Are you a beginner? A casual observer? Or do you fancy yourself to be a junior Carl Sagan? If you're a beginner or a casual observer, this guide's for you. You'd probably like to see the moon up close, observe the planets or get a good look at a passing comet, but you're not ready or interested in spending the time or money to do astrophotography just yet. You might also like the telescope to have some portability so you can take it to dark sky locations or with you on vacation without much trouble.
The best way to start is with a pair of binoculars. Binoculars have several advantages. They're lightweight, portable, easy to aim, have low powers of magnification and a wide field of view. Just in case you lose interest, they can be used for other observing pursuits. Also, they tend to be less expensive than a telescope and you might have a pair already.
Binoculars are typically rated by their power of magnification and the size or aperture of the front lens. This information is contained in those cryptic numbers on the eyepiece end of the binoculars. They read as 7X35, 7X50 or some combination of numbers on either side of a 'X'. To define the numbers, let's start with the 'X'. It stands for the binoculars' power of magnification. So in 7x35 we have 7 power magnification. Typically, binocular power is usually 7, 8 or 10. The higher numbers like 10, 15 and the occasional 25 power are rarer and usually quite expensive. They're also impractical for casual observing because you'll need a tripod to hold them without jiggling at 10 power and higher.
The other number is the diameter measurement of the front lenses in millimeters. This number can range from 35 to 70 and higher. As with the magnifications though, the higher the number, the more expensive and more difficult they are to handle. This brings us to our first tenet of buying binoculars or telescopes: Always buy the biggest aperture you can afford. Reason? The bigger the lens or mirror in your optics, the more light you can gather. The primary misunderstanding about telescopes and binoculars is that magnification is your ultimate goal. While magnification is nice, you can't magnify what you can't see. In other words, if you aren't gathering enough light from an object to make it visible, what's the use of magnification? With binoculars purchased for bird-watching or enhancing the view of the game from the nosebleed section, you can also do some casual skygazing. Ordinary binocs will give excellent views of the moon and greatly improved views of Jupiter, Saturn, and comets. Good binocs start at around $65 and go up from there. Check to make sure they're built well and test them by looking at something with lots of vertical lines. You want to make sure that the lines don't curve near the edges of the field and that they have a hard, sharp focus. Did you know that most comet discoveries are made by amateurs with binoculars? Or that those old binocs you have in the drawer at home are more powerful than Galileo's first telescope? Try them. They might just be the best way for you to begin.
Telescopes come in two basic types - refractors and reflectors. Refractors use lenses to gather light from dim objects while reflectors use a mirror. Refractors are the long narrow tubes with the eyepiece at the back end. Reflectors are usually shorter, fatter and open at the top. In most styles of reflector, the eyepiece is placed near the top. Refractors provide good, sharp images and are good for observing pinpoint light sources such as stars and planets, but tend to be on the expensive side. Reflectors are good for observing more diffuse light sources such as galaxies and starclouds. They also provide good, sharp images but they are much less expensive because reflectors reflect the gathered starlight from just one surface. In refractors, the light has to pass through two lenses and the surfaces of both (four altogether) have to be polished as close to perfectly as possible. This is both time-consuming and labor-intensive. Going by our first tenet of "Always buy the biggest aperture you can afford", you can almost always purchase a much larger reflecting telescope for what you'd spend on a smaller refractor.
In many stores telescopes are sold by their power of magnification. It actually has very little importance in purchasing a telescope because of the second tenet, "You can't magnify what you can't see". : as magnification increases, resolution decreases. The rule of thumb for magnification limit in telescopes is 50 power per 25 millimeters(25mm = 1 inch). It works like this: the main lens or mirror of a telescope gathers light from an object and forms a small image near the eyepiece. The eyepiece magnifies the small image. Different eyepieces have different magnifications. The numbers on the eyepiece refer to their focal length. If you divide the focal length of the main lens by the focal length of the eyepiece, the resulting number is the magnification you'll get with that particular eyepiece. For example:
900 mm focal length divided by 25 mm focal length = 36 power magnificationThe lower the eyepiece numbers, the higher the magnification. Remember: the rule of magnification is 50 power per 25 mm of aperture. In a 60 mm scope, the best you can do is 100 power. In a 100 mm scope, 200 power and so on. If the advertised magnification is 500 power, the scope size HAS to be 250 mm or more of aperture, otherwise the loss in resolution will degrade the image dramatically. It's also just plain better to begin with low powers anyway. Telescopes have high magnification and a small field of view - just the opposite of binoculars. Using lower powers of magnification in a telescope make it easier to find and keep track of the objects in view.
It's also better if your telescope has a finderscope - a smaller telescope mounted adjacent to the eyepiece. The finder, with much lower power and much wider field of view makes object location much easier.
For casual observing, motors to drive the telescope for tracking objects aren't really necessary. After just a little practice you'll find that guiding a telescope by hand as you observe is quite easy, especially at low powers of magnification. Motors are important if you're planning to do astrophotography. For beginners, complication of operation and price are reduced if you don't have a motor drive - save it for your next, larger telescope.
The mount or tripod of the telescope is as important as the optics. It should be firm and steady. A massive tripod is less prone to wind vibration. The simplest telescope to set up and use is the Dobsonian reflector. The optic tube just sits in a giant "lazy-susan" type mount. It's very easy to operate and could be the best value for your dollar. You'll find them advertised in the magazines like "Astronomy" or "Sky and Telescope".
If you'd like to spend even less money for a good telescope, consider a used telescope. The periodicals mentioned above have listings of used equipment and if you have access to the Internet, check out www.Astromart.com. You'll find endless listings of equipment for sale. Good luck, start looking early and don't forget ; caveat emptor - Let the buyer beware!
Derrick H. Pitts
Chief Astronomer
Franklin Institute Science Museum
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