Check out the online classified advertisements at http://www.astromart.com/telescope_warehouse.html. This will give you a good overview.

Also look at Sky and Telescope or Astronomy magazines for advertisements and catalogs that you can order form.

There are many good companies that make fine telescopes. Their adds can be found in Sky and Telescope and Astronomy magazine.

Remember telescopes are rated by their aperture,, the size of the mirror for reflecting telescopes and the size of the lens for refracting telescopes. The larger the aperture the fainter the star that you will be able to see.

How well a telescope resolves fine detail, say on the Moon, or how well the telescope resolves close double stars depends both on the size of the aperture and the quality of the optics. How well the mirror or lens has been ground.

The focal length of the telescope is the length from the main mirror or lens to the point where the light is focused. The focal length of refractors and Newtonian reflectors is approximately the length of the telescope tube. The focal length for Schmidth-Cassegrains is much shorter than the tube length because the optical path is folded back upon itself. The longer the focal length the higher the power the telescope will produce with any particular eyepiece.

Focal Ration

Telescopes like cameras are given a speed ratio, called the focal ratio or simply the f ratio. The f ratio equals the focal length divided by the aperture diameter. A 100 mm telescope with a focal elngth of 1,000 mm has an f ration = 1,000 mm/100 mm = 10. A low f ratio means faster photography, an f/ 4 telescope would be faster than an f/8 telescope. Faster systems do not by themselves produce brighter images. The brightness of an image is determined solely by the size of the aperture.

Magnification

The magnification of a telescope is controlled by both the focal length of the instrument and the focal length of the particular eyepiece that is being used. The magnification equals the ratio of the focal length of the objective divided by the focal length of the eyepiece. Power or magnification is not an important consideration, because you can always increase the magnification by using an eyepiece (lens you look through) with a shorter focal elngth. The telescope magnification should not exceed 50 times the aperture in inches or 2 times the aperture in millimeters.

The Dobsonian Telescope is a good starter telescope. It is a Newtonian reflector with a thin light weight primary mirror mounted in a simple tube with an altazimuth mount. This telescope is not good for astrophotography and does not track the stars across the sky. The cost of a Dobsonian with a 6 inch mirror is about $350. For more information see the Sky and Telescope web site http://www.skypub.com/tips/telescopes/buying.html.

Refracting telescopes use lenses to collect light. Small refractors are generally expensive for the light gathering power that you get. A minimum size is about 2 1/2 inches ( 60 mm). You can get a 2 1/2 inch (60 mm) refractor for about $160.

Edmund Scientific has a unique telescope the Astroscan 2001. It is highly portable 4 1/8 inch Newtonian RICH-FIELD reflected that provides a 3° field of view. It is highly portable an durable and the scope and mount weight just over 12 pounds. It is in the $400 range. For an Edmund Scientific catalog call 609-573-6259.

If you are able to spend more you can look into Schmidt-Cassegrain or Maksutov-Cassegrain telescopes. These telescopes use a combination of mirrors and lenses and are highly portable. They have equatorial mounts which allow them to track the stars and can be used for astrophotography. They are more expensive, however. The 8 inch version costs around $1,000. See below for more information.

a newcomer learn the constellations and how to locate scores of interesting objects. This is essential knowledge for using a telescope. Also if you want to spend less than $200 then you should consider getting a good pair of binocular. A 7 (magnification) x 50 (size of objective in mm) are good for night work. These are referred to as night glasses. If you divide the objective size by the magnification you get what is called the exit pupil, the diameter of the shaft of light that enters the pupil of your eye. Young healthy eyes dilate to about 7 millimeters in dark skies so a model yielding a 7 mm exit pupil gives the brightest image for night work.

For additional information about telescopes consult Sky and Telescope’s web site at http://www.skypub.com/tips/telescopes/telescopes.html or the book The Backyard Astronomer’s Guide, by Terrence Dickinson and Alan Dyer.

Inexpensive Diffraction Gratings (they act like a prism and spread light out into its colors) are available from Edmund Scientific: A set of two 50 x 50 mm diffraction gratings catalog number M39,359 is available for $2.75. Call 1-609-573-6270 or write for an annual school catalog to Edmund Scientific Company, 101 E. Gloucester Pike, Barrington, NJ 08007-1380.