Telescopes

This paper explains a few of the basics about telescopes. Obviously, it's good to have some knowledge of what's out there if you are considering a purchase. I've tried to be objective.
It must be noted that the comments presented here refer to telescopes for astronomy. Telescopes for terrestrial use are similar but not exactly the same.

Avoid the temptation to buy a telescope on the basis of "power" or "magnification" ("power" and "magnification" are generally used to mean the same thing). Although magnification is important, it is far too often over emphasized by beginners. Besides, with most telescopes, you can change the magnification by switching to another eyepiece.
Lots of magnification does not mean that you'll get a better view! To the surprise of many beginners, high power is frequently undesirable. To see many large star clusters and galaxies, it is essential to use a low power.
The largest moons of Jupiter can often be seen at powers below 10 and the rings of Saturn can be seen at about 30 power (although they do look better at higher powers).
Much useful observing is done at magnifications below about 100. Magnifications over about 250 aren't useful as much as a lot of people seem to think.
During good conditions, you can get a maximum of about 50 power per inch of a telescope's aperture (about 2 power per millimeter). Powers beyond that make things worse because of magnification of both turbulence in the atmosphere and limitations in the optics. (Premium telescopes can go to 100 power per inch of aperture and beyond under the very best atmospheric conditions but that is definitely the exception.)
The "aperture" of a telescope is the diameter of its main mirror or its front lens (don't confuse this with "focal length" or "focal ratio" which are totally different).
If the telescope's aperture is 3 inches (about 80mm), you can usefully get about 150 power out of it under good atmospheric conditions. Small, low cost telescopes with apertures of about 2 or 3 inches are often sold with claims of 300 to 500 power and more. It is beyond question that such a high magnification on a small telescope is terribly unrealistic and will be absolutely useless. Sadly, I've met more than one would be observer who became discouraged by a telescope like this. If you have one of these, you may be able to make it more usable by acquiring some low power eyepieces.
Frequently, the atmosphere makes it impossible to get anywhere near 50 power per inch. Turbulence in the atmosphere causes things to look bad. It's like standing on the deck of a swimming pool and looking at something at the bottom of the pool. It will look wavy because the water ripples bend the light. The earth's atmosphere has the same effect on the things that you want to look at with your telescope.

Often, you will hear that the most important thing about a telescope is its aperture. If you've done much checking about how to choose a telescope, you've probably had that pounded into your head. The bigger the aperture the more light the telescope will collect. This allows fainter things to be seen. Also, it will be better at resolving detail if atmospheric conditions are good.
However, aperture is only the most important thing if everything else is equal--which is not always the case! The most important thing about a telescope is its quality--PERIOD!! You are far better off with a small high quality telescope than a large lower quality telescope. Get the highest quality telescope you can afford not the biggest.

A solid support for the telescope is essential.
Any movement of the telescope is magnified by the telescope's power making whatever you're looking at appear to jump around as you try to look at it. This makes observing an irritating and frustrating experience.
A sloppy support will also make finding things difficult because the telescope won't stay where you put it.
A very good mount that's capable of moving the telescope so it can easily follow things across the sky will account for much of the price of a quality telescope setup. This type of mount is called "equatorial". It has one axis parallel to the earth's axis so it can follow objects across the sky with a minimum of fuss. To do this, the mount must be properly setup when the telescope is used. They're often motorized so they can automatically follow things across the sky.
There are two common types of equatorial mounts: the German and the fork. The German type is better balanced but a well made fork will provide a stable support, too.
There's another less expensive mount called "alt-az" which is short for altitude-azimuth. This is the "up-down-left-right" arrangement. It cannot easily follow things across the sky. Whether this is annoying is a matter of personal preference. Some alt-az mounts are computer driven and will follow objects across the sky automatically with an easy to do setup. This type, however, cannot be used for long exposure photographs without special add ons for the camera.

Consider the size and weight of a telescope. If taking it out and setting it up is too difficult, you won't use it very often regardless of how good it is.

There are three broad categories of telescopes: refractor, reflector, and catadioptric.

The refractor is the type that most people think of as a telescope. It has a lens in the front that collects and focuses the light toward the back where the image is viewed. There are two types of refractors: the achromatic and the apochromatic. The apochromatic design shows a better image.
Theoretically, the refractor is capable of the best images for a given aperture but only if it's very well made. Truly high quality refractors are very difficult to manufacture making their cost per aperture very high. A premium refractor will show images that are spectacular (often better than much larger telescopes of other designs).

The reflector uses mirrors to collect and focus the light. There are several types of reflectors.
The most popular type is the Newtonian. It has a large curved mirror in the back of the telescope to collect and focus the light and reflect it forward. A small flat mirror near the front then reflects the light through a hole in the side of the telescope where the image is viewed. This type offers the most aperture for the money because, optically, it's the simplest to manufacture.
The "dobsonian" is a reflector with an alt-az mount and is often--but not always--a reasonable choice for a beginner. They are easy to setup and reasonably priced. The dobsonian, however, is considered difficult to use by some people because it has a manual alt-az mount. If you aren't sure what this means, you probably shouldn't buy one until you learn if it's really for you. What it means is that you have to constantly grab the telescope and move it to keep whatever you're looking at in the eyepiece although some have the capability of a motor drive.

The catadioptric uses both mirrors and a lens.
Light enters the telescope through a single element lens that does some preliminary focusing of the light. The light then heads to a large curved mirror in the back which does more focusing and reflects the light to the front. A small curved mirror in the front continues focusing the light and reflects the light back through a hole in the main mirror where the image is viewed
There are several types of catadioptric designs. The most popular is the Schmidt-Cassegrain followed by the Maksutov-Cassegrain. The catadioptric offers the most compact telescope for a given aperture because the light is bounced back and forth inside the telescope tube. The compact design makes them a very popular choice.

The eyepiece is a very important part of any telescope setup.
Small, low cost telescopes usually have 0.965" diameter eyepieces. This is not a very popular size and does not provide a very good selection of quality eyepieces.
The most popular size is 1.25" which offers many eyepiece choices. (There are adapters that will allow many, but not all, 0.965" type telescopes to use 1.25" eyepieces.)
The next step up is to 2" diameter eyepieces. These are usually found only on large or expensive telescopes although they have been becoming more popular on more modestly priced telescopes.
There are larger eyepieces than the 2" but they are very rare on amateur telescopes.
There are many different types of eyepieces available and they vary greatly in quality. Describing all of them is far beyond the scope of this paper. Get the best ones that you can afford because they have a tremendous impact on the final image quality.
Popular brands of eyepieces are Tele Vue, Celestron, Meade, and Orion. In my experience, the best in this group is Tele Vue. Their top of the line Nagler eyepieces are exceptional--and have prices to match! (The very best eyepiece I've used is made by Leitz but these are difficult to obtain and, with prices in four figures, very expensive.)

There is another thing to consider that can make astronomy frustrating: finding things to look at!
Finder scopes help but only if you already know where to look.
A big aid in finding things is a low power eyepiece and a smooth, steady mount.
The moon is an easy target and so are most of the planets at various times of the year. Finding other things, such as galaxies and nebulae, can be a challenge.
Computerized telescopes and digital displays are available to help guide you to a particular object in the sky. These add to the cost but they're a considerable help. I strongly recommend them. There remain a few people who see these as "cheating" but that attitude is pure nonsense and has faded a lot over the years. The anti computer philosophy is that you should learn to find things on your own. My question is, "Why? Why not take all the advantages you can get?" A slightly bent analogy would be that I disconnect the fuel gauge in my car so I can learn how to determine when I should get gas without the use of any electronic gadgetry. Nonsense!
The setting circles on equatorial telescope mounts can be a big help but they must be setup properly when the telescope is used.
Finding things without a microprocessor or setting circles requires patience, experience, patience, a good star atlas, and more patience. You have to learn where to look. That doesn't mean you have to memorize the things in the sky. It means you have to learn how to use the resources you've got.*

So, which telescope is best? There's no answer to that.
The best quality telescopes I've used have been made by Astro-Physics and Takahashi (these are expensive brands).
Celestron and Meade are very popular with amateurs. They both sell decent, moderately priced telescopes that are easy to use.
You must realize that no telescope can be all things to all people. It's been well said that the best telescope is the one that gets the most use. Don't jump in head first. Take your time deciding.

Occasionally, a parent has asked me what telescope they should buy for their child. Often, the kid is maybe 7 to 12 years old and the parent would like to get an inexpensive telescope for the child for Christmas or a birthday. Because of the age of the kid, the parent isn't too sure how interested the kid is. So, they usually don't want to spend too much money.
Sadly, I can't offer a good solution to this. A cheap telescope might be used to look at the moon occasionally, the bright planets if the kid knows where to look, and, perhaps, to look at a mountain or hill during the day. But a toy will be tough to use to find things like galaxies, etc. (I never could). Sometimes, though, looking at the moon is enough to get the kid interested.
Other times, a parent will come to me after buying a cheap telescope and want to know why they can't see anything with it. So far, I haven't been able to think of a good answer that doesn't hurt their feelings or disappoint them. Their problem is usually that the telescope isn't very good but it's hard for me to tell them that. The more perceptive ones realize what I don't want to say. Sometimes, the telescope isn't too bad but they don't have a clue about how to use it and, for some reason, they won't practice with it during the day by looking at a distant tree or something even after I urge them to do just that. Usually, though, the problem is that the telescope is just plain crummy. I've seen some that were so bad that even viewing the moon was nearly impossible.

What should you do?
Go to a book store and buy an issue of both Astronomy and Sky & Telescope magazines. These magazines can be very helpful when it comes to learning what types of telescopes are available. Consider subscribing to one (or both) of them. This will be a better way to spend your money than on a low quality telescope. There are also some books about different types of telescopes and their pros and cons.
Join your local astronomy club! These clubs usually have a club telescope (or maybe several) that you can borrow. You will also be able to get direct advice from people who've been doing this for years. So, spend the $25 or so to join. It will be money well spent; certainly a better investment than a crummy telescope. I've heard some horror stories about clubs that snub beginners but I've never actually seen that happen. On the contrary, I've seen just the opposite.
You can also find information on the internet. Two good places to start are with Astronomy and Sky & Telescope magazines' web pages. They have periodically updated information and links to a lot of other sites. They are at:
http://skyandtelescope.com/
http://www.astronomy.com

There are a few stores that sell telescopes. I suggest checking in the ads in the magazines.
You can buy telescopes at camera stores and other stores that don't specialize in telescopes but the sales staff there may not have very much experience with telescopes or astronomy and too often the telescopes they sell aren't very good (if the label on the box mentions some huge power, don't buy it).
Lots of manufacturers sell their telescopes mail order/online which is often the only way to get them. Be sure that you understand the warranty and return policy before you buy.
Another option is to buy a used telescope but this may be a bit risky. Before buying a used telescope, insist that it be checked out by someone who knows what they're doing. If the seller balks at that, consider keeping your money in your wallet.

You don't need a telescope to enjoy astronomy.
There are many things that you can see with a pair of binoculars. Even ordinary 7x35 binoculars will show the larger moons of Jupiter when they are well placed in their orbits (the "7x" refers to the magnification and the "35" is the diameter of the front lenses in millimeters). Binoculars will also show many star clusters, nebulae and even galaxies. As always, you get what you pay for and cheap binoculars won't show you what you'd see with good binoculars. Many experienced observers use binoculars a lot because binoculars have a low power enabling them to take in a wider patch of the sky than a telescope. There are books available about observing the sky with binoculars.
Many people are surprised at what they can see just with the unaided eye. You just have to know where and when to look! I can assure you that you've seen most of the planets whether you knew it or not. Frequently, Venus and Jupiter are the brightest "stars" in the sky. For some reason that I can't understand, people often point to one of them and proclaim that they've found the North Star. (The North Star is the 49th brightest star in the sky. If you're looking at the brightest thing in the night sky, you're not even close to looking North.) Don't worry about memorizing the night sky. I never have and neither have most other amateur or professional astronomers. Too often people have the idea that they need to memorize the names of the stars and constellations, etc. You don't need to do that any more than you need to memorize a map of the city you live in. You learn a few important landmarks and intersections and find you way around from there with a map. You can do the same thing with the sky.
Take a look at Astronomy and Sky & Telescope magazines for articles about what can be seen. Check the library for books and videos about astronomy.

Oh, yes. One more thing. You can't see through an overcast sky or through rain; even with a telescope. There's no magic.

3/15/08



*It doesn't have much to do with telescope choosing but you might have been told to get a star map and memorize the constellations and bright stars. This makes about as much sense as getting a map of where you live and memorizing it. You only need to learn a few "landmarks" and you can find your way around from those using that map. Of course, if you want to memorize it, go ahead!