.
This does not apply to department store telescope, though. Really.
Under $100
Get a pair of binoculars. The only telescopes in the double digit
range are pure junk. On the other hand, you can get a good pair of
binoculars. Orion sells a pair (the 7x50 or 10x50 Observer (17mm and
14mm eye relief respectively)) for $109 specifically designed for
astronomy. The Bushnell "SportView" are a possibility as well.
Under $300
Orion has a 60mm Refactor for under $200. For just under $300
Stargazer Steve has a 4.5in Newtonian DOB that comes as a kit,
and is very easy to do and is also a fine telescope and Edmund
Scientific Co. has one for around $230. There are some others on
the market now, but once more you have to be carefull, for a lot
of them are still the 'Made in Red China' junk scopes.
>From $300 to $500
Edmunds Astroscan 2001 ($290 - $340). It is a very portable 4"
Newtonian with the distinctive shape of a cylinder thrust into a
sphere. The sphere rests in an aluminum base and the telescope can
be pointed in any direction. Uses 1.25" diameter eyepieces.
Supplied with a 28mm eyepiece giving 16x and a 3-degree field of
view, wide enough to do without a finder scope. The drawbacks are
that it is not very good for planets; and that it's difficult to
track at high power. Also the "permanent collimation" the Astroscan
comes with probably isn't. One respondent's seems to have come
slightly out of alignment; this is unnoticeable at low power (e.g.,
the 16x it gives with the eyepiece it comes with), but is noticeable
and rather objectionable at about 100x. Since it's permanently
sealed up you can't go in and tweak the mirrors the way you can with
most reflectors; you have to send it off to Edmund so they can look
at it. Even if you could tweak it yourself, getting it all aligned
would be tricky; short focal-length reflectors (which the Astroscan
is an example of) are much more sensitive to minor alignment errors
than longer ones. Accordingly, your high power images may be on the
fuzzy side.
$500 to $900
Now you are moving into the range were you will start to find a
bigger range of telescops.
Orion has both 6" f/8 and 8" f/6 and 10" f/4.5 DOBs along with
their 4.5" Skyview Reflector and 90mm Refactor telescopes. If you go
bare bones, you can even get one of their 12.5"DOB's for just under
$900.
I've not seen them up close, but Meade also makes some DOB's that
fall into this price range too. Meade also has a line of Equatorial
Reflectors that fall into this price range. They also have a line of
Refactors too. You have to find a Dealer that handles Meade as they
do not do mail order like other companys.
The next company is Celestron, they now have 2 DOB's under the
'Star Hopper' name, a 6" f/8 and a 8" f/6 model, they both are in
this price range. I belive that Celestron has a Refactor or two in
this price range.
Another company is Murnaghan instruments, now making the Odyssey
line of DOB telescopes. They range from 8inch f/4.5 to 13.1inch f/4.5
The price range is :
MODEL # TUBE (Length X O.D.) AND WEIGHT PRICE
SIZE Improvements [in brackets] now apply to COMPLETE
F-RATIO ALL ODYSSEY (tm) SCOPES! ($ U.S.)
=========================================================================
ODYSSEY 6
6 in. 48 X 8 in. Double-Strength Sonotube, 37 lbs. $279.95
f/8 [Rock-Solid Dob mount, big Teflon bearings,
Improved Micro-Focuser]
ODYSSEY 8
8 in. 37 X 10 in. Ultra-Compact-Deep Space, 39 lbs. $399.95
f/4.5 27 mm Coated eyepiece. [Easy-Adjust Rear
Cell, Provisions for Cool-down Fan option]
ODYSSEY 8L
8 in. 57 X 10 in. Lunar/Planetary Scope, 59 lbs. $399.95
f/7 27 mm Coated eyepiece. [Improved stability,
Mount points for optional Finder Scope\par
and Balance Kit installation.]
ODYSSEY COMPACT
10.1 in. 45 X 13 in. Extremely portable, only 65 lbs. $499.95
f/4.5 Large Aperture, Wide-Field, 1-person setup.
[Improved low-obstruction spider, secondary.]
ODYSSEY I
13.1 in. 59 X 16 in. a BIG scope for LESS! 97 lbs. $799.95
f/4.5 Better Optics, Large Aperture and Improved
Mount System for Maximum Big-scope Stability.
Coulter Optical (tm), a part of MURNAGHAN Instruments Corp.
1781 Primrose Ln., W. Palm Beach, FL 33414 U.S.A.
Ph. 1-561-795-2201, Fax 1-561-795-9889, E-mail murni@bix.com
There are now some smaller companys that are marketing telescopes
that are in this price range. See the list.
$1000
Orion, Meade, and Celestron all have telescopes in this price
range.
You'll find some of the bigger DOBs and SCT's around this price.
Around $1500
Now you are getting up into the range of telescopes that are either
bigger or have more extras. This is the low end of the price rage of
the Meade SCT's, like the one below.
The Meade 2120B The cheapest 10" Schmidt-Cassegrain I could find,
except for the 2120A, which appears to be the same scope, but
without the coatings. The 2120A sells for $1500 from the
discounters, so the B is almost certainly a better buy. The 2120B
appears to be an f/10 scope with a fork mount.
Around $2000
The Meade 2120 model 40 ($2000 - $2150). A 10" f/10 Schmidt-
Cassegrain system with "Smart Drive" and a hand controller. The
motor works in the Right Ascension direction but not declination
(the declination motor costs extra). Do NOT opt for the super wedge.
It costs about $300 extra, and can be bought separately (ie.,
later when you decide you actually could use it) for about $300. The
same logic applies to all the nice things you get with the model
50 it costs as much to buy them packaged as to buy them
individually. The issue is that beginning astronomers do not need
all the fancy equipment. The big disadvantage, which I did not
appreciate until I bought this telescope, is that while the optical
tube weighs only 45 pounds, it is unwieldy as hell in the case
they give you. I find that I cannot maneuver it around corners in my
house, so I either have to get my wife to help me,or I have to carry
it by holding the forks, which do not give as good a purchase as one
would like, given that one is holding a $2000 piece of very
sensitive, and reasonably heavy, junk. It also takes up enough room
in the back of the car that it won't fit if we are filling the car
for a camping trip.
Around $2500
The TeleVue Genesis (? - $1600) and Systems Mount (? - $900). A 4"
Fluorite Refractor, which many people rave about. The Genesis II has
been designed to fit into an airline overhead rack.
The Meade 10" LX200. Tons of wiz-bang features (see above), for not
a lot more than the Premier 2120s. The finder scope looks like the
feeble one that came with my 2120/40, but you can certainly live
with that for a while.
Telescopes over $3000:
The following list is ment to be helpful information and not a sales
pitch for any particular company.
Listed below are telescopes by three companies,Astro-Physics,
Takahashi, and Obsession. Several of the optical tube assemblies are
under $3000 but with a new mount most of the complete telescope
assemblies cost more than $3000.
These are not the only telescopes costing more than $3000, but they
are the ones I am more familiar with.
APO REFRACTORS
APO (Apocromatic) refractors, have special objective lenses that
produce images essentially free of false color, with very high
contrast.
ASTRO-PHYSICS:
Astro-Physics makes some of the finest APO refractors in the world.
Their telescopes are made in the U.S.A. Their objective lenses are
multi-coated triplets with Super ED glass. Astro-Physics offers a
full line of accessories for their telescopes.
You should call Astro-Physics Inc. for current prices (815) 282-1513.
Optical Tube Assemblies by Astro-Physics:
Aperture / f ratio tube length O.T.A weight
dew cap retracted
105mm f6 EDT APO 19 inches 9 lbs
130mm f6 EDF APO 28.5 inches 15 lbs
130mm f8 EDT APO 36 inches 16 lbs
155mm f7 EDF APO 41 inches 23 lbs
155mm f7 EDF APO * 41 inches 23 lbs
180mm f9 EDT APO 60 inches 35 lbs
* astrograph with 4 inch focuser and 4 inch flat field lens (the
other Astro-Physics OTAs have 2.7 inch focusers)
Astro-Physics mounts:
All Astro-Physics can be used with JMI NGC-MAX computers.
400 German Equitorial Mount
Refractors up to 5.1", reflectors to 6", Cassegrains to 8"
Weight of equitorial head, 20 lbs (9.1 kg).
600 German Equitorial Mount
Refractors up to 6.1" f9, reflectors to 8", Cassegrains to 10"
Weight of equitorial head, 27 lbs (12.3 kg).
800 German Equitorial Mount
Refractors up to 7.1", reflectors to 10", Cassegrains to 12"
Weight of equitorial head, 45 lbs (20.5 kg).
900 German Equitorial Mount
capacity not listed
Weight of equitorial head, 38 lbs (17.3 kg).
1200 German Equitorial Mount
capacity aprox. 90 lbs.
Weight of equitorial head, 72 lbs (30.9 kg).
TAKAHASHI:
Takahashi makes some of the finest APO refractors in the world.
Takahashi uses fluorite rather than ED glass to achieve excellent
color correction. The three listed below are fluorite doublet APOs.
Takahashi makes a number of different types of telescopes and
accessories. They are imported in the U.S.A by Texas Nautical Repair
Company/Land Sea and Air (713) 529-3551. Call your local
Takahashi dealer for current prices and specs.
Optical Tube Assemblies by Takahashi:
call dealer for current prices.
Aperture / f ratio tube length O.T.A. wieght
78mm f8.1 FS-78 APO 28 inches 6.5 lbs
102mm f8 FS-102 APO 36 inches 11.5 lbs
128mm f8 FS-128 APO ? about 22 lbs
Takahashi mounts:
Listed below are only two of several mounts available.
EM-10 German Equitorial mount refractors to about 15 lbs.
EM-200 German Equitorial mount refractors to about 25 lbs.
"High End" Dobs Telescopes:
Obsession Telescopes.
You should call Obsession for current prices and specs
(414) 648-2328.
Aperture f ratio primary mirror maker
15" f4.5 Nova
18" f4.5 Nova/Galaxy
20" f5 Nova/Galaxy
25" f5 Nova/Galaxy
30" f4.5 Nova
SENT 01-12-96 FROM NEUSCHAEFER_RICH
Tele Vue Pronto
A beautifully made 70mm f6.8 ED doublet Semi-APO refractor.
It is a small astronomical telescope that can also be used as a
spotting scope. With very sharp optics it can easily show much
lunar detail, banding on Jupiter, the rings of Saturn, as well as a
number of deep sky objects.
Weight: aprox. 6 lbs., length: 18 inches. Front cell can take a
standard 82mm photo filter. Comes with protective travel bag.
Can get the Pronto with 2" Tele Vue mirror diagonal or 1 1/4"
diagonal or 1 1/4" 45 deg correct viewing prism.
Tele Vue Ranger
A beautifully made travel scope for astronomy or wildlife spotting
with the same sharp optics as the Pronto.
It is a 70mm f6.8 ED doublet Semi-APO refractor like the Pronto but
it is lighter and uses a unique, silky smooth, in-line helical
focuser.
Weight: aprox. 3.5 lbs, length about 18 inches. Comes with either a
1 1/4" mirror diagonal or 1 1/4" correct viewing prism.
For more info see Tele Vue's ad in the Astro-Mall at:
http://www.rahul.net/resource/
Ultra-Priced Scopes
David Smith contributes the following about the NGT (about $9000-not
an inexpensive choice): I have spent a couple of evenings with an
acquaintance who has an NGT-18. It is a very good scope. It's
comparable in size to a Dobsonian, and I don't need a ladder to see
into the eyepiece. I could see dim stars among the Trapezium which I
couldn't see in other scopes nearby (4" refractors and 8-10"
Newtonians and SC's). The rotating nosepiece works well, although it
places increased demands on accuracy of physical and optical
axes: the view was sharper from one rotation of the nosepiece than
from another. Disadvantages of the NGT-18 are price, time to set up
and take down, and lack of fine adjustments for polar alignment.
7. OK, Where Do I Buy My Telescope?
Well, there are three basic places:
A Store
Yes, the obvious-you find a store (NOT a department store) which
sells telescopes and write a check (or, if they won't give you a
cash discount, use a credit card that offers buyer protection, or
gives you bonus miles, or some such).
The advantages of this method is that you have someplace to return
the telescope to if you have problems with it. Some places even
offer your money back if you change your mind within some grace
period.
The disadvantage is that you generally pay more for the telescope
itself, and you pay sales tax.
Mail Order
There are two sorts of mail order: the discount stores that sell all
sorts of stuff through the mail, and telescope stores that sell
through the mail in addition to selling from their store.
The advantages and disadvantages of mail order are obvious: you
cannot take the merchandise back easily if something goes wrong, but
it's cheaper and you probably pay no sales tax.
Other People
You can find some great deals in used telescopes. Many people buy
expensive telescopes, use them two or three times, get bored and
sell them. The advantage is strictly monetary: you pay significantly
less (and,of course, no tax).
The disadvantage is that you are buying something "as is" which you
may want to think twice about doing if you are buying an expensive
telescope. Also, both Meade and Celestron offer (limited) lifetime
warranties on their optics, which are not transferable.
All that having been said, here is a list of places you can buy
telescopes, with comments as applicable. Note that all will sell
director will ship.
Orion Telescopes
P.O. Box 1158\par
Santa Cruz, CA 95061
(also San Francisco and Cupertino)
800-447-1001
sales@oriontel.com
Orion Telescopes carries a wide selection of binoculars, telescopes,
and accessories (Celestron, Tele Vue, and their house brand; they do
not carry Meade). They have a 30 day "no questions, satisfaction
guaranteed" refund policy, which they do seem serious about. A fair
number of people (myself included) have bought at Orion and all are
very satisfied with the way they were treated. If you need technical
assistance when you call, ask for Steve or Eric. They have a very
good service and support record.
Lumicon
2111 Research Dr. #5
Livermore,Ca.94550
While I have not had any dealings with this company,the messages
I've seen on sci.astro.am have all had good things to say about them.
Astronomics
2401 Tee Circle Suites 105/106
Norman, OK 73069
Higher prices than Adorama and Focus (see below),but lower than
Orion and Lumicon. Enthusiastically recommended by a couple of
people on the net. As with all mail order, make sure the shipping
price is included.
Pocono Mountain Optics
104 N.Plaza
Moscow,Pa.18444
Enthusiastically recommended by a few people on the net.Owned by
Glenn Jacobs who goes to most of the astronomy get-togethers in the
NY-NJ-PA-CT area so you actually meet him if you live in the area.
Often willing to cut a package deal if you are buying big ticket
items. No problems returning things with which you are dissatisfied.
Roger Tuthill
11 Tanglewood Lane
Dept. ST
Mountainside,N.J. 07092
Enthusiastically recommended by a person on the net. Not the least
expensive, but top-notch service. Roger unpacks, inspects and
collimates every scope he sells, and is very good about refunding
your money if you are dissatisfied.
University Optics
P.O.Box 1205
Ann Arbor,Mi.48106
A few people have reported using University Optics, and all report
receiving good service. I have heard no complaints.
7.1. What About Buying Used?
If you decide to buy used, get a subscription to The Starry
Messenger and/or the E-Mail AstroMart.
It appears that most people want to get about 75% of list when
advertising in the astronomy mags (Starry Messenger, S&T, etc). This
is probably not enough of a discount to make it worthwhile. If you
can find something at 50% of list, you might want to think about it.
A used telescope is just as good as a new one if it's been properly
stored, transported and used.
Then there is also the AstroMart Ad service.
Astromart is a free resource to the Internet community with 1,700
subscribers. It is strictly Astro Classified ads. All ads are
distributed via electronic mail as well as going on the web with
look-up keys and a search engine. It is the best way to buy and sell
used astronomy stuff.
_____________________________________________________________
-=> A S T R O M A R T <=-
http://www.astromart.com/
Your Free InterNet Astro Classified MarketPlace
= TO SUBSCRIBE =
Send an E-mail to astromart-request@lists.best.com
In the message body type: subsingle
For the digest version type: subscribe
= TO PLACE AN AD =
Send your ad to: Ads@Astromart.com
------------------------------------------------------------
7.2. What About Building A Telescope?
This section was written by Andy Michael.
We just took a rather unusual approach to getting a beginning
telescope: we took John Dobson's telescope building class and built
an 8"and a 12.5" reflector on Dobsonian mounts (of course). We went
this way for a few reasons: to get large aperture for seeing deep sky
objects and higher magnification with good resolution when compared
to small refractors in this price range, to keep the price down, and
to soak up John's wit and wisdom. The down side is that these
telescopes are not suited for astro-photography (at least not without
building a different mount) but that didn't bother us. Also they are
large. The 8" tube we broke into two pieces for easy portability,
but the 12.5" one will probably go on the roof rack. These are about
f/7 telescopes so the tube lengths are 56" and 7' respectively. Of
course, when you build yours you can make whatever size you want. On
the other hand you can pack your clothes in them; try that with an
SCT. The cost was about $250 for the 8" telescope, $450 for the
12.5"er plus about 24 to 30 hours of work and 16 - 24 hours of
class. It's a challenging project but the first time you focus on
something with a mirror you ground is an incredible thrill. Another
benefit is that we now know a lot about telescope design and if we
ever have problems with them we know how to fix them.
If you don't have access to John's (or other peoples) classes then
you can try building one by reading his book and by watching the
video. Our class was the first to see parts of the video and had
great success at finishing the telescopes fast and without needing to
correct the mirrors very much. Coincidence? Class consensus was no.
The book (excerpted from the order form): "How and Why to Make a
User-Friendly Sidewalk Telescope" by John Dobson with Norm
Sperling.To appreciate why Dobson makes each factor just so, learn
how he thinks about it. His philosophy of star-gazing perfuses his
telescopes and his book. The book includes the only detailed
biography;wonderful vignettes from the Sidewalk Astronomers'many
expeditions;their own special way of describing celestial objects;
and, of course,complete details for making a Dobsonian. 169 pages;
154 clear,friendly line drawings; 9 photos. Hardbound in plywood,
Dobson's favorite material. Exclusive source. Send $39.95 + $5.00
shipping to Everything in the Universe, 185 John Street, Oakland, CA
94611.
The video (also excerpted from the order form): For the first time
on video, John Dobson shows how you can build your own low-cost
Dobsonian Telescope. The 90-minute video is a complete step-by-step
guide, covering telescopes from 8 inches to 16 inches in diameter.
$39.95 +$3.50 shipping.
Parks Optical
270 Easy St.
Simi Vally,Ca.93065
A couple of people have mentioned that shipment can be pretty
delayed,but the quality of their equipment appears to be high, and
improving.Salespeople vary from knowledgeble to bubbleheaded.
Adorama
42 West 18th Street
New York, NY 10011
orders: (800) 223-2500
info: (212) 741-0052
Along with Focus Camera (see below), the lowest prices you will
find. Expect no dealer support, and make sure you find out how much
they will charge for shipping before placing your order. And pray
that the optics arrive intact. I really would recommend that you not
buy telescopes from these guys. Eyepieces and other accessories,
however, are probably worth the risk if the price difference is
significant.
Focus Camera
4419-21 13th Avenue
Brooklyn, NY 11219
orders: (800) 221-0828
info: (718) 436-1518
Refer to Adorama. Same comments apply.
Pauli's Wholesale Optical
Danbury, CT
A lot of bad reports, order at your own risk!
Also there is the AstroMall.
****************************************************************
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** A S T R O - M A L L **
** **
** ONE STOP ASTRONOMY SHOPPING AND PRODUCT INFORMATION **
** **
** See in-depth product information for such companies and **
** products as: **
** **
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** Lumicon, Software Bisque, Astro-Cards, Astronomical **
** Adventures, Bethany Sciences, Equatorial Platforms, **
** Jim Kendrick Studio, Science Software, Celestial Products, **
** Gnome Technologies, Custom Ophthalmics, Analytical **
** Scientific, Galactic Images, Murnaghan Instruments, Crazy **
** Ed Optical, Celestial Scripts, Deepsky 2000 and more... **
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8. What Accessories Will I Need?
In addition to a telescope, you absolutely must have a mounting and
a tripod. You will also need a few eyepieces, a telescope with only
one eyepiece is like a piano with one key.
These accessories don't come cheap, expect to pay as much for the
mounting and tripod as you paid for the optical tube. For a first
telescope,you probably will want to buy an entire system it tends to
be less expensive that way.
Which eyepieces should you start with? I'd suggest three or four,
maybe a 30mm, 25mm, 20mm, 8mm and a 2x Barlow (which will give you
coverage of 30,25, 20, 15, 12.5, 10, 8, and 4 mm). Buy eyepieces
of like quality to your telescope. Putting a $300 Nagler eyepiece on
a $150 telescope is pointless (it would also probably tip over the
entire telescope).
9. What Are Digital Setting Circles
This section was written by Jim Van Nuland
9.1. What Are They?
Digital Setting Circles (DSCs) are a small special purpose computer,
mounted on or near a telescope. The scope has shaft encoders
attached to sense the motion of the scope's axes, and the computer
then converts these motions to the position of the telescope, and
displays it (for instance) in Right Ascension (RA) and Declination.
An 8-conductor cable runs from the computer to the encoders, with 4
wires to each encoder. RJ-45 telephone connectors are used at the
computer.
They do NOT move the scope. You push it by hand, and the DSCs tell
you which way to move and how much.
What makes DSCs so desirable is that they work on alt/az-mounted
scopes; and even with equatorial mountings, it is not necessary to
polar align the mount. (However, it's desirable to have the mount at
least roughly polar-aligned so it follows an object.)
Additionally, most models have an internal catalog and a "guide"
mode.One selects an object (or, in some,a planet), and the DSCs tell
which way to move each axis.
They are marketed by Lumicon, Jim's Mobile, Inc., Celestron, and
Orion Telescope Centers. The various brands and models differ mostly
in their internal catalogs of celestial objects. All are actually
manufactured by the same company, Tangent Instruments of Palo Alto,
California, USA, who, however does not sell directly to individuals.
I own the NGC-MAX from JMI, so some of my statements may not apply
to other versions.
9.2. Must the ground board be leveled?
No. An alt/az mount must have a fiduciary mark such that the tube
can be placed accurately at 90 degrees to the elevation axis.
One way to do this is to (one time only) level the ground board,
then the tube. Make the mark in such a manner that it can be
adjusted when something changes. Some models of DSCs allow an
alt/az mount to be initialized in a vertical position. When
starting the DSCs, the tube must be set horizontal (or vertical),
and then two stars are used to align. The stars must be at least
20 degrees apart in the sky (90 is ideal), and the first may not be
Polaris.
9.3. How does one set up an equatorial mounting?
If the mount is known to be accurately polar aligned, you may still
use two stars as mentioned above. Or you may set the DSCs to take
advantage of the known alignment, and it will require only one
object,and no zero degree reference mark is needed.
If an equatorial mount is not polar aligned, it must have a
reference mark at zero degrees declination, and must use the
two-star setup. For a German mount, the mark may be on either side
of the scope (tube pointing east or west), and the DSCs set to
correspond. The mount may be driven or undriven. As for an alt/az
mount, the stars must be at least 20 degrees apart, and the first
may not be Polaris.
9.4. Do the DSCs support a Poncet platform?
Probably depends on the model. The NGC-MAX provides telescope type
ET(equatorial table). It assumes that the table is carrying an
alt/az scope, and that the scope is initialized with the tube
horizontal. I believe that an equatorial mount could be used, but
have not tried to simulate it.
9.5. How accurate is the device?
The position of the scope is displayed to one minute of RA and 10
minutes of dec. Guide mode displays position error to 0.1 degree of
arc. The actual accuracy depends on the care with which the alignment
was done, the accuracy of the mounting, accuracy with which the shaft
encoders were installed, the resolution of the encoders, and a bit
of luck. If the level or zero was not set accurately, the system will
work poorly, and it should be re-started. If star settings were done
carelessly, one can simply re-do one or both of them.
The "luck" factor stems from the digital nature of the shaft
encoders. If the encoder is on the verge of a step, you could be off
by one step.
The absolute theoretical resolution is three encoder steps, assuming
everything else is perfect. In practice, I get about 0.2 to 0.3
degrees, and closer near the alignment stars. If I move a long way
across the sky, the error is perhaps 0.5, but then I re-align on a
convenient nearby star. It's not too unusual to get 0.1 if all has
gone especially well during alignment. This with 4000 step encoders.
Accuracy is best between the alignment stars, and the DSCs calculate
a "warp" so as to spread out the error. When re-aligning, only one
star sighting is needed. The DSCs retain only the two most-recent
star settings, provided they are at least 20 degrees apart in the
sky.
9.6. What objects are in the internal catalog?
This is the major difference between models. All have a few dozen
named stars, used especially for initial alignment. Some have the
planets. The Lumicon models have a catalog of planetary nebulae,
which is Dr. Jack Marling's specialty.
The NGC-MAX version 3.94 (July, 1992) has the planets;28 user
defined objects; the Messier catalog (including M40 and M110); the
full NGC,including the so-called "non-existent" objects; about half
of the IC catalog; a catalog of 951 interesting stars (multiple,
red, variable);and a list of 367 additional deep-sky objects, many
of which are very faint.
For each object, the catalog has the position, magnitude, size
(diameter or separation), constellation, name (if any) and/or
catalog number, and the type of object. Some have a word or two of
description. This probably varies with the brand and model.
9.7. May I add my own objects? Comets, for instance?
The NGC-MAX accepts user objects, and I presume most other high-end
models do as well. I like to put in the Sun and Moon, so that I can
align during the day. This must be done carefully, with the Sun
filter attached. THIS IS DANGEROUS, as the filter must be removed
when sighting on the Moon, and if you come back to the Sun, you MUST
have first re-attached the filter! The moon is a poor alignment
object because it has up to a degree of parallax, and it moves about
0.5 degrees per hour. But it provides a start, and it may be enough
to locate some bright stars, and re-align.
9.8. What is "identify" mode?
Identify mode is present in the NGC-MAX, and probably other models.
One specifies the class of object, and the faintest magnitude, then
the DSC selects the nearest to the telescope's position. Very nice,
but in the Realm of Galaxies, alignment is critical and then there
are too many to be certain. To check, read out the magnitude and
description, and go to Guide mode and see how far away the object is.
It's especially useful in clouds, as one may point the scope into a
clear spot, then ask what is nearby. One must separately search for
galaxies, clusters, etc.
Identify mode runs continuously, so that, as the scope is moved,
the DSCs will (after a few seconds), indicate the new (or nearest)
object.
Some models allow alignment on ANY catalog object, which is helpful,
but I find that accuracy is best on stars or very round objects. I
find that planetary positions are especially suspect. The computer
carries only the date, not the hour. (Use UT date.) I have often had
poor alignments when using planets, and do so only for daylight set-
ups; I re-align on stars as soon as I can find any. Open clusters are
especially unreliable; galaxies are not much better.
Jim Van Nuland, San Jose (California) Astronomical Association
9.9. Can it replace star charts?
For comparatively easy objects, probably. In a crowded
field, no.Some models support the Tiron Atlas 2000 and
the Uranometria 2000, by indicating, for each object,
the page on which it (the object) will be found.
These models also indicate the chart corresponding to
the position of the scope, regardless of specific object.
9.10. What other functions are present?
This varies heavily with model. The NGC-MAX (here we
go again) has two that have not already been discussed.
"Timer" counts up in hours, minutes, and seconds.
It can be stopped,reset, and re-started, but can't
be restarted without first being reset.
"Encoder" shows the encoder positions in degrees.
If an alt/az scope was pointed north when the DSC
was powered up, then encoder mode will read elevation
and azimuth, if the scope is also standing reasonably
level.
9.11. How is it powered? How long does the battery last?
There is an internal 9-volt transistor battery. The
load is 18 to 40mA (NGC-MAX), depending on how bright
the display is. I suppose this might depend on the model,
too. The maker claims 30 to 50 hours on an alkaline battery.
They do last a good long time. There is a "low battery"
indicator which would turn on at about 4.5 volts, but in
practice, I get "encoder error" messages before that.
Some models have a second connector (serial port) by which
external 9- 15 volts DC may be supplied. This does not
require the internal battery to be removed; the two
supplies are in parallel with diodes to prevent back-circuits.
It does not recharge the internal battery.
9.12. How accurately SHOULD the mount be constructed?
The brief answer is, as accurately as you'd like the DSCs
to operate.For an equatorial mount, there must be little
flexure; the RA axis must be perpendicular to the dec
axis, which in turn must be perpendicular to the optical
axis of the tube.
For an alt/az mount, the ground board must be rigid,
the azimuth bearing surface must be flat, dent-free
and stiff; and the side bearings must be the identical
height, that is, the elevation and azimuth axes
must be accurately perpendicular. In addition, the optical
axis of the tube must be perpendicular to the elevation axis.
There is a terrible irony here: the Dobsonian mount works
precisely because its kinematically stable design does
NOT require that it be accurately constructed!
9.13. How accurately should encoders be installed?
Again, the short answer is, as accurately as you'd like
the DSCs to operate. One can't do the job with a hand-held
drill. OTOH, careful work with a modest lathe and drill
press is quite sufficient, especially if performed by a
modest machinist. Most astronomy clubs have such a person.
Best accuracy is obtained with high-resolution encoders.
Standard encoders have 2048 steps per revolution, and
high-res type has 4000.One can also use gears to provide
greater resolution, but see below.
If the encoder is connected directly to a shaft, the
hole in the shaft must not be oversize. It must be straight,
well centered, and parallel to the axis. The body of the
encoder must be held so that it cannot rotate with the shaft.
If it is connected by gears, the shafts must be parallel,
and there must be no backlash.
Encoders are not especially delicate, but they do not
like to be bent.They require very little torque, and rotate
continuously. The setscrew should not deform the shaft.
The 4-wire connector should be looped so it does not pull
on the encoder. They may be mounted such that the shaft is
stationary, with the body moving, or the usual way;the
direction is set in the DSCs' setup option.
In an alt/az mount, the azimuth encoder is typically
mounted atop the center bolt. In this case, the bolt
must be nicely perpendicular to the ground board,
and the comments about shaft mounting (above) apply.
If the rocker box has any side play, it will be nearly
impossible to avoid some runout. This can be reduced by
using a very long lever arm to hold the body of the encoder.
Both side bearings must be round (especially the one with the
encoder), the center must be carefully located, and the
encoder shaft parallel to the elevation axis. Any runout
There will cause serious inaccuracies when moving across the sky.
9.14. How accurately MUST the mount be constructed?
Please don't feel that only a million dollar mount can be equipped
with DSCs. My 1972 Optical Craftsman (German) mount works very
well,even with about 0.5 degrees of error if I shift the mounting
and return to an object. This was the economy model! A machinist
friend helped me drill the holes for the encoder shafts.
I used UGMA grade 10 precision gears to step up the dec shaft speed.
The designer of the DSCs was amazed at that, and admitted that he
used UGMA 4 with adequate results. I don't know how to calculate how
much more accuracy I might be getting from my expensive gears.
My alt/az mount, crafted of wood in my shop with only hand tools,
carries a 108mm f/4 scope, and *always* puts an object in a
low-power field. OTOH, if I re-collimate the scope, I must also
re-position the vertical mark. I usually re-align after moving far
across the sky.
If the mounting is less than perfect, it means that you will need
tore-align more often. But if the mount is *really* sloppy, it
probably will not be satisfactory.
9.15. Can I connect the DSCs to my own computer?
Yes, for some models. The NGC-MAX, and probably others,has a serial
port that may be used with an external computer, so that the screen
shows a dynamic star map, identifies objects, etc.
But the attached computer must take over ALL functions, including
the prompting for "level me," pointing at particular alignment
stars,guiding, calculating the conversions for RA and Dec, etc. I
understand The_Sky, from Software Bisque, does all this, but I have
not seen it in use nor heard from a live user.
The port is a modular telephone connector (RJ11). It has four
wires:B+, data in, data out, and ground. External to the NGC-MAX,
the cable must route DTR back to the attached computer as DSR, CD,
and/or CTS,as needed by the attached computer. The 4th wire is
+Battery, a 9 to 15 volt external power supply, which does not
charge the internal battery. It is not necessary to remove the
internal battery,
When the NGC-MAX is operating in "BOX" mode, it blanks its own
display, and does nothing but pass the shaft encoders' values over
the serial port. It multiplies them by the encoder ratios (the
latter set in the NGC-MAX setup function), and scales them such that
00000 is the position at power-on, and 32767 is just under 1
rotation.
Communication is at 9600,8,N,1. When the NGC-MAX powers on, it sends
a hello message such as "V2.94". When the attached computer sends a
character (the sample program uses "Q" but anything seems to work)
down the port; and the NGC-MAX replies with 13 characters of the
format "+00000t+00000" where the "t" is ASCII 9, and the 00000s
are the two encoder values.
I don't use this facility, but I'm too curious not to have tried
it.I used my modem program to supply the computer side. I use the
NGC-MAX whenever I'm doing general observing, and I like it very
well.But I don't have a portable computer to use with it, and don't
too much see the need. OTOH, if I fell into a laptop, I'd surely
want to try connecting them.
10. Why Should I Start With Binoculars?
The quick answer is because you already have them, so you do not
have to spend any money. Certainly going right out and buying the
Fujinon 25x150 Astronomical Binocular ($11,000 list price) would be
a pretty stupid thing to do, no matter how good the binoculars are.
You should also avoid the quick-focus binoculars, as they are easy
to de-focus as well.
The remainder of this section was written by Paul Zander.
Based on my experience, I suggest that you start with a pair of 7x50
binoculars. This is the most popular size and hence good ones are
available from many stores, even some of the discounters. Be sure to
get ones that have anti-reflection coatings on the mirrors and lens.
If you wear eyeglasses, you may be able to find binoculars which can
focus without them (unless you have significant astigmatism). Make
sure the image is sharp at the center and edges at the same time.
"7x" is the magnification. Most people can hand hold these without
needing to bother with tripods, etc. The "50" means 50mm (~2 inch)
objectives (aperture). This gives light gathering ability similar to
many small telescopes. Many advanced star gazers regularly use
binoculars to either locate items to focus telescopes on, or just
for the wider field of view.
When trying to view near the zenith, use a reclining lawn lounger:
you can lie back and support your arms on the chair, giving a
steadier view. You also will not get a crick in your neck.
You might also use a plastic pad to lie on.
10.1. How Do I Hold Binoculars?
This section was written by Jay Freeman.
If you don't have a tripod (and tripods are sometimes a little
clumsy, and are often difficult to use when the binocular is
pointing near the zenith), it is important to know how to hold a
binocular correctly to achieve maximum steadiness.
The way most people tend to hold a binocular is with one hand on
each side of the middle of the body-roughly where the prisms are in
a conventional 7x50, say, so that the left hand is directly to the
left of the center of gravity of the instrument and the right hand
is directly opposite it, to the right of the center of gravity.
For most people, there is a better position. Imagine that you are
holding the binocular to your eyes, with your hands positioned as
just described. Now, slide your hands along the body of the
instrument,toward your face, until only your pinky and ring fingers
are curled around the back end of the binocular body. In this
position, the binocular feels a little nose-heavy, because you are
supporting it behind its center of gravity.
Now curl each thumb up as if you were making a fist, and flex your
hands so that the second bone in from the tip of your thumbs are
pressed up against your cheekbones (counting the bone in the part of
your thumb where the thumbnail is, as the first bone). This makes a
quite solid structural connection between the body of the binocular,
through your hands and thumbs, to your face, and markedly improves
how steadily you can hold the instrument. Similarly, curl the first
and middle fingers of each hand around the corresponding binocular
eyepiece, to provide a little more structural connection (and perhaps
also some protection from stray light). In this position, your hands
are not far from where they would be if you brought them to your
face to block out stray reflections while peering through a store
window at night.
For most people, this position leads to markedly steadier viewing,
but if the binocular is especially long and heavy (say, a 10x70 or
an 11x80), the out-of-balance position can be quite tiring. In that
case, move *one* hand out to the objective end of its side of the
binocular, so that you are supporting the instrument on opposite
sides of its center of gravity, but with some structural connection
between it and your face; namely, the other hand. When the hand way
out there gets tired-just switch hands.
For each person, there is a limit to how heavy and / or how powerful
a binocular can be, before there is no way for that person to hold
it steady enough. I am an averaged-sized adult male in reasonable
physical condition, and I find I can hold a 10x70 (Orion's) steadily
enough to use indefinitely on astronomical objects. But I have
an old Celestron 11x80, that doesn't look much bigger or heavier
than the 10x70,that I can only use for a few minutes before my arms
get tired. As a 12-year old I am sure I could have used a 7x50
indefinitely with no problem, but at a younger age I might have had
difficulty using one continuously. Your experience may vary with
your strength, size and condition. Try before you buy, if at all
possible.
10.2. What Are Some Eye Relief Figures?
If you need to wear eyeglasses while looking through binoculars
(presumably you have astigmatism, but if you require many diopters
of correction you might need to as well) you need reasonably good eye
relief. Dana Bunner contributes the following table:
Model Advertised ER Measured ER
Bausch & Lomb 7x26 Custom 16 15
Celestron 10x50 Pro 15 10
Celestron 7x42 Ultima 23 19
Celestron 7x50 Ultima 20 16
Celestron 10x50 Ultima 19 17
Celestron 8x56 Ultima 21 11
Fujinon 8x40 BFL 19 17
Fujinon 7x50 FMT-SX 23 20
Fujinon 10x70 FMT-SX 19 17
Minolta 7x50 Standard 18 16
Minolta 10x50 Standard ? 9 (FYI)
Minolta 10x50 XL 18 16
Nikon 8x30E Criterion 13 13
Nikon 7x50 Windjammer 16 16
Optolyth 10x40 Touring 13 12
Pentax 8x24 UCF 13 8
Pentax 7x35 PCF 14 9
Pentax 7x50 PCF 20 10
Swift 8x25 Micron 13 11
Zeiss 7x42 B/GA T Dialyt 19 18
Zeiss 20x60S ? 14 (FYI)
11. What Books and Star Charts Are Recommended?
If you don't know the constellations, you might want a book that
will help you learn them. A "fun" book for those just learning the
stars is The Stars, A New Way of Seeing Them by H. Rey, which
presents a non-orthodox way of drawing the constellations so
they are easier to visualize.
You will probably want a beginner's guide, such as the book by
Sherrod mentioned above. Sky Publishing has some introductory
materials which would probably be as useful, which you get for free
when you subscribe to Sky and Telescope.
Petersen's Field Guide to the Stars and Planets comes highly
recommended. It is very inexpensive ($13), small and handy to use at
the telescope. It has a good discussion about stars, planets,
nebulae, and galaxies; and has a very complete albeit small-scale
star chart, along with a the usual tables. It hasm long lists of
deep-sky objects for each area of the sky.
You will need a bigger star chart than is included in Petersen's.
Try Sky Atlas 2000.0, by Wil Tirion. The field edition, which has
white stars on a black field, is probably more useful than the desk
guide. It is also printed on heavier paper, so is more resistant to
dew and the rigors of the night. For beginners, buying Uranometria
2000.0 is probably a mistake. Yes, it is the "best" star chart, but
the scale is impossibly small- when the Orion constellation takes up
four separate pages it is really hard to use for beginners.
Burnham's Celestial Handbook ($36). This three volume set is billed
as "An Observer's Guide to the Universe Beyond the Solar System"
rather all-encompassing claim, which it manages to live up too.
Information on every item of interest you can think of: galaxies,
double stars (optical and binary),variable stars, nebulae, etc. More
information than you could use in a lifetime. I consider this a
necessity.
Sky and Telescope's 100 Best Deep Sky Objects. About $5, which is
kind of expensive for a list, but it sure makes it easier to figure
out what to look at when you are just beginning. The items are
sorted by Right Ascension, which makes it real easy to figure
out which ones are currently up.
All the materials listed are available from:
Sky Publishing Corporation
P.O. Box 9111
Belmont, MA 02178-9918 USA
Their catalog is free.
11.1. What About Computer Programs?
There have been a lot of changes in the computer software
since the FAQ was started. There are programs for most computers. If
you have any programs for ANY computer, please write up a short
review of it and e-mail it to me.