Telescope Buying Guide

1. As the observer you will need to answer the following questions
What do I want to use the telescope for? Daytime? Nightime? Sun? Planets? Comets? Stars? Galaxies?
Where will I be using it? City? Suburbs? Country?
Will I be carrying it often? Will I be transporting it often?
Will I be taking night-time pictures with it?
Will I be using it mostly for fun or for some amateur scientific exploration?
2. Background for the questions
If you will be carrying it often you will need to decide how much weight is sufficient to tire you out. Twenty-five pounds is enough to tire out anyone if it's more than 30 yards from the car or home. If you want it for sports events or nature photography then it better be very light.
The fainter the object is that you want to observer the larger the primary lens or mirror should be. Also if there is a fair amount of light pollution, there might be too much light scatter to make some object visible at all.
The mount and optical design are important, but these are secondary to the light gathering capabilities of the telescope and the magnification capabilities of the eye-piece system.
Decide whether you will be carrying the telescope once a day or evening or whether you can keep it setup in one place for a while. Setting up a telescope takes a bit of time.
Galileo had a telescope that was weaker than some of the small finder scopes attached to the larger telescopes - so any telescope on the market today can be used for fun or science.
3. How to make your choice
Observing Sun Spots (indirectly!!!) can be done with a small refractor (1 1/2" - 40x) bought at a children's store - provided you have a good projector attached.
Most night-time observing requires less than 200x magnification, so if you plan to carry your telescope and mount around go for the largest telescope and mount you can carry. For the average person this would be in the 3" to 5" range for the primary mirror. Some of the lower end telescopes are named "sporting" scopes.
If you plan to do a lot of night-time city observing and you won't be travelling regularly with your telescope, buy the largest telescope you can. This would be anywhere in the range of 6" to 16".
4. New or used
You can find some great deals for used equipment, but you can also find some great deals for broken used equipment. Even new equipment might have bolts that are too tight or too loose, or a mount where one leg doesn't slide out properly. You need to be aware of all the moving pieces and make sure the system works as a complete tool. Purchasing without touching should be discouraged. And at least there needs to be full disclosure of problems and a return policy.
When buying it, remember to ask: Can I carry it as I will during my viewing sessions? Is it powerful enough for the viewing I want to be doing? Is it easy to setup and take down if I am not leaving it in one place?
5. Eye-pieces
The eye-piece is what gives you the magnification. You calculate the magnification of the system by dividing the focal length of the primary telescope mirror/lens by the focal length of the eye-piece . For example a telescope primary with a 2000 mm focal length and an eye-piece with a 20 mm focal length with produce a magnification of 100x.
NOTE WELL: Unless you have bought a super-scope and you will be observing from the top of a mountain beside a tranquil body of water, during a power-grid failure, don't waist your money on an eye-piece that promises to deliver more than 300x magnification with your primary mirror/lens.
6. Having fun
I promise you that once you look at Jupiter or Saturn, or look at a projection of a Sun spot you will be hooked on Astronomy. Don't kill the passion that you have by going overboard with a top-of-the-line system right off the bat. A system that takes too long to setup or is too heavy to move or carry, or even one that is too expensive may be a quick reason to stop observing. There stars have been there for a billion years - they'll wait for you to make a good decision on a first telescope.
7. Checklist
main telescope (size based on carrying, viewing)
several lenses giving you some maginfication possibilities for 40x, 80x, 120x
a rugged mount like Alt-Az for viewing or Equatorial for star tracking or photography