sci.astro.amateur Telescopes FAQ: Introduction

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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 for up to 300+ 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. What is the single best piece of advice to give to someone thinking about buying a first telescope?

Editorial Note: This information comes from Jay Freeman.

Occasionally, amateur astronomers ask me for recommendations about telescope buying, learning the sky, and so on. I thought I would write some thoughts down for future reference.

(Somewhere I should state credentials. I am a moderately experienced visual observer: Off and on over 40 years I have logged about 5000 observations of about 2000 different objects, and used perhaps a dozen different telescopes enough to know them well. I have completed some ten optical surfaces to 16-inch diameter. (A sphere - my biggest paraboloid was 8 inches.) My particular forte is deep-sky work; my most "gosh-I-don't-believe-I'm-seeing-that" observations were of the Sculptor Dwarf Galaxy (10x70 binocular) and of Maffei I and Leo II (Celestron 14). My interests led me to a physics PhD, studying the interstellar medium with spacecraft instrumentation. Thus by training I am an astrophysicist. However, I have maintained amateur standing in visual wavelengths, my thesis work was done in the extreme ultraviolet.)

What to do First.

First, some meta-advice. Written words do not substitute for hands-on experience. Your first step should be to find an astronomy club, join it, go to some observing sessions and try out other peoples' telescopes. You will learn a lot, and will find other enthusiasts eager to talk about features of various designs.

To find local clubs, ask at science stores, museums, planetariums and the like. Physics and astronomy departments of local colleges may know, even though club activity isn't really their line of business. The two big English-language popular astronomy magazines, _Astronomy_ and _Sky_and_Telescope_, both publish comprehensive annual directories of clubs, stores, observatories and such. Look for the magazines on the newsstands, or go to a library and page through some back issues.

Been to a club already? Honest? Okay, you can keep reading...

Some Basic Questions.

If you are considering buying a telescope, you face bewildering choices, many expensive. To help straighten out the confusion, I suggest you first ask yourself some questions. We'll see as we go along how the answers might figure into your decision.

  1. How much intellectual effort are you willing to put into learning the sky? If you know the constellations well, and have practiced finding things by "star-hopping", just using charts instead of dial-in or punch-in coordinates, you will probably be able to make good use of a telescope which is less expensive, more compact, lighter, and easier to set up than one which uses precise alignment or computer control to help you find celestial objects.
  2. 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 are you willing to put up with to do so? Differences in not only size but also optical design create vast differences in telescope portability, and any telescope that you do take out and use will be far better than any telescope that sits at home in the closet because it is too heavy and too cumbersome.
  3. 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 money for advanced and sophisticated features, even if you don't really need them? If so, that's fine - lots of amateur astronomers like neat equipment, including me. But if not, be careful that technology enthusiasts don't sell you things you don't need.
  4. Do you want to take photographs of celestial objects, or obtain CCD images of them? "Astrophotography" is a very expensive word, particularly for time exposures of faint fuzzies. I have never been into this side of the hobby, so haven't much to say; however, I have friends who are crack astrophotographers, and it has typically taken them several telescopes and several years before they were satisfied with their setup and its results, and they have usually spent lots more money than visual observers do.

Some Realities.

With these thoughts in mind, I can now attempt some general comments.

(A) The most important practical matter in determining the optical performance of a telescope is the diameter of the beam of light that goes into it- its so-called "clear aperture". Obviously, the more light, the fainter the things you can see, but much less obviously, the amount of detail present in the image is limited by clear aperture for reasons that have to do with physical optics - bigger telescopes are capable of producing sharper images, just because they are bigger.

Now, there are some important qualifiers here. First, clearly, bad craftsmanship can make any telescope perform poorly. If the optics are not well made, they won't work. Fortunately, it is not too hard to make optics of the sizes and types commonly used in amateur telescopes, and most of the manufacturers routinely turn out optics that are okay; occasionally bad ones turn up, but major manufacturers will often fix or replace a real lemon, if you have the wit to recognize that you have one, and the will to complain. (Most of us have neither; that's how some manufacturers make money!)

Second, there are differences in optical performance of different designs of telescope. Schmidt-Cassegrains, Newtonian reflectors, and various kinds of refractors all have different good and bad points, and people who love telescopes, or people who make their living selling them, will be eager to debate such matters. However, all these differences are relatively minor: It is usually an adequate approximation to the truth to assume that all telescopes of a given clear aperture and a given quality of optical craftsmanship have identical optical performance: Real differences will correspond to changes in aperture of only perhaps 10 to 20 percent. (Shabby optical work will increase that difference enormously.)

Third, atmospheric turbulence ("seeing") affects the ability of a telescope to show detail, and sky brightness affects its ability to show faint objects. Poor seeing usually hits large telescopes harder than small ones. When you observe from an urban or suburban area where the sky is bright and the seeing is lousy, 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. Fortunately, many of us have found good dark-sky stable-seeing sites within a reasonable drive of where we live-from my favorite sites near the San Francisco Bay Area, sometimes I have to stare through the eyepiece of my Celestron 14 for several minutes before I realize that there is any atmosphere at all between me and what I am looking at.

Notwithstanding all of these caveats, APERTURE WINS, and wins big. Thus if you buy the finest 3.5-inch fluorite refractor that exists in the world, do not be chagrined if a junior high school student shows up with a so-so home-made 6-inch Newtonian that blows it clean out of the water in optical performance: The so-so 6-inch I made at age 13 puts my world-class 90 mm fluorite to shame; there isn't even any contest, and it's not because I was a master optician at 13, either, it is because six inches is bigger than three and a half inches, and therefore intrinsically better.

(B) There are several hundred deep-sky objects big and bright enough to show interesting views through apertures of two inches or so, at very low magnifications. Thus medium sized binoculars - 7x50 or 10x50, say ("7x50" means "7 power, 50-mm aperture", and so forth) make relatively inexpensive, highly portable, easily set up beginner's instruments. Perhaps you have one already. To use them well, though, 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:

(D) For all that they are pricey and cumbersome, small refractors are pretty durable and relatively difficult to get out of whack. They thus may make respectable beginner instruments, particularly for beginners with a surplus of thumbs. And a good small refractor provides a wonderful way for an experienced observer to embarrass folks with humungeous Newtonians and insufficient observing skills to exploit them.

(E) Altazimuth mountings tend to be cheaper, lighter, less clumsy, and quicker to set up than equatorial ones, but in order 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 the material in (C). You might say that there are variety of ecological niches for telescopes, corresponding to somewhat different uses and requirements. I have identified six:

  1. Big Iron: This is the giant Dobson-mounted Newtonian, or humungeous Schmidt-Cassegrain, that fills up all the space in your garage. To transport it requires a small trailer, a pickup truck, or a panel van, and setting it up on-site calls for the concerted efforts of three used fullbacks and a circus elephant. The ladder to climb to the eyepiece is so tall that you need supplemental oxygen to keep from succumbing to 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.
  2. Largest Conveniently Portable Telescope: The definition here is the most telescope that will fit easily in your regular vehicle without having to hire a bulldozer to clean out the back seat and/or cargo area. What that is, depends on what your vehicle is - if it's a ten-speed, or a subway train, you have a problem. An eight-inch to eleven-inch Schmidt- Cassegrain is the right size for many people; that is one of the reason these telescopes are so 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 would just barely fit crosswise across the back seat of my car. I used it a lot till I bought a smaller car. I now drive a Geo Metro, and my Largest Conveniently Portable Telescope is presently a 90 mm f/9 Vixen fluorite refractor on an altazimuth mount. A C-5 or a somewhat smaller Dobson than my 8-inch would work equally well, and would have greater performance for most purposes.
  3. Public Star Party Scope: This is probably 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 my Vixen on a Great Polaris mounting and use it, but I set the tripod legs to maximum length so the expensive fluorite objective is way up out of reach. So far, no one has slam-dunked a rock.
  4. Quick Look Scope: The idea here is to leave something all set up in your entrance hall, or perhaps hidden under a stack of old _Sky_and_Telescopes_ in the back of your car, so you will have a telescope available on two minutes notice if a truly close comet comes whizzing by unexpectedly, 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 or may not 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 C-90 (Maksutov, not refractor) on one that is a bit heavier.I don't have a quick look 'scope at the moment.
  5. Binocular: A good binocular is very useful, and can do most of the work of a Quick Look Scope. I have too many binoculars; ones I use for astronomy include the 7x35 Tasco ($29.95 at Sears) that I keep under the seat of my car for bird-watching (oops, lost observer points there), an old Swift Commodore Mark II 7x50 (long out of production), which was one of the first binoculars I ever saw with BAK-4 prisms, and an Orion 10x70 with BAK-4 prisms and fully multicoated everything (up to but not including the case). At regular star parties I tend to wander around with one of these dangling from its cord around my neck. I tried two, but that failed for insufficiency of eyes.
  6. High-Tech Conversation-Stopper: This is the one you use to put to shame those grass-chewing hillbilly clodhoppers 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 of theirs put together, and it cost more than all of 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 are always asking if you know how to walk the doggie. Some folks like Questars, but I am not among them.
    My present High-Tech Conversation-Stopper is the 90 mm Vixen fluorite refractor that I mentioned earlier. It is not big enough to be as impressive as I might want, and is rather lacking in knobs, but I can usually talk fast enough to make up the difference, and then some.

What about accessories?

Well, I have already said most of what you need to know about accessories, which is that (A) aperture wins. If you are planning a budget for a telescope, and eyepieces, finders and other such devices 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 magnifying glass used 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 your personal limit of telescope portability, or if you have lots of money, or if you like fancy 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:

Clear sky, and enjoy your telescope.

Jay Reynolds Freeman

freeman@netcom.com
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Last Modification: December 16, 2000