Astronomy - Which Telescope should I buy??


A guide to help you decide.

I am often asked, "What is the power of your telescope?" The common belief is that the telescope magnifies the image. It does not! The eyepiece is the magnifier. The telescope collects light and puts the image at a point a specific length from the Primary mirror or lens. This length where the point is, is at the focal length of the objective. The eyepiece will magnify the image presented. The amount of magnification depends on the focal length of the eyepiece. To figure this out, we need to learn some optics.

Optics is the way a piece of glass or mirror bends or refracts light so that we can magnify objects at the focal length. The math here is pretty simple. Addition, Subtraction, Multiplication and Division are all that is required. There are no fancy algebraic formulas or anything too esoteric. We will keep this simple. I would suggest you be familiar with the Types of telescopes   and be somewhat familiar with the terms in the glossary of terms (given in "Astronomy Jargon").

All these principals apply to any telescope regardless whether a refractor or reflector.

First, a few terms:

Aperture : This is the diameter of the primary objective usually expressed in inches. Here we will refer to aperture in millimeters (mm). There are approximately 25 mm to the inch (to be precise, it is 25.4 mm), but we will use 25 here. We are only approximating so that you can do this stuff in your head. Some common apertures converted to mm are:

4.5 inches = 112.5 mm

6 inches = 150 mm

8 inches = 200 mm

10 inches = 250 mm

12 inches = 300 mm

The larger the aperture, the brighter the object at the focal length. A larger aperture gathers more light. The larger apertures, 14" or more are sometimes called "light buckets" because of this phenomenon.


Focal Ratio : you probably have heard people refer to their telescopes in inches and F ratio. Take the example of a Newtonian 8" F/7 telescope. The F/7 is the focal ratio. This means that the focal length of this telescope is 7 times the diameter of the primary objective, or a 56-inch focal length. In mm it is 56 X 25 = 1400 mm. Some common apertures and focal ratios converted to focal lengths are:

4.5" F/10 = 45" = 1125 mm

6" F/10 = 60" = 1500 mm

8" F/4.5 = 36" = 900 mm

8" F/6 = 48" = 1200 mm

10" F/5 = 50" = 1250 mm

12" f/4 = 48" = 1200 mm

Make note of the fact that a 6" F/10 has a longer focal length than the 8" F/6 or even an F/7.


Magnification : Here we are at the crux. The magnification is determined by dividing the focal length of the eyepiece into the focal length of the primary aperture. Bang! That's it. The focal length of the eyepiece is written somewhere on the eyepiece and is always given in millimeters. Common ones are 12mm, 13mm, 20mm, 26mm and 32mm. You can get them down to 4mm and on up. I have seen 50mm and 75mm eyepieces. Now lets work out some examples. I own 12mm and 25mm eyepieces.

Lets take the 12mm and see what happens in the examples above:

1) 6" F/10 - 1500mm focal length

1500 / 12 = 125 I will be magnifying at 125X (X is pronounced "Diameters")

2) 10" F/5 - 1250mm focal length

1250 / 12 = 104.17 (we can dispense with the .17) or 104X.

"WAIT! That is a lower magnification than the 6" telescope!" Yes, with the same eyepiece. "Then I should buy a 6" telescope, right?" WRONG! You are forgetting the light gathering power. A 10" telescope gathers almost twice as much light as a 6", so the magnification may be higher with the 6", but the object is twice as bright with the 10" than the 6".

Other phenomena that goes along with higher magnification is:

The higher the magnification, the smaller piece of sky you are looking at called the "field of view" (more on this in an upcoming page on Eyepieces, Stay Tuned). Soon your image may be larger than your field of view and you will not be able to see the whole object. Also the higher you go with magnification, the dimmer the object becomes. There is a practical limit of magnification, which is about 50X per inch of aperture. That is an 8" has a maximum useable magnification of about 400X, and a 6" is 300X. After that, the image is too dim to see. The limiting magnification is extremely difficult to use unless you are under perfect viewing conditions, and that rarely happens.