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s.a.a. FAQ

The sci.astro.amateur FAQ I have maintained here is very out of date. It was last significantly rewritten in 1998; though amateur telescope engineering and design are largely unchanged, the commercial aspects (such as the buying guide and list of vendors and references) are quite badly obsolete. It is now kept in the archives. I will keep this set of files here indefinitely, but will not make any changes to them. The original author of the FAQ does maintain a more up-to-date version, in a different format, at his personal Web site.

The archived s.a.a FAQ is not licensed under the license generally applicable to It is copyright 1995-2006, Slc. Dennis Bishop.

A couple of years ago, I compiled some of my own suggestions for newcomers. Here's a summary, which is still quite applicable today:

My first suggestion is the same as many other people's: Go visit an astronomy club or a public star party and ask the people there. Folks at public star parties are usually quite friendly and will often let an interested person try out their scopes for a little while. Once you've seen how different types of scopes behave (how they move, what they show, how easily they are set up and put away), you'll have a much better idea of what you want.

Additionally, I highly recommend getting familiar with the night sky. Learn the major constellations; be capable of recognizing a dozen or so when the sky is clear and there isn't much moonlight. If you have a pair of binoculars, even little 25mm "pocket" models you take to football games, spend some time exploring the night sky with them. You'll be surprised at what you can see, and you'll learn more about how to observe things without even being aware of it.

A good beginner's scope will have the following features, regardless of price, design, or accessories:

  • Easily set up. Not too big, too heavy, or too complex. Ideally, it's something you can take outside in one trip, at most two, and then set up in just a few minutes.
  • Reasonable quality. To an extent, you get what you pay for, but even fairly inexpensive scopes will perform well if competently designed.
  • Good for a wide range of targets. After you get some observing under your belt, you'll have a better idea of what kinds of objects you like to observe. At the beginning, though, you'll want something that shows pleasing views of a wide range: the major planets, double and multiple stars, and at least the brighter star clusters and nebulas.
  • Good basic accessories. At a bare minimum, you will want at least 2 eyepieces (ideally, 3) and a finding device of some sort. Many scopes only come with 1 eyepiece, and some inexpensive (but otherwise perfectly good) scopes come with an inadequate finder. If your prospective purchase lacks some of these items, you'll want to budget some extra money for them.

Some Things I Suggest

Given all this, I generally make two suggestions to new

  1. A 6" (150mm) or an 8" (200mm) Dobsonian reflector
  2. A 4" / 100mm medium to long focus refractor (roughly, f/ratio of f/8 to f/12)

At the moment (early 2006), good vendors of the Dobsonians are Orion Telescopes and Discovery. For the refractors, look at Orion Telescopes as well, and then look at any dealer that carries Celestron or Meade telescopes, such as Astronomics, Anacortes, or Oceanside Photo and Telescope. For higher but still fairly reasonable prices (about 75-100% more than Orion, Celestron, or Meade), Stellarvue is a fairly new refractor manufacturer with a very good reputation.

I have a definite bias towards the reflectors. The extra aperture really makes a big difference, even (I might say especially) from a light-polluted city. Don't believe what people might say about light pollution being worse in larger apertures than in smaller ones. Under given conditions, whether light or dark, larger telescopes will show fainter objects, and normally greater detail in bright ones. Also, the Dobsonian style of mounting and aiming a telescope is straightforward, and beginners can quickly learn their way around the constellations with one.

However, they have a few minor drawbacks. In particular, reflectors of all types require occasional maintenance to give sharp images. I don't find reflector maintenance to be a big hassle, but not all observers are alike.

Hence, my second suggestion, a 4" or 100mm refractor. With a good small refractor, you will end up with very sharp images with little or no tweaking. Also, assuming equal optical quality, refractors can produce slightly brighter images than other scopes of the same aperture; modern multi-coated refractor lenses generally transmit more light than the combinations of mirrors used in reflectors. This difference is fairly small (10 - 20% typically), but still enough to be worth noting.

Refractors of a given aperture are significantly more expensive than other types of telescopes, and usually larger and heavier, so it's hard to get as much aperture on a budget. A good 6" Dobsonian might cost $400; a cheap 6" refractor will cost twice as much, and be much bulkier and harder to use. So, for refractors, I suggest a smaller, more manageable scope. For many people, having a scope that requires next to no maintenance is a significant plus, worth some loss of aperture. If this is the case, then a 4" refractor would make a good starting scope. If it's not, and you want to get the most bang for your buck, stick with the Dobsonians.

A Little Off the Beaten Path

  • Cash-Savers: The scopes I describe above will cost at least $300 to $500. If you are really strapped for cash, you can get a slightly smaller scope such as an 80mm refractor or a 4.5" Dobsonian. There are some very good deals out there. For example, Orion sells two 4.5" Dobsonians, a larger (f/8) model a smaller (f/4) model (called the "StarBlast") for about $200 or even a little less. Both of these would make very good starting scopes for a person on a budget.
  • Catadioptrics: Another possibility, which is a little more expensive for a given aperture, is a catadioptric telescope. This is just a telescope that uses both lenses and mirrors to form the image. Usually they have a large mirror in the rear, like a reflector, and a large corrector lens at the front. I don't recommend them as much for beginners as the other two designs, because most catadioptrics have very long focal ratios (f/12 to f/15), making them less suitable for wide field views than the other scopes I've described. Also, they tend to be relatively expensive for the aperture. A 5" catadioptric might cost $600; for the same $600 or so, you can get a 10" Dobsonian that will be generally more powerful all around. They are, however, incredibly compact for the aperture, so someone who really wants or needs an especially portable telescope, and plans to observe things like double stars or planets most of the time, might want to consider one.
  • For the Ambitious: Finally, if you know that amateur astronomy isn't just a passing fancy, and have some extra cash to spare, push my suggested aperture limits a little further. Try looking at a 10" Dobsonian, a 7" or 8" catadioptric, or a 120mm (4.7") refractor. This is as big as I'd go with a first telescope, and in any event, try a scope in this size range before buying. A 10" Dobsonian is probably the largest medium-cost scope you can fit in a typical car, for instance. Refractors larger than 4", of conventional length (over f/8), are much bigger than they seem in a photo.

Some Things I Don't Suggest

  • Department-Store Hobby-Killers: Avoid the small (50mm and 60mm) refractors that sell for $150 or so in the department stores. These almost invariably have poor eyepieces, hopelessly inadequate finders, and a wobbly mount and tripod. For just another $150 to $200 you can get a much better quality telescope that will serve you well for years.
  • Very Small Refractors: Actually, don't get a 60mm telescope from anywhere. For only a small amount more money, you can get a larger telescope that will show much more.
  • Small Reflectors: Ones smaller than 4" / 100 mm. Just don't; very small reflectors tend to be rather poorly made.
  • Pseudo-Catadioptrics: Any telescopes that appear to have a corrector lens built right into the focuser (as opposed to a true catadioptric, which has a corrector lens at the front of the tube). These are almost always a disaster. Some ultra-short reflectors, including some sold in department stores on online shopping centers, use this design. Unfortunately, some otherwise reputable vendors, such as Orion Telescopes, sell scopes like this.
  • "Short Tube" Refractors: Usually 80mm to 120mm, with an f/ratio under f/8. These actually aren't poor scopes, just rather specialized ones that are sometimes a little oversold. They are optimized for low-power, wide-field views of things like star clusters and big nebulas. They are not very good for planets, which are important targets for all observers, particularly new observers from a city. Also, all inexpensive short-focus refractors suffer from color distortions at high magnification. This can get annoying on the Moon and planets in a hurry.

    This last rule is not as absolute as the others (in fact, I do have one of these telescopes myself). It can definitely be bent if you plan to do terrestrial observing as well as astronomy, for example; the ubiquitous 80mm f/5 short tube refractor easily mounts on almost any camera tripod, making a nice spotting scope too. Also, wide-field telescopes complement larger scopes, especially those over about 8", very nicely. In particular, there are always nights where you want to observe something, but don't want to set up the "big" telescope. You may want to consider a short-tube refractor as your next purchase, after your first telescope.

And Finally, Don't Forget

A few other things you might want to get, depending on your experience and the scope you get:

  • Eyepieces. You'll want at least two. A very general guideline: have at least one eyepiece that magnifies about 6-8 times per inch of aperture (for low powered, wide-field views), and another that magnifies about 15-20 times per inch (for medium- to high-power views of things like planets and double stars). So if you get an 8" telescope, look for magnifications of around 50x and 150x. You'll eventually fill in the gaps as you get more experience. For example, if you observe the planets a lot, you'll probably want to get an even higher powered eyepiece (say, 30 power or more per inch of aperture); if you enjoy wide-field views, you might want an ultra-low power eyepiece (one giving 3-4 times per inch).

    For scopes of the sort I suggest as beginning scopes, you don't need expensive, wide-field designs like Naglers or Panoptics (of course, go ahead and buy them if you really do want them and have the money). Plössls and Orthoscopics will be fine, and you'll save a bunch of money (cost: $40-$80 per eyepiece).

  • Finder. If your scope has a 40mm or larger finder scope, it's probably enough for now. Otherwise, you may wish to upgrade. If your scope has no finder, you'll want to get one soon. Trying to find objects by sighting along the tube gets to be a royal pain in a hurry. Either a magnifying finder scope (looks a bit like a rifle scope) or a "unit power" finder like the Telrad or Rigel Quickfinder will do, though I recommend trying both before purchasing, since most folks have a preference for one type or the other.
  • Star charts. You'll need these to find anything besides the planets and the brightest stars.

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