Introduction to Telescopes

There are many factors to consider when choosing a telescope. Making the right decision can be the difference between it ending in the closet for years vs spending many nights in the backyard or observing site. There are a lot of different types of telescopes and each has a specialty. This web site contains several pages that will help you make the decision that is tright for you. Before you head out to make your purchase I urge you to at least read this first page.

While cheap department store telescopes are often sold by marketing their "power" or magnification, the truly important factors are aperature and configuration. Any telescope can give a highly magnified image but if that image is blurry or washed out who cares.

The terms defined below will help you understand what is really important when trying to determine the quality of a telescope. You should understand these terms before choosing a telescope.

Aperture
How fat is the telescope tube? Aperture is the measure of the diameter of the main mirror or objective lens of a telescope. In general, the larger the aperture, the better the resolution and the fainter the objects you can see. The most important factor for gathering light is aperture. When you look the tube of a telescope, a "fatter" tube has a larger aperture. In a refractor telescope, the circle at the end of the telescope closest to the stars determines the aperture.

Telescope manufacturers usually advertise telescope aperture measured in millimeters or inches. You may see an ad in a magazine for a 8" LX200, or a 12.5" inch Newtonian. On the other hand you may see a 80mm refractor, or a 125mm mak-newt. These numbers weather in millimeters or inches represent the diameter of the primary mirror in a reflector or the primary lens in a refractor. When it comes to aperture a reflector gives the most for your dollar. This is because it is easier and thus less expensive to create a large mirror than a large lens. This is why the very large telescopes you see at observatories are mirror based. On the other hand a good quality refractor while expensive will often give better definition or contrast.

Focal Length (determines the power of magnification)
The focal length is the distance from the primary mirror (or objective lens, for refractors) to the focal point (which is also called the prime focus). The focal point is where the light rays converge. The longer the focal length, the more magnification you will get for a given eyepiece.

Focal Ratio
Simply the focal length divided by the aperture. You often see this describes as the F value of a telescope i.e. F4 or F5. You need to be sure you are using the same units when performing the division.

Resolution
Angular resolution is the ability of a telescope to show fine detail. The greater the aperture of a telescope, the more detail it will reveal. The theoretical angular resolution of a telescope is equal to 4.56 divided by the telescope aperture in inches. The lower this number is the better the resolution. This number is referred to as "Dawes Limit."

Definition
Definition is a telescope's ability to reveal the contrast between two areas having nearly the same brightness in an extended object image. This is especially important in planetary observing. Dust, bad coatings, thermals, poor atmosphere ic conditions (seeing), or optical aberrations can upset a telescope's contrast factor, thus diminishing its ability to reveal low-contrast detail. Of these, poor seeing is by far the most frequently encountered.

Magnification
Magnification (Power) is calculated by dividing the focal length of the telescope by the focal length of the eyepiece.  This means the shorter the focal length of the eyepiece is the higher power is will be.  This also means a given eyepiece used in different telescopes of difference focal length will produce different magnifications.

Magnification is useful when viewing the moon and planets.  Power is also useful when using a telescope terrestrially.  But there are limits.

During the daytime, typically you will not be able to view at a distance at more than 60 power. (Spotting Scopes usually to not exceed 60 power).  This is because as you increase the magnification, you increase the magnification of everything, including turbulence in the atmosphere.  As a rule if a magnification produces a unstable image you probably will want to reduce the magnification a bit.

Seeing
Astronomers use a term for the stillness of the atmosphere called “Seeing”. “There is good seeing tonight.” “There is bad seeing tonight.”  “Seeing” is basically a relative measurement of the stillness or turbulence of the atmosphere.  If the stars are twinkling, the seeing is probably not good. When seeing is poor, less magnification is used to steady the view through the telescope.

Optical LImits
A good practical rule-of-thumb is to figure about 50 power per inch of aperture of the telescope for telescopes under 6 inches of aperture. For example, a 4.5 inch (114mm) Newtonian Reflector telescope will theoretically max out around 225 power, sometimes refered to as "225x".  The practical limit is probably around 100-200 power for high power views of the planets and moon, depending on the “seeing” and the quality of the optics.  You can put even higher power eyepieces in (This is how some inexpensive department store scopes advertise 300, 500 even 900 powers.), but the views will be unsatisfactory and useless.

In the above example you could theoretically reach the 225 power or even exceed it a little, if you have good optics and incredibly still atmosphere – a rare occurrence in most places.