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I've noticed quite a few decent telescopes for great prices on craigslist, but I am a complete novice and don't know the first thing about them. I thought I would be able to pick up what I needed from reviews, but when I hit sentences like this:

It has a 150mm clear aperture and a 1500mm focal length. The C6-A XLT comes with a Vixen dovetail mounting rail attached and is available packaged with a CG-5 go-to German equatorial mount...

... I knew I was a stranger in a strange land.

What questions should I ask when buying a used telescope, and what answers should I look for?

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It depends on what is your primary target, for example observation or photography would affect the importance of the mount type, also the type of objects; planetary versus deep space affect the importance of the features of the telescope. Maybe if you complement your current question with some information about your target, it will be easier to answer. –  Omar Salinas Dec 27 '11 at 0:38
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closed as not constructive by Waffle's Crazy Peanut, Emilio Pisanty, Manishearth Jan 18 '13 at 11:31

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I wouldn't recommend that a beginner buy their first telescope from CraigsList, eBay, or similar, as they are almost certain to get stung. Most of the stuff there is out and out junk, and it takes a pro to tell which, if anything, is a good buy.

The safest and best place to buy a telescope is from a local telescope store. This way you get to see and try the telescope, and you get support from the dealer after purchase. Telescopes are different from most consumer purchases in that they really do require specialized knowledge to evaluate.

Another alternative is to join your local astronomy club; their members often have telescopes for sale, and these are much safer buys because the seller will have to meet you face-to-face for years in the club. A third option is to buy from a reputable online dealer, such as Orion: http://www.telescope.com/ Their web site is a wealth of information and, if you phone during business hours, you can usually get knowledgeable advice from a real person. There are several other reliable dealers, but Orion is the one I've done the most business with, and I've always been pleased.

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Thank you. Precisely what I needed to know. –  Brien Malone Dec 28 '11 at 22:18
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Something else that I found useful: skyandtelescope.com/equipment/basics/12511616.html –  Brien Malone Dec 28 '11 at 22:23
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Many people who decide to purchase a telescope to penetrate the mysteries of the universe. But often feel disappointed to see that the instrument does not serve to those who wait.

First of all there is to know the possibilities of a commercial telescope and above all must learn the mechanics and "geography" of heaven. As before getting permission to drive a vehicle must dominate a little traffic rules for space transit routes should take some classes "astroschool." Only in this way our adventure will not end in total disaster.

To choose well you should know that the telescope has some key parameters:

The diameter or aperture is the size of the target. Indicates the true potential of the telescope. The wider, the greater the amount of light that captures and therefore allows you to see fainter objects and far. In the characteristics shown as $D = X mm$., Where $X$ is the number in millimeters. For a buyer nobel can indicate that the acceptable minimum aperture is $60 mm$ in a refractor and $100$ or $114 mm$. a reflector.

If the budget allows and the hobby is promising is advisable to choose 80 or 90 mm. and 150 mm in refractor. in the reflector.

Larger sizes are recommended only for observers with some experience. Therefore, the system sold aperture $Schmidt-Cassegrain$ is the $8-inch (203 mm.)$, As fans normally acquire advanced knowledge.

It is not uncommon to see small telescopes that are typically sold in non-specialized stores with great enthusiasm to announce that we will $500x$ or $750x$ increases. The reality is that never achieve much in a small telescope magnification: the images will turn dark and blurred, as there is a proper limit for each telescope, which is well below these vast amounts of magnification.

The amount of increases that we can give our telescope is directly dependent on the diameter of the lens. That is, the size of the input light having. In the case of a reflecting telescope (composed of special mirrors) is the primary mirror diameter of the involved. In the case of a refractor telescope (compound lens) the diameter of the lens. By nature, the lens diameter determines the amount of useful magnification that will give us a telescope. To make a simple quick calculation, the maximum amount is usually estimated as twice the diameter in millimeters, if we see a refractor telescope with a main lens diameter $70 mm$, we estimate that the maximum increase that we will be about $140x$, comfortable and enough to see the major planets and close-ups of the moon.

In short, more increases is not synonymous with better. Remember that what we want are clear images. And keep in mind that commercial telescopes exaggerate this issue as part of the sales strategy. Just do not pay attention.

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