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What telescope should I buy? Page 2

Focus on refractors

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First things first: DON'T buy one at a "big box" store!

Although some "big box" retailers are starting to carry SOME decent telescopes, most of them are, well... garbage. 9 times out of 10, the main lens or mirror optics are okay. But what is garbage is what comes with it - a shaky mount, cheap eyepieces that give truly HORRIBLE images and are nearly impossible to see through, and a finderscope that is less than useless. This is a recipe for [b]guaranteed frustration[/b], and you will have spent $100 or more on a telescope that will wind up as a clothes hanger. I can think of a lot less expensive clothes hangars, so... buy a GOOD telescope, not a clothes hangar!

When most people think of a "real" telescope, they think of a refractor - or a telescope that light enters through a lens (or lenses) at the objective - which then bends the light to a focal point and comes into focus using an eyepiece at the other end. Refractors have some advantages: In small apertures, they are cost-effective, require little - if any - maintenance, and are generally good performers. Some of the disadvantages are: They have small apertures, it can be hard to tell a "good" one from a "bad" one in the store, and inexpensive ones are made in longer focal lengths, meaning the small mounts they're on will magnify vibration that much more.

So what do I look for?

Aperture

As with binoculars, the aperture of the objective (large) lens is more important than ANY magnification number in big red letters (675x POWER!!!). Basically (although with some overlap), refractors pick up about where binoculars start to leave off, or around 50 or 60mm. As with binoculars, that aperture lens will show you literally thousands more stars than your eyes alone. But where "larger is better" with telescopes is in seeing fine detail - to a point. The larger the aperture of a telescope lens (or mirror, more on those next page), the better its resolution will be, or ability to resolve smaller and smaller details. It also allows the lens (or mirror) to gather more light, increasing the contrast of what can be seen. So point #1: More APERTURE is better.

Focal length and focal ratio

Focal length is the distance from the lens to the point at which the light rays converge. For small refractors, it is typically between 400mm and 900mm. The ratio itself is determined by dividing the focal length by the aperture of the lens. There are variations on these; some are termed "slow" and some are termed "fast." A 720mm focal length refractor with a 60mm lens would have long f/ratio of 12 (720 / 60 = 12) and be considered a "slow" f/ratio. A 400mm focal length refractor with and 80mm lens would be an short f/ratio of 5 (400 / 80 = 5) and considered a "fast" scope.

Eyepieces

The eyepiece is the part of the telescope that determines magnification (and by extension, the field of view). ANY telescope can be "made" to magnify 500x or 600x or even 2000x, but that won't matter if you can't see through the eyepiece or the image is horribly blurred. Realistic magnification for almost any telescope - regardless of aperture - is about 150x to 200x, on good nights (for exceptionally stable "seeing" through the layers of our atmosphere, it's possible to go up to 400x, about once or twice a YEAR).

There a different kinds of eyepieces too. Most cheapie, department-store style telescopes use Huygens lenses. In a word, these eyepieces are terrible. Look for Modified Achromat (MA) eyepieces, or Kellner (K) eyepieces. These are at least decent, although still inexpensive designs. Better still are the Plossl style eyepieces. Eyepieces have different focal lengths too - short ones provide higher magnification and smaller field of view, and longer ones provide lower magnification, but a larger view of a piece of sky. A basic starter set will often include 25mm and 10mm eyepieces.

Magnification

Magnification is determined by dividing the focal length of a telescope by the focal length of the eyepiece. For example, with our "long/slow" 720mm telescope above, we could expect magnifications of 29x and 72x with our "starter set" of 25mm and 10mm eyepieces:

For the "short/fast" telescope of 400mm, the overall magnification would be lower:

Why would we want lower magnification? Well, just like the binoculars, lower magnifications equals wider fields of view, AND also means less movement in the eyepiece due to vibrations. Sometimes we actually want a very large field of view to see wide, extended objects like the Andromeda Galaxy (M31, which spans nearly 5 degrees of sky) or the Pleiades star cluster (M45, which spans more than a full degree). High magnification is most useful for planetary or lunar observation, but much of "deep sky" observing of nebula, galaxies and star clusters is done with lower magnification, due to the wider field of view.

So which one is best? Long/slow, or short/fast?

Well, that depends. If you have a set of binoculars at home, a long focal length scope might be a good complement to it. If not, even a 400mm "short" refractor will show the four Galilean moons of Jupiter, Saturn's rings, and detail of craters on the Moon. Here are some good "starter" refractors to consider:

• Orion 60mm f/12 Alt-Az refractor - about $100*
• Orion 70mm f/10 Alt-Az refractor - about $140*
• Meade 70AZ 70mm f/10 Alt-Az refractor - about $75, this is a very good overall deal on a low cost starter telescope.
• Celestron Powerseeker 70mm, f/10 Equatorial mount refractor - a pretty good deal on an equatorial mount IF you're willing to learn how to use it, AND you promise me to throw out the 4mm eyepiece as soon as you get it (it will be crap, trust me). You'll have a barlow lens to get higher magnifications with the included 20mm eyepiece.
• Orion 80mm f/4.3 GoScope refractor - about $100*

* - all prices are circa 2009; please click links for current pricing

There are others out there, but please, please PLEASE promise me you will NOT buy something like this!! You'd be better off burning 30 one dollar bills in winter.

"But, but - I want something I can see MORE with!

Alrighty then... click over to the next page and we'll talk reflectors.

One last thing: Please be sure to turn off your outdoor lights, or if you must have an outdoor light on, be certain it is a full cut-off fixture, and the minimum wattage required. No sense in lighting the underside of planes at night - it just ruins the night sky for everyone else! Besides, doing so really will save you money on your electric bills. For more information about light pollution, please visit the International Dark Sky Association.