My Guide to Astronomy on a
Budget
Disclaimer: Choosing a telescope is somewhat like choosing a car. You'll get as many different opinions as there are models to choose from. Part of the difficulty lies in the fact that different people want different things from a telescope. The often heard truth is that there is no such thing as the perfect telescope. The following advice is just that: advice. It is based on reading and actual observing experience by one amateur astronomer. It is not a definitive guide to every question, and is no substitute for a visit to a knowledgeable retailer.
There is an art involved in purchasing a telescope. For a scientific field, there should be a more scientific method for choosing equipment. While there are some guidelines, most of astronomy is subjective. Beware the individual who claims one telescope is superior to all others. Often, they haven't taken into account the different needs of different users. A 24-inch dobsonian may reveal the sky better than a 3-inch refractor, but the average 12 year old will be able to move the latter better than the former. A 3-inch refractor may reveal great detail on planets, but is totally useless for viewing faint galaxies.
There are several factors that must be balanced when buying a telescope. Some are obvious. Some are not. Price is often listed as the primary factor, but price is actually the least important aspect. Fail in any other area, and the price of the telescope will not console you. The best telescope is the one that gets used the most, not the one that cost the least. Compromise on price if you must, but recognize the sacrifices being made. Price should be the last factor in decision making, simply because it has the least to do with satisfaction once the telescope is purchased.
Buying a telescope can seem a tremendous undertaking, and it becomes easy to forget that the process of buying is not the purpose of buying. You buy a telescope to use it, not to buy it. Just as you wouldn't choose a car based solely on price (as evidenced by the lack of Yugos on the road today,) you shouldn't make the mistake thousands have made in choosing their telescope (or their children's telescope) based on price. The fact that department store telescopes are made and still sell is evidence that price not only can be, but is overrated in the field of astronomy. A good telescope can last a lifetime, offering thousands of hours of viewing. Spending too little will cause problems and frustrations that will either turn you off to the entire hobby, or cause you to feel as though the money was wasted when you finally buy the telescope you knew you really wanted all along.
That said, I've provided a small guide for people who want something now and don't want to wait until they've saved the thousands of dollars needed for their "dream" scope.
A primary factor is portability. There are two subfactors in this field: how much portability you need, and how portable the scope is. If you plan on setting up a permanent observatory in a dome, portability is only a factor when moving the telescope from the shop to your site. If you plan on moving the telescope from location to location, say on the Appalation trail, portability is a major factor.
The physical size of a telescope has a lot to do with how portable it is. If the pieces it breaks into are large and heavy, the telescope will be less portable than if it breaks into small, light pieces.
The weight of a telescope is also a factor in portability. A lighter telescope may be a requirement for a young or old person, or someone who has a back problem. A telescope that is too heavy to lift will get little use without a dedicated observatory. It almost goes without saying that different people can handle different weights easily. I may struggle with a 30 pound telescope, while Brutus the Sanitation Engineer throws around 120 pound mounts like last week's garbage.
This area is perhaps the hardest to define, because so many factors come into play. Performance varies by target. A large, low quality mirror may be ideal for deep sky, but views could be horrible for planets. Likewise, two telescopes of equal size, maybe even the same manufacturer, may perform drastically differently depending on the manufacturer's quality control.
Before you can answer the performance question, you need to determine where your interests lie. Larger isn't always better, especially if quality is sacrificed.
In general, a higher quality mirror or lens will outperform a larger, poorer quality mirror or lens. This is why some people swear that a 7-inch Maksutov outperforms an 8-inch Schmidt Cassegrain. If the Maksutov's mirror is of significantly higher quality, it will undoubtably outperform larger telescopes. The only approach that seems to differ on this point is the "light-bucket" approach which says that the larger the mirror, the better the deep sky performance. However, there is no denying that a "perfect" 24-inch mirror will outperform a shoddy one every time. Be careful about accepting poor quality for more aperature. Light buckets are primarily deep sky telescopes, and often do poorly on planets and double stars.
A factor often overlooked is maintenance. Some telescopes require more hours of maintenance than others. Your willingness to clean, adjust and tweak a telescope should be considered. Some designs are inherently easy to maintain, while others require almost nightly changes. For the enthusiast who counts astronomy as their only hobby, cloudy nights can be utilized for most maintenance activities. For the casual observer, a telescope that is ready to use as soon as it's placed in a field may be better than one that requires significant adjusting.
The old adage, "The best telescope is the one that gets used," holds here. If you don't have the patience or time to do the necessary maintenance, your telescope will quickly collect dust or fail to perform the way you expect it to. Try to be realistic about this. How many hours a week do you have to spend on astronomy, and how willing will you be to spend an hour adjusting your telescope before you can use it?
I include this factor because it is often overlooked. Different manufacturers put out different quality products. Check sci.astro.amateur for discussions of experience with various companies. Try to find the majority viewpoint. Disgruntled customers can complain loudly and often about a company that hundreds of others have had no problems with. If you're just starting out, stay away from the controversial companies.
Also in this category is the do-it-yourself route. If part of your enjoyment is in knowing that you built the telescope yourself, then that will play a major role in choosing your manufacturer!
There are three basic kinds of telescopes: reflectors, refractors, and catadioptrics. There are a few basic differences between the types. Knowing the advantages and disadvantages (remember: there is no perfect telescope!) of each type will help in making the decision.
Reflectors are very popular, mostly because they offer the most aperature for the dollar. They come in two basic mount styles: Dobsonian and equatorial. The first, the Dobsonian is a very solid, cheap mount. By cheap, I don't mean poor quality. Instead, the low center of gravity and minimal materials required make the Dobsonian mount inexpensive. Equatorial mounts are more complex, have a higher center of gravity, and often require heavy counterweights to balance the telescope. A $100 Dobsonian mount is almost always more stable than a $500 Equatorial mount.
The primary advantage of a reflector is price. A medium quality 8 inch reflector costs less than $500. A similar quality Schmidt-Cassegrain is at least $1000. A refractor would be well over $3000 (and probably much more!) This price advantage comes with some downsides, however.
Optically, the reflector is excellent. Although difraction (distortions of the image caused by a central obstruction) is present in reflectors, the central obstruction is much smaller than a Schmidt-Cassegrain. Chromatic abberations are almost non-existant. A recent article in Sky & Telescope magazine described how to reduce the optical disadvantages of a reflector to negligible levels.
The disadvantage of a reflector is maintenance, and to certain extent, portability. The reflector tube is open, making the primary objective subject to degradation from environmental factors. Over time, the shiny coating that makes the primary mirror reflect will become tarnished. The only solution is to realuminize the mirror or pay someone else to do it. Also, the mirrors can become misalligned, requiring collumation. The faster the telescope (the lower the number after the f), the more sensitive the telescope is. An f/8 is relatively forgiving compared to an f/4. Portability is also a negative factor, since the focal length is only one trip through the tube. For an 8" f/6, the tube length is over 4 feet!
Having owned a 4.5" equatorial reflector, I can honestly say that reflectors are superior to cheap department-store type refractors. Although moving the telescope was a challenge, the views were very exciting, even from a light polluted city suburb. Planetary detail was disappointing, although this would be greatly enhanced with a larger mirror.
Refractors are an interesting study, since they represent both the cheapest and the most expensive telescopes money can buy. Most often, cheap refractors are referred to as "department-store" telescopes. Although they have no central obstruction, these telescopes are rarely worth the money. I'd rather have a decent pair of 7x50 or 10x50 binoculars than a cheap 60mm refractor boasting 575 power. I still use my 7x50s. I don't even have my department store special anymore.
Parents are often attracted to department store telescopes for some very simple reasons. Little Tommy or Susie is interested in astronomy and wants a telescope. "Real" telescopes cost hundreds or thousands of dollars, more than most parents are willing to spend on a child's passing fancy. Along come the department stores with the perfect solution: an affordable telescope. The initial joy of having the telescope is quickly forgotten after just one evening spent trying to find anything squinting through tiny eyepieces. The moon, a relatively large object, can take minutes to spot through the eyepiece for an inexperienced user. Soon enough, the telescope is collecting dust in the closet, Tommy or Susie has moved on to other interests, and the parents feel justified in not having spent more on a forgotten hobby.
As a young teenager, I longed for a telescope. Any telescope. After months of reading Astronomy magazine, I still hadn't really gone outside looking with anything other than my naked eyes. My dad's binoculars sat unused in his closet, since I was convinced I couldn't see anything with them. I went back through those magazines a while back when my childhood interest was rekindled by comet Hyakutake and discovered that the same advice was there that I now give: a decent pair of binoculars, even ones costing only $40, can give a youngster a better experience than any department store telescope. Even for those parents willing to spend $300-$500, a decent pair of binoculars is still almost a necessity for learning how to use a telescope.
Now that I've given my sermon on the evils of cheap refractors, which no doubt has convinced some of you that the entire class should be ignored, I'll tell you why some refractors are expensive. The problem with the department store telescope isn't the design, it's the materials. Usually, the telescope comes with cheap (worthless) eyepieces, a flimsy mount unsuited for holding a flower vase, let alone a highly sensitive scientific instrument, and no coatings to counteract the optical drawbacks of a refractor. A decent refractor, costing $1000 or more, will have a stable mount, coatings to reduce chromatic distortion, and maybe one good eyepiece.
The problem with the refractor is that the primary objective is a lens, not a mirror. As any physics student will tell you, different wavelengths of light travel through glass at different speeds. This is why a prism breaks up light into its various wavelengths. A refractor's lens will spread the light out slightly. The longer the tube, the more this distortion becomes a problem. Even a 700mm long tube is enough to seriously distort the color of Venus, the moon, and other bright targets. The solution is to use coatings that reduce or eliminate this spreading of the light. Another, much more expensive solution is to use a low-dispersion form of glass. Some of these are called 'florite' refractors, referring to the type of glass they use. Since it is costly to manufacture this type of material, high-quality refractors are the most expensive telescope per inch of aperature.
The obvious drawback of a refractor is the cost. The larger models, say 5" or greater, are also quite large and heavy. Another drawback is deep sky performance, since the refractor usually lacks the aperature of a reflector or Schmidt-Cassegrain. Although some claim a refractor can show more detail on a deep sky object than a reflector with twice the aperature, this is probably more a function of the long focal length of the refractor than a serious claim of superiority for the reflector. Finally, the larger models are not especially portable. A 7" refractor can require a sport-utility vehicle to transport.
The advantage of a good refractor is superior performance on planets and the moon, as well as minimal maintenance. No other design gives as sharp a view, especially since there is no central obstruction. Since the tube is sealed, collimation and aluminizing are not required. If you have unlimited funds, the refractor is a wonderful choice.
The Catadioptric is a compromise telescope. Catadioptics have two major categories: the Muksutov and the Schmidt-Cassegrain. Both operate similarly internally, traversing the length of the tube 3 times before being viewed, giving a larger effective focal length in a physically shorter tube. Although neither is optically perfect, both have become popular because of how well the compromise is made.
Earlier, I said that all telescopes are a compromise, because there is no perfect telescope. The catadioptic compromises performance for portability. A refractor or reflector will outperform a catadioptic of similar quality. But neither the refractor nor the reflector will work well with an 8" aperature in a 24" long tube. Because Catadioptics are popular, there are more accessories available for them than any other design.
Schmidt-Cassegrains have a corrector plate that attempts to correct some of the difraction caused by the relatively large central obstruction. Some books claim that this obstruction is as much as 33% of the aperature. It is important to understand that the area of the objective is more important than the diameter. For instance, a 4" aperature has twice the diameter of a 2", but it has over 4 times the area. Most obstruction percentages are based on diameters, not area, for simplification. Thus, a "33% obstruction" is not reducing the light gathering potential of a telescope by 33%.
After passing through the corrector plate, the light bounces off a parabolic mirror at the base of the tube, pointed towards the central obstruction, which is actually another mirror. This second mirror (the third surface the light has come in contact with) is pointed towards a hole in the first mirror, where the light actually escapes into the eyepiece. Remember that the hole is actually in the "blind spot" created by the secondary mirror/obstruction. It does not contribute to a reduction of aperature.
The Maksutov differs in that the corrector plate is curved inwards and has a much greater effect on the light path. The secondary mirror/obstruction is actually an aluminized spot on the back of this plate. The primary mirror is spherically shaped, leading to less difficulty in manufacturing. In smaller aperatures, the Maksutov is cheaper to make. As the aperature grows, however, the work of creating the corrector plate becomes more time consuming.
The short tube lengths make the catadioptrics very portable. I found that a Schmidt Cassegrain can be easier to carry than a reflector half the weight, due to the easier to handle package. Although difraction from the large secondary mirror is greater than the others, the portability often offsets that drawback.
Although not as affordable as a reflector, the maintenance is greatly reduced. The tube is sealed, meaning the primary mirror should never need work. The compact size and equatorial mounts can make the catadioptrics ideal photographic telescopes. Also, due to popularity, catadioptrics retain most or all of their value over time, making resale less of a problem.
Here ends the informative part of my telescope introduction. Hopefully, it has helped to make a decision easier. If you're still unsure, check out some of the following sites for more information.
My Guide to Astronomy on a
Budget
