INTRODUCTION TO ASTRONOMY

This article featured in the February 2003 Beginners Magazine

MORE ABOUT TELESCOPES

In the previous articles in this series we considered the first step for the astronomy beginner to take, this was to buy a book with sky maps and become familiar with the night sky. The second was a look at binoculars as the first instrument needed to improve the view of the night sky. In the third article the two main type of telescopes were explained these are the refractor and reflector.

In this article we look in more detail at the telescope and how it works. In future articles advice will be given on how to use an astronomical telescope and advice on buying your first telescope.

CREATING THE IMAGE

The purpose of the main optical element of a telescope (primary mirror or lens) is to gather light from an object and concentrate the light into an image. This image is a fixed size but the size depends on the focal length of the optics, the longer the focal length the larger the image will be. The focal length is the distance from the mirror or lens to where the image is formed. A 150mm (6 inch) telescope with a focal length of 1000mm (39 inches) may create an image, of the full Moon, 10mm in diameter. A 150mm telescope with a 2000mm focal length may produce an image, of the full Moon, 15mm in diameter.

CALCULATING THE MAGNIFICATION

The magnification of a telescope is determined by the focal length of the eyepiece being used and takes into account the focal length of the main optics. The magnification is calculated by dividing the focal length of the main optics by the focal length of the eyepiece being used. For example a 25mm eyepiece used on a 1000mm focal length telescope will give a magnification of 1000 divided by 25 = 40. A 10mm eyepiece used on the same telescope will produce a magnification of 1000 divided by 10 = 100. The two eyepieces in the example above used on a 2000mm focal length telescope will give 2000 divided by 25 = 80 and 2000 divided by 10 = 200. A long focal length eyepiece will produce a low magnification but have a wide field of view whereas a short focus eyepiece will have a higher magnification and a narrower field of view.

THE BARLOW LENS

A special lens called a 'Barlow' can be inserted into the light path before the eyepiece to double or in some cases treble the magnification of the eyepiece being used. A 2x Barlow used with the 1000mm telescope and the 25mm and 10mm eyepieces will give additional magnification of 80x and 200x.

FOCUSING THE TELESCOPE

All people have differences in their eyesight and various makes of eyepieces have a difference in their focusing position. This means that the focusing of the eyepiece has to be adjustable to suit the person using the telescope and the eyepiece being used. Telescopes are therefore fitted with a Focuser to adjust the position of the eyepiece. The focuser usually has a knob which when turned moves a tube in or out of the focuser housing. The eyepiece is mounted into the tube and normally clamped using a thumbscrew. The Barlow Lens mentioned above is mounted in a housing which can be fitted into the focuser and an eyepiece then fitted into the Barlow mounting.

THE FINDER

Telescopes usually have a quite long focal length and therefore have a narrow field of view. That means they can only view a small area of sky even with a low power eyepiece fitted. With only a small area of sky visible through the telescope it can be very difficult to find and object to be observed. Most telescopes have a smaller low powered telescope called a Finder mounted on the tube to help find the object. A finder has a view similar to that seen through a pair of 10 x 50 binoculars. The finder is used to locate the object to be viewed and may have cross hairs to enable the object to be centralised in the finder. Once the object has been centralised in the finder it should be somewhere in view in the eyepiece of the main telescope. It will of course be necessary to align the two telescopes so they are looking in the same place. This can be done, by finding a bright object such as the Moon, a bright planet or star that will be quite easy to find in the main telescope, using a low power eyepiece. The finder is usually mounted in a housing, fitted with two sets of three adjusting screws. By adjusting the screws the finder can be aligned to the object located in the main telescope and then locked in position.

TELESCOPE MOUNTINGS

Telescopes are usually supported in one of two kinds of mounting, these are an Altazimuth or an Equatorial. An altazimuth is the type of mounting that a seaside telescope would be mounted on. It will usually have a fork mounting with a bearing on each side of the tube to allow the telescope to be aimed up and down rather like the trunnions on an old cannon. The fork mounting will in turn be supported on a turning bearing which will enable the telescope and fork mount to be swivelled to the left or right. The Dobsonian design is the most popular type of altazimuth mounting used on astronomical telescopes. This design is in the form of a wooden box section mounted in a turntable with trunnion bearings fitted to the top of the box section. This provides a simple, cheap but very stable mounting.

THE AUTHOR's 14 INCH DOBSONIAN TELESCOPE

The disadvantage of the altazimuth mounting is that it is not suitable for tracking stars and other objects across the sky using motorised drives. Because the axis of Earth is tilted at about 26º to the plane of the Solar System, objects appear to move in an arc across the sky as the Earth rotates. To compensate, the Equatorial mounting has its rotational bearing tilted to the same angle as the tilt of Earth's axis. The advantage of this design is the mounting has only to be driven in one axis to track a star across the sky. It is especially suited to astrophotography where tracking for long exposures is required.

LATEST TECHNOLOGY IN TELESCOPES

In recent years the cost of telescopes has dropped enormously. The technology of optics and the incorporation of computer control systems has greatly extended the range of telescopes available to the amateur astronomer.

There is now a huge market for the Schmidt Cassegrain design of telescope. This type of telescope combines, through very cleaver optics, a long focal length with a short compact tube. Some of these telescope are supplied with computer guidance systems containing a database of thousands of stars and deep space objects. The very latest instruments of this type are fitted with a GPS satellite location system that can determine the position of the telescope anywhere in the world to within a few metres. The GPS also gives the telescope the exact time so in combination with its internal compass, the telescope can align itself on almost any object the astronomer asks it to find.

In the next article we will consider buying a telescope and look at some of the options open to the beginner to astronomy .

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