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 observing sessions, and try out other peoples' telescopes. You will learn a lot, and will find other enthusiasts eager to talk about equipment.
To find clubs, ask at science stores, museums, planetariums and the like. Physics and astronomy departments of local colleges may know, though clubs aren't their line of business. Two big English-language popular astronomy magazines, _Astronomy_ and _Sky_&_Telescope_, publish comprehensive annual directories of clubs, stores, observatories and such. Look for the magazines 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.
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 these 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 home in the closet because it is too heavy or 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, take care 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 an expensive word, especially for time exposures of faint fuzzies. I have never been into this side of the hobby, but, 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 results, and they usually have spent lots more money than visual observers do.
Some Realities.
With these thoughts in mind, I can make 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. 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 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 variations 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 percentage 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 home-made 6-inch Newtonian that blows it clean out of the water in optical performance: The 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:
(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 the short, stubby tubes, but for small apertures -- say, four inches or less -- portability of all types is dominated by the 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?
The answer is often a high-quality refractor.
(C.2') What gives most optical performance for a given car to carry it?
The answer is often a Schmidt-Cassegrain.
(C.3') What gives most optical performance for a given budget?
The answer is often a big Dobson.
(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 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.
(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 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 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 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 having to hire a bulldozer to clean out the back seat and/or cargo area. What it 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 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 the back seat of my car. 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 Super Polaris German equatorial (I have hardware to fit both), but at present I use a six-inch f/10 Intes Maksutov on the Super Polaris. A somewhat faster Dobson than my 8-inch f/5 would work equally well, and would have greater performance for most purposes.
(F.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 the Intes or the Vixen fluorite on the Super Polaris, but I set the tripod legs to maximum length so the expensive optics are way up 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 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.
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 way 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.
(F.6) High-Tech Conversation-Stopper: This is the one you use to 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 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 always ask 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 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.
(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 wide-angle finder, 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. 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 of objects 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 the skill and equipment to program the control interface will have a field day doing things most of us have never dreamed of.
I do not presently own a CyberScope. That's probably because I write programs for a living, and far too many of them. In my hobby activities I prefer to avoid anything even suspected of containing electrons.
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 if 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 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 limits of telescope portability, or have lots of money, or like whizzy 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. You will need a low-power, wide-field eyepiece both for finding things in the first place and for low-power views of big, diffuse objects. Such an eyepiece 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 probably 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.
With a good low-power eyepiece on hand, 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 third eyepiece will depend on what kinds of things you like to look at. I myself would pick a focal length nearer the low-power eyepiece than the medium-high-power one -- I use 32 mm on my C-14 or 12.4 mm on my 8-inch Dobson. Others might want a power nearer the medium-high-power end, say 20 or 7 mm on the same two instruments.
In general, "fast" f-numbers, typical in Dobson-mounted Newtonians, require fancy, expensive eyepieces to give good views, simply because the wide, steeply converging light cones of these instruments are difficult for an eyepiece to cope with. Slower instruments can use simpler eyepiece designs. It is a "Catch-22" of amateur astronomy, that cheap telescopes (fast Dobsons) need expensive eyepieces, whereas 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. And it used to be that Barlow lenses, also called telextenders, which multiply the focal length of the telescope with which they are used, generally worked well only with telescopes with large f-numbers, where they were not needed -- another "Catch-22". I have heard reports that there are now Barlow lenses that work with faster telescopes, where they are indeed needed, but I urge a try-before-you-buy approach when selecting one.
For over fifteen years I used an eyepiece set I bought in roughly 1980. It featured no whizzy 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 view of the Erfles was overwhelming enough that I was untempted to buy wider-field types. Besides, a big Erfle is already heavy enough that I had to rebalance the telescope when I put one in. I did actually 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 a variety of additional eyepieces, mostly out of curiosity. I found that decent Plossls are roughly comparable to decent orthoscopics. I bought several Vixen "Lanthanum" eyepieces, which all have built-in matched Barlow lenses to give uniform 20 mm eye relief, even at such short focal lengths as 2.5 mm. I don't need to wear glasses when I observe, but even so, the long eye relief makes viewing more relaxed -- I'm not forever worrying about bumping the eyepiece. The long eye relief also facilitates public viewing -- I can focus with my glasses on, and tell everyone in line to leave theirs on and not try to refocus.
As a lark, I once bought 6, 12 and 25 mm eyepieces of the Ramsden design, one of the oldest, simplest, and cheapest. They were about ten dollars each in 1980. I carry them around at star parties, and try them out on other people's telescopes without letting on what they are. The field of view is a lot narrower than for more modern types, but on axis, and at slow f numbers -- say, no faster than f/10 -- they give up exactly nothing to even the whizziest designs; the 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 it might be reasonable to 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 shown on the charts you use to observe with. It helps a lot in identifying what you are looking at through the finder, if every star in the finder is on the chart, and vice-versa. Once the right pattern of stars is in the finder, you can then point the finder crosshair to the place in the pattern where the object you want to look at lies, even if that object is too faint to see without the main telescope.
In dark sky, the 10x40 finder on my C-14 shows stars to about magnitude 9.5, which matches my big set of 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), and so matches any number of naked-eye star atlases.
Zero-power finders, such as the Telrad, are very useful if the sky is dark enough to show plenty of stars. These devices allow you to stare at the sky with both eyes wide open and see a dot, circle or crosshair of light where your telescope is pointing. A simple peep sight made by taping bits of cardboard to the ends of your telescope tube will work equally well and be much cheaper. I have a zero-power finder (Orion's) on my Vixen refractor, but I find I rarely bother to turn the LED on, I just sight through the tube.
(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.
(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; 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 that you rush right out and buy a computer, but if you already own one, you might bear in mind that there are many programs on the market that will turn your console into a window onto the simulated heavens, with all manner of 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 only 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 that 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 flashlight is probably too bright.
(e) A logbook. This item is not necessarily for everyone, but I have found it fun and useful to record my observations, even if I don't do anything other than note that I saw such-and-such an object with a certain telescope and magnification. These make for fun reading when it is cold or cloudy, and often there will be reason to look up something long after the fact.
- Jay Reynolds Freeman