Picking a Telescope [Updated April 13, 2007]
Jul 06 '00 (Updated May 05 '07)
The Bottom Line Research online, then go see and use some scopes yourself. Observing events are held all over the country; they are a chance to see what you like before you buy.
One of the most difficult questions in amateur astronomy is choosing your own telescope. The number of factors which will affect your final satisfaction is quite large and unfortunately, a bad experience with a first telescope is often a final experience with astronomy.
At the same time, most people are so completely overjoyed when they discover a type of telescope they like that they go out and advocate it for everyone without realizing telescope fit is as individual as taste in cars. A more detailed description of the technology is in my Telescope Primer.
The factors in deciding to get a telescope I address here are:
Cost
Size
Performance
Appeal
Growth
Dealing with the above
My hope is that by addressing these criteria, I can at least help people to have a satisfactory experience with their first optical instrument and help them discover astronomy for themselves.
Cost:
This is the first factor everyone sees. In optics, you really do get what you pay for. However, the question of getting what you want for what you paid is just as important. It is far better to get nothing than to end up disappointed. There really is a lot to see in the night sky, even with the naked eye. In my own experience, I found an inexpensive pair ($50) of wide angle 7X35mm sports binoculars were a fantastic investment. They make it possible to find many objects in the night sky and amplify just enough light to make nebulas and galaxies become visible. The best advice I can therefore give on spending is to spend nothing until you know what you want.
Size:
Size is everything for telescopes, but it doesn't quite work the way you'd think. Size not only governs what a telescope can do, it also governs what you will do with the telescope.
Size will determine what your telescope does in your life. If a very large telescope, such as the CPC 1100 is difficult and time consuming to take outdoors and set up, you rarely use it. Even though that telescope may excel at your favorite kind of viewing, few people bother unless they are really in the mood to observe and have the time to commit several hours to observing that evening. In comparison, a small telescope on a simple mount, like the NexStar 5i, while inferior in every performance measure there is, may be a far more meaningful addition to your life because you will use it. If taking the telescope out doesn't mean moving an 85lb. system in pieces and carefully aligning it, but rather means picking up something weighing 11 lbs., walking out, and pointing it at something, then stargazing may be a lot of fun since you can go out for a 20 minute look around and come back in. In my own experience, I found getting a small lightweight telescope and mount transformed my experience of the night sky. There were many more nights I was willing to pop outside with the small telescope for half an hour and thus began to see the movement of the heavens since I was looking every few nights. The second discovery this led to was the friendly neighbors I had never met.
The big telescopes have a large following since the performance is there. However, you should notice the ads in Sky and Telescope for what is called an Apochromatic refractor- this is a telescope designed to give maximum performance for its size with cost as no object. People pay $2500 for high end small telescopes only 85mm in diameter because nothing will outperform them for the size- and they know they will use them at that size. More recently, equivalents to these telescopes have become far more reasonable in price, and small scopes such as the Onyx8 80ED and the AT66ED work very well at a small fraction of the price of the original APO scopes. The right size telescope for you will therefore depend on a balance of the following criteria, which you must decide for yourself:
(1) Performance requirements- What are you satisfied with?
(2) Area- Do you need to move it somewhere to see a dark sky?
(3) How gung-ho are you?
(4) How strong are you? Do you have someone to help?
Performance:
The most common performance number people have seen is magnification. However, many of the most impressive objects in the night sky are actually quite large and really just need to be made brighter to become visible. Distortion from earth's atmosphere makes extremely high magnification views impossible except in areas with exceptionally clear skies (hence the practice of putting observatories on mountain tops or in orbit). At the same time, magnification actually dims the image a telescope produces. For example, a 90mm telescope, when compared to the 7mm diameter of a dark-adapted human eye, has 165 times as much light gathering capacity. However, if the image is magnified to 25X, then the image your eye will see looking into the telescope is only 6.6 times brighter than what the unaided eye would have seen. Because of this effect, the magnification of 165X is called the Effective Magnification- the magnification at which objects are the same brightness as your eye would have seen without the telescope. Thus, the common 60mm refractor has an effective magnification of 73X. Although it is possible to put in eyepieces which will make the object seen through the telescope 150X, or more its original size, the image will appear dark and brownish in addition to being very blurry. Magnification beyond the Effective Magnification is often referred to as Empty Magnification because it provides no benefit in the image.
For a beginner, I would strongly recommend getting nothing rather than one of the 60mm telescopes advertised as 525X telescope, 600X telescope or the like and sold in department stores. The money would be better spent another way. These telescopes are actually more of a deterrent than a benefit since they give the user the impression there just isn't much to see when, in fact, instruments very close to these in cost and a little larger in diameter (but in many cases actually lighter and more portable), such as the NexStar80 series, open up the entire sky to the observer.
The resolution a telescope can see is proportional to its diameter. This is why the Hubble Space telescope takes such beautiful pictures- it's huge. A 90mm telescope is capable of seeing object details around 8% of the size of those the unaided eye can see. The 60mm scope can resolve objects 12% the size of those the unaided eye can see. Now, this effect is independent of magnification. That means when you enlarge an image in a telescope, at some point you will make the full image resolution visible and further magnification only makes the larger image dimmer and blurrier. Or, to put it another way, when the telescope's image is magnified to the point where it has less resolution than your eye, it will always look fuzzy and blurry. The larger the telescope, the more you can magnify the image before this happens. A similar effect can be seen when enlarging photographs. As a result, most people find a telescope has its most pleasing view at a magnification of about 10-15 power per inch of aperture, or about 4-6 power per centimeter of aperture. This balances light gathering capability with image resolution so you get a view which is brighter than what you would see with the naked eye and is larger and more detailed, too.
What you will see in the telescope, the field of view, is a function of its diameter and magnification. Telescopes have focal ratios based on diameter. For example, a 90mm diameter telescope with a 500mm focal length has a focal ratio of 500/90 or f/5.6. The magnification a telescope produces is its focal length divided by the focal length for the eyepiece being used. For example, a 20mm focal length eyepiece is referred to as a 20mm eyepiece. When placed in a 500mm focal length telescope, the magnification is 500/20 or 25 power. A telescope with an f ratio below 8 is usually called a fast telescope, which means its optics are optimized for high brightness low magnification views. It is called Fast because if it were used with a camera, the exposure times required would be shorter than for a telescope with the same diameter and a longer focal length (and hence higher magnification and lower brightness).
The lower the magnification a telescope operates at, the wider the field of view. For example, a 90mm diameter, 500mm focal length telescope with a 20mm eyepiece has a field of view equal to the apparent field of view seen in the lens (how much of your own eye's field of view it looks like) divided by its magnification. If the view in the eyepiece appeared to be 50 degrees across, then the actual field of view the telescope was seeing would be about 2 degrees across, about four times the size of the full moon (compare with a pair of low light marine binoculars, which commonly have fields of view over 7 degrees across). If the 90mm telescope had a focal length of 1000mm, or an f ratio of about f/11, then the magnification with a 20mm eyepiece would be 50X and the field of view would be only 1 degree. This telescope would do a good job at average magnifications, but would have trouble encompassing larger celestial objects, like the Beehive Cluster or comets, which the 500mm telescope could fit into its field of view .
Very low focal ratio telescopes, such as f/3.6, tend to have such strong curvatures in their optics that they can't produce sharp images at high magnifications due to buildup in optical errors. Telescopes with very short focal ratios also tend to have fish-eye type curvature in their images. Thus, low focal ratio telescopes are typically seen as only fit for low power use for nebulas and galaxies. As a result of this lower limit on focal ratios, larger telescopes necessarily have longer focal lengths than small telescopes and hence can only see smaller patches of the sky at one time.
Longer focal lengths make the shape requirements for mirrors and lenses more forgiving, so telescopes with longer focal lengths are desirable for high magnification images of bright objects such as planets or the moon. Therefore, telescope designers can choose what a telescope will be good at when they decide on focal length. As a result, the more extreme the telescope's focal ratio is, the more specialized its application. Many amateur telescopes, are produced which have intermediate focal ratios of f/10 and diameters of 8 inches (203mm) to try to make them general use instruments. In practice this results in telescopes which can't quite fit large nebulae into their field of view on one end and can't quite get the very sharpest views of planets at the other, but can at least do something at both ends of the viewing spectrum. Of course, making versions in smaller sizes, such as 5inch diameter and 6inch diameter scopes does result in a wider field of view, and these scopes can use Focal Reducersto get a wider and brighter image.
One other size effect telescopes face is urban light pollution. All of the street lights, gas stations, and other lights in our cities fill the sky with a glow as the air is lit from below. Large telescopes are actually at a disadvantage in such circumstances because their large light gathering ability amplifies the background light along with the subject's light, thus washing contrast out of the image. Small telescopes (under 6 inches; or 152mm) are less susceptible to this effect since they collect less light. Higher magnification is also a defense against this effect. However, the most effective defense is to change regulations and lighting standards to ensure lights shine on the ground where light is needed instead of illuminating the sky, or are deleted where they serve no purpose. Good examples of telescopes which do this are the NexStar 5, NexStar 6SE, the 80ED refractor, and the AT-66ED refractor. A secondary defense is to use a < a href="light">http://www.epinions.com/content_141056577156">light pollution filter (LPR), though I personally don't like them since a fair amount of image goes with the light pollution.
If someone asked me to name a telescope for beginners, I'd look at telescopes somewhere between 80mm and 127mm in diameter as a starter telescope. These telescopes are powerful enough to see the most famous objects in the sky such as the planets out to Neptune and the Messier catalog. They also are easily portable, take up little space, and are large enough to give reasonably satisfactory views of many objects. There are many economical small 80mm telescopes such as the ones mentioned in this article, and since computer drives have come down in price, around $350 can see you set up with a starter system able to do a tour of the night sky.
Appeal:
Believe it or not, there is some amount of aesthetic bent to the telescope question. If you want to have it out in your living room, you'll want it to have a look you find pleasing. It may sound silly, but you might really need to give this some thought- if the telescope is hidden in a closet because you find it about as appealing as a washing machine, then it may spend a lot of time in the closet instead of getting used. For those with more of a Lexus streak in them, a top of the line telescope may be a statement in their home as well as an instrument for exploring the universe from the patio. Others keep telescopes in their garage with a tarp over them where they are instantly available, but otherwise invisible.
Growth:
One important principle I would like people to keep in mind is they don't have to think of a telescope as final when they get one. If you buy a nice telescope, it has a surprise waiting for you when you sell it- unlike computers and cars, good telescopes hold their value because they will work just as well when sold as when they did new unless they have been mistreated and damaged. In fact, there are some telescope brands which sell for more used than they do when new because new ones can only be purchased after going on a indefinite waiting list of five years or more. However, given the quality of the scopes available at reasonable prices now, I caution that buying a really high price instrument early on just isn't necessary. Many people find themselves moving between scopes or owning several as their interests change and grow.
Dealing with the above:
I realize I've just talked about a lot of issues and expressed a lot of concerns most beginners didn't even suspect were waiting for them. The solution to deciding what you want, however, may be a lot less stressful than you fear: Take a look for yourself. The best advice I can give on finding a telescope you will like is to keep the issues I've mentioned in the back of your mind and go look through a lot of telescopes. Don't let someone tell you what looks good- take a look for yourself by joining your local astronomy club and going to some of their observing events. You will discover astronomers are a sociable lot and really enjoy getting out with a few telescopes a few times a month and looking at the sky with other people. People without telescopes are usually strongly encouraged to come and get a look. This way, there is a chance to see many different kinds of equipment and see what it is like to use them, what the view through them looks like, and also get some pointers on where some neat things are in the sky. Ask at a local telescope shop, the college or university science department, or look on the web to find your nearest astronomy society. Above all, don't be afraid to ask questions or for a peek through the eyepiece. There is an awful lot to see above us- start looking and the rest will follow.
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Member: Rich W.
Location: Tucson, AZ
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About Me:
Dad, Engineer, Scientist, Astronomer, Traveler; order may vary.
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