Telescope Basics

© 1995 by Robert C. Moler

This is the time of year, with less than 60 shopping days left till Christmas, that some amateur astronomers young and old are putting a telescope on top of Santa's list. I personally don't agree that a telescope should be a gift, unless Santa really knows exactly what the recipient wants. This assumes the recipient really knows what he or she wants. So, let's talk about telescopes.

Everyone knows that a telescope makes distant objects appear nearer. This quality is known as magnification. Ten power will make an object 10 times bigger than when seen with the unaided eye, or appear ten times closer. Except for telescopes without removable eyepieces, this is not a fixed value. A telescope has two or more optical elements, which may be lenses or mirrors. The element optically nearest viewed object is the objective. The element nearest the eye is the eyepiece.

The objective's job is to gather light from the object and bring it to focus as some point from it. The distance from the objective to the image is called its focal length. Telescopes with objective lenses are called refractors, because the light is refracted or bent by the lens. Telescopes that use a concave mirror as an objective are called reflectors. A telescope is rated by it's objective diameter, also called aperture.

The size of the objective is very important for two reasons. A larger size allows more light into the image, making it brighter. Thus, telescopes are used to see objects much dimmer than can be seen with the unaided eye. Also the resolution or image sharpness increases with objective diameter. Resolution isn't magnifying power, but governs the amount of power that may profitably be used. As a rule of thumb there is no gain in seeing fine detail by using a magnification more than 60 power per inch of aperture. That's the general formula, I notice image degradation at 30 power per inch. That means a telescope, that is advertised in bargain catalogs with a 60mm (2.4") aperture can only support decently sharp images up to 144 power. These telescopes are usually touted as having up to 400 power. The power may be there, but the image quality is horrible at 400 power. Besides the image will be very dim and shaky with the poor quality mounts these telescopes have.

Luckily, hardly anyone has need of that much power. I prefer a smaller sharp image to a big fuzzy one anyway. Most astronomical objects are dim, so the bigger the aperture the better in seeing these dim objects. In fact most telescopic observing is done with the lowest available magnification. High power is needed for viewing the details of planets and for splitting double stars. For most everything else low power (50 to 100) is fine.

The magnification of a telescope is calculated by dividing the focal length of the objective by that of the eyepiece. So a 900 mm objective with a 12 mm eyepiece gives 75 power. The longer the focal length of the eyepiece, the lower the power. To make things more complex, there are a wide range of eyepiece designs from cheap to expensive. In this short article, all I can say is that you get what you pay for.

A vastly overlooked part of a telescope is its mounting. A mounting must be sturdy, not flimsy. The telescope must stay where it's pointed. There is nothing so frustrating than trying to aim a telescope on a flimsy mount. It is especially frustrating for a beginner.

There are two basic mountings: alt-azimuth and equatorial. The alt-azimuth mount is generally sturdiest of the two, and the easiest to point. Its axes are altitude, or up and down, and azimuth or horizontal. The equatorial mount must be adjusted for one's latitude and the polar axis set parallel to the earth's axis to obtain its benefits. The other axis is called the declination axis. Properly set up, an equatorial mount will follow an object in the sky by motion of the telescope about only one axis. That axis, the polar axis can be fitted with a clock drive motor to track any celestial body, excluding artificial satellites and meteors.

The simplest equatorial mount is called a German mount, in which the axes cross in a T, with the telescope balanced on the declination axis by counterweights. Pointing and tracking accuracy is affected by how well the telescope is balanced. The German mounted telescope takes a bit of time to learn how to point. It's surprisingly difficult at first. A more intuitive equatorial mount is the fork mount. The telescope is suspended between the tines of a fork on its declination axis. The axis of the fork is the polar axis. This type of mount is used for physically short telescopes, like reflectors.

Refractors are pretty straight forward, optically with the objective at one end of the tube and the eyepiece at the other end. Reflectors come in a wide variety of optical configurations.

The simplest reflector is the Newtonian named after that gravity fellow who invented it. It is a large open tube with the primary mirror at the bottom, and a small flat diagonal mirror near the top to reflect the light out the side to an eyepiece. These can be very inexpensive when combined with a Dobsonian mount.

The Cassigrainian comes in several forms itself, but has a small convex secondary mirror which reflects the light through a hole in the primary mirror, so the telescope looks like a fat stubby refractor. The kind made by Celestron and Meade are of the Schmidt variant and are rather reasonable in price. Questars are of the Maksutov variant, and very expensive.

Space here doesn't allow for further discussion of terms like Schmidt, and Dobsonian. Check out an astronomy magazine or a telescope book from the library.

Let's look a bit more about basic differences between reflectors and refractors. I'll list them, and let you sort them out.

Happy telescope hunting.


Questions? Send Email to me at bob@bjmoler.org

Uploaded: 07/14/96