Telescope Buying Guide

The Basics

The function of a telescope is to collect light. The more light that is gathered, the fainter the objects you can view.
 
Aperture (inches) Limiting Magnitude
2.0
4.0
8.0
14.0		 10.5
12.0
13.5
14.7

Determine what you want to look at. Certain telescopes are better suited for looking at deep sky objects, like galaxies, versus other telescopes that are designed for looking and planets, the moon, and double stars.

Objective - the part of the telescope that gathers light
Aperture - the size of the objective
Focal Length - the distance it takes for the objective to form an image
Focal Ratio - focal length divided by the objective diameter
An f/3 - f/8 is a fast system giving a wide field of view. These telescopes are great for viewing galaxies and nebulae.
An f/10 - f/20 is a slower system with a smaller field of view. Viewing the moon and planets is better in these telescopes.

Magnification is the least important factor in buying a telescope. To determine the magnification power: divide the focal length by the eyepiece size.

Telescopes

The Refractor
This telescope uses a lens to gather light at the front and goes straight though the tube assembly to be focused and magnified at the back. The views are clear and crisp and hold up well under magnification. Best for viewing planets and the moon. The major drawback is its cost.

The Reflector
The reflector uses a mirror to reflect light to a secondary mirror, which then directs it to the eyepiece. While the views are not as crisp as the refractor, what you can see is still breathtaking.

Newtonians and Dobsonians are similar in their design. Newtonians are usually an f/5 to f/8. Dobsonians are a faster system, usually f/5 or faster. They are also nicknamed "light buckets" because they can gather so much light and have a wide field of view. The cost is more in the price range of most astronomers.

The Catadioptric
The "cat", also called a schmidt-cassegrain system, is a very popular telescope. It is a type of reflector that uses a correcting lens to eliminate imperfections in the mirror. The light is actually folded back on itself and brought to a focus at the back of the optical tube assembly. This is a good all-around telescope. They are stored more easily than other telescopes and are fairly reasonable in price.

Mounts

Altazimuth
These are decent mounts, but can have wobbly tripods. Slow-motion controls are commonly used to track the stars. They can only move in two directions (up and down, left and right). They do not have to be polar aligned. Dobsonians also have an altazimuth mount. The movement with these is usually smooth. If you are interested in astrophotography, this is not the mount for you.

Equatorial
These mounts make tracking easier by making one axis parallel to the celestial pole. They will either have slow-motion controls, a clock drive that will track for you, or both. For accurate tracking, the mount must be polar aligned. There are two versions: the German equatorial mount and the fork mount.

Eyepieces

Eyepieces are an important part of your telescope. The exit pupil is what you need to go by when purchasing an eyepiece, not magnification. Depending on the speed of your telescope, you can decide what eyepieces are best for you. If you have an f/10 and want to view the moon, you would need an exit pupil size of about 2. Multiply the speed of your telescope (f/10) by the exit pupil size 2. You would need about a 20mm eyepiece for viewing the moon. Below is a chart you can use for your telescope.
 
Exit pupil size (mm) Object viewed
5-7
3-4

2
1

 Nebulae and Galaxies
General viewing and Clusters
Planetary nebulae
Moon and planets
Maximum for moon and planets
Close doubles

Telescope Products Manufacturers and Suppliers


Anacortes Telescope & Wild Bird
9973 Padilla Heights Road
Anacortes, WA 98221 USA
Phone: (360) 588-9000 Fax: (360) 588-9100
E-Mail: info@buytelescopes.com
http://www.buytelescopes.com/

Astronomics
2401 Tee Circle
Suites 105/106
Norman, OK 73069
(405) 364-0858
1-800-422-7876
www.astronomics.com

Celestron International
2835 Columbia St.
Torrance, CA 90503
(310) 328-9560
www.celestron.com

DayStar Filter Corp.
5110
Diamond Bar, CA 91765-5110
(909) 591-4673
Solar hydrogen-alpha filters

Edmund Scientific
101 East Gloucester Pike
Barrington, NJ 08007
(609) 573-6250

Meade Instruments Corporation
6001 Oak Canyon
Irvine, California 92620
(949) 451-1450
www.meade.com

Orion
P.O. Box 1815-S
Santa Cruz, CA 95061
1-800-447-1001
http://www.telescope.com