Purchasing Amateur Telescopes FAQ
Last Updated: 06-29-02
Copyright (c) 2002 Slc.Dennis Bishop All rights reserved.
|This FAQ is under construction. There may be some sections|
that are not totally done yet.
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?
7.3 What is the Best Mount?
7.4 Binocular Telescope
8. What Accessories Will I Need?
9. What are Digital Setting Circles?
10. Why Should I Start With Binoculars?
11. What Books and Star Charts Are Recommended?
11.1 What About Computer Programs?
11.2 E-mail Lists
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, David Knisely|
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.
2) 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.
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 humongous 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 humongous 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 Dobinsons 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.
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 Dobinsons) 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 telex tenders, 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 Plossel, 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 Plossels 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 4 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. (Editor note: Atari Planetarium / or Big Sky works on all ST/TT/Falcon computers)
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.
Clear sky, and enjoy your telescope.
-- Jay Freeman
3. What Does All the Jargon Mean?
OK, by popular request, here is a glossary of common astronomy terms encountered in amateur astronomy.
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.
The diameter of the objective.
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.
A set of Dobsonian telescopes mounted so their eyepieces form a Binocular 3D view of the sky.
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.
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 nearly perfect.
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 have 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).
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).
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 parties.
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.
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.
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.
f/10, f/6.3 See Focal Ratio
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 star hopping 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.
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.
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.
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.
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.
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.
A common slang term for a large aperture. The cure for "Aperture Fever."
Meridian An imaginary north/south line passing through the zenith.
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.
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.
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 .
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.
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.
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 bird watching, or as a powerful telephoto lens on a SLR camera.
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.
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.
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.
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 Star charts.
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 when in it's orbit it's closer to Earth. 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 Hubble Space Telescope. If a $2000 telescope could get these, nobody would have spent billions of dollars to put the HST in orbit.
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. Remember, many of them are millions of Light Years away.
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 companies which make good telescopes. A lot will depend on just how much you want to spend for a telescope. The Major companies that make and/or sell telescopes are as follows: Orion Telescopes, Meade, and Celestron, but you have to be careful with what you buy from even these companies, as they ALL are selling telescopes which are coming from factories in 'RED CHINA' and are the same as the Junk department store telescopes. There are other smaller companies that make good scopes too. There are some Japanese companies that are selling some very good telescopes and also some poor ones too.
Televue has a very good reputation, at a somewhat higher price.
Tasco is now owned by Bushnell. Another maker of Junk telescopes.
Bushnell: I have looked at this company's telescopes 1st hand and I do not believe that they would withstand one full night of usage viewing the sky. They are even WORST than Simmons! They are so bad they make Tasco junk look good!
There are now a lot of smaller companies popping 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. Even the bigger companies are now selling scopes made in 'Red China'.
There are some companies 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.
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 telescope. This does not apply to department store telescope, though. Really.
As the numbers of companies who now either make and/or just sell Telescopes of ALL price ranges, the list is just to much to put into this FAQ, instead, the next section will list a number of both large and small companies that market telescopes. The best idea would be to contact the companies and find out what kind of telescope they market in your price range. Then if you can, Find one of those telescopes at a Star party.
7. OK, Where Do I Buy My Telescope?
Well, there are three basic places:
1) 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.
2) 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.
3) 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 not all will sell or will ship. To you, some you must go to a store.
P.O. Box 1158
Santa Cruz, CA  95061
(also San Francisco and Cupertino)
Orion Telescopes carries a wide selection of binoculars, telescopes, and accessories They have a 30 day "no questions, satisfaction guaranteed" refund policy, which they do seem serious about. Orion seems to be heading for the low end of the telescope market now.
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.
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.
2835 Columbia St.
Torrance, CA  90503
This company is no longer part of TASCO, they no longer sell DOB's. Main line is GOTO scopes.
615 S. Tremont St.
Oceanside, CA  92054
They make not only Dobs, but also equatorial reflectors and equatorial refractors too. Their scopes are made in USA and are very good scopes.
Mag 1 Instruments
16342 Coachlight Dr.
New Berlin, WI  53151
Markets their 'Portaball' style DOBs in 8in and 12.5in size.
Meade Instruments Corp.
6001 Oak Canyon
Irvine, CA  92620
Markets many types of Telescopes, from junk to High End. No longer lists Dobs.
P.O. Box 804a
Lake Mills, WI  53551
Markets Dob's from 15in to 30in!
1752 Rutherglen Cr.
P3A 2K3 Canada
Markets a 4 1/4inch DOB in both Kit form and/or ready-to-use. Both under $300.00. Also a 6inch DOB kit for $424.00 with shipping.
3228 Rikkard Dr.
Thousand Oaks, CA  91362
Markets DOBs from 8in to 30in.
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.
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 knowledgeable to bubble-headed.
42 West 18th Street
New York, NY 10011
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.
4419-21 13th Ave.
Brooklyn, NY 11219
orders: (800) 221-0828
info: (718) 436-1518
Refer to Adorama. Same comments apply.
Pauli's Wholesale Optical
A lot of bad reports, order at your own risk!
For Both Used and New Scopes and other goodies you can try:
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 per fuses his telescopes and his book. The book includes the only detailed biography; wonderful vignettes from the Sidewalk Astronomer's 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.
Note: This book is no longer being printed. If you find a copy 4 sale anyplace GRAB IT!
7.3 What is the Best Mount?
EQUATORIAL Vs. ALTAZIMUTH: THE TRUTH
The various telescope mounting systems available for use by amateur astronomers have been discussed at length on sci.astro.amateur. There has been a great deal of debate, a little ill-informed opinion, and some real misconceptions concerning each of the basic mounting schemes, so perhaps it is time to clear the air. One basic and irrefutable fact must be stated up front: NO MOUNT SYSTEM IS PERFECT FOR ALL SITUATIONS! Any attempt to champion one mount scheme over another without considering all the facts is doomed to failure. Below are the true advantages and difficulties of the two most popular mounting system.
The ALTAZIMUTH (i.e.: Dobsonian, etc..). This mounting system has gained considerable popularity over the past 20 years, evolving from the old "pillar and claw" system originally used only in inexpensive small telescopes, to a modern well-designed one which boasts of supporting some of the largest apertures in amateur astronomy today.
EQUATORIAL MOUNTS: These mounts are aligned to the celestial coordinate system, and have been the mainstay of serious amateur and professional astronomical telescopes for over a century. They come in a variety of designs: German Equatorial, English Yoke, English Cross-axis, Polar disk, Fork, Split Ring, etc..
NOTE: none of these disadvantages will eliminate a mount design from use by the amateur. For strictly visual use (especially for the beginner), the altazimuth can easily be recommended, while for long-exposure photography, the equatorial is often the mount of choice. For very large apertures intended for easy portability, the altazimuth almost has to be used. However, the compact split-ring equatorial design can also remain fairly portable even with telescopes as large as 18 inches. Computers and computerized driving systems have narrowed the choice between the two mounting systems (and driven up their prices), but their basic characteristics have not changed. In any case, both the altazimuth and the equatorial have a firm place in amateur astronomy.
7.4 Binocular Telescopes
There have been some people who have built their own Binocular Telescope, this is two telescopes who are the same in every way that are mounted together and are used more or less like a pair of binoculars but are much more powerful. During the last few years a bino viewer has been made that turns a normal scope into a bino-scope.
Now there is at lest one company that is making and selling their own Binocular Telescope. While I have not been able to use such a scope (I don't have that kind of money!), I would think that the images from it would be awesome!
1157A Bucke St.
Canada N5Y 3S2
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.
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.
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. A. 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 has 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 Corp
P.O. Box 9111
Belmont, MA 02178-9918
Their catalog is free.
11.1. What About Computer Programs?
There are too many types of computer programs that I can NOT review, as they do not run on my machine. I think there should be a FAQ just for all the computer programs.
11.2. There is another world where a lot of info can be gotten, that is the world of Cyberspace. Not only web sites, but the world of E-Mail lists too. Here are a few:
12. About this FAQ
This FAQ is a copyright work. You have my permission to reproduce it however you like, as long as you leave all the attributions and the copyright notices. If you find this FAQ useful a small Donation sent to the address below would help.
470 20th West #23
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Created on ... June 29, 2002