Astronomical Telescopes

Astronomy Kits for GCSE

Educational

Our popular K. 1 ASTRONOMY KIT FOR GCSE COURSES is intended for those people who are studying Astronomy as a GCSE course, in which they are required to build a telescope as part of the coursework.

The kit comes complete with pages of useful information about the construction of the telescope and how to measure magnification, true field of view and apparent field of view.

The kit contains a 50mm diameter achromatic objective lens with a focal length of 13". It is brand new and is anti-reflection coated. To build the eyepiece for your telescope the kit contains a pair of plano-convex singlet lenses, suitable for making a Ramsden eyepiece.

The photograph is only for the purposes of illustration - the exact nature of the lenses in the kit may vary from time to time, but all of them will build-up into a super little telescope.

Please note that this kit is also suitable for building a finder telescope for mounting on a larger astronomical telescope.

Using the small telescope kit in your GCSE project

This information is intended for the GCSE project audience. A more comprehensive guide to making telescopes can be found in our Making Telescopes section.

Getting started

It is a good idea to make the eyepiece first. The two planoconvex lenses need to be put with their curved faces towards each other, and spaced about 25mm apart. You first need to make a short tube to act as a spacer, to keep the lenses the correct distance apart. So, make a cardboard tube 25mm long, with the same outside diameter as the lenses. The inside of this tube needs to be lined with a matt black surface, such as black cartridge paper or painted matt black.

Now make two shorter versions of this spacer say, about ¼” (6mm) long and the same diameter. These will be the retaining rings that will stop the lenses from falling out of the eyepiece.

Once this is done you will have five components for your Ramsden eyepiece - a spacer, two retaining rings and two lenses.

You will need to make another tube, with an inside diameter the same as the outside diameter of the two lenses and the three spacer rings. This tube is to hold the five eyepiece components – the two lenses and their spacer, with one retaining ring at the front and the other retaining ring at the rear. The complete assembly will be the eyepiece for your telescope. This type of eyepiece is known as a RAMSDEN eyepiece.

Once the eyepiece is made you can investigate how far it needs to be placed from the objective lens to give a sharp image. That distance decides the length of your telescope.

The total length of the telescope when it is in focus will be the focal length of the objective lens PLUS the thickness of the assembled eyepiece - about 25mm in this case.

The big lens at the front of the telescope (the OBJECTIVE lens) needs to be fitted into the telescope with its most curved surface facing outwards. The less curved surface needs to be inside the telescope, facing the eyepiece.

Safety is an important consideration in any project that involves making things. Please read, mark, learn and inwardly digest the following safety panel.

SAFETY FIRST: Do not observe the Sun with the telescope that you have made, nor leave it around so that others might do so. Permanent damage to the eyes would result.

Children or school pupils must never be left to work with this kit unless there is a qualified adult in attendance, to ensure that no-one attempts to observe the Sun using lenses.

Glass lenses may have sharp edges, so there is a danger of cuts.

Glass lenses are fragile and may shatter if handled roughly, giving rise to the danger of cuts from the sharp fragments.

How to find the true field of view of your telescope.

To find the TRUE field of view, you need an Ordnance Survey map of your area. Look through the telescope and find two prominent objects that are at the extreme edges of the field of view. Locate these on the map and measure the angle they subtend at your vantage point.

How to find the apparent field of view of the eyepiece

To find the APPARENT field of view (the angle through which your eye needs to move in order to view the entire scene through the eyepiece) just multiply the true field of view by the magnification of the telescope. Expensive eyepieces have apparent fields of view from 55 degrees to 80 degrees.

How to find the focal length of a lens

During the daytime, stand indoors opposite a window. Hold the lens against the wall opposite the window. Have the less curved face of the lens towards the wall, and the more curved face towards the distant scene.

Move the lens away from the wall until it produces, on the wall, a sharply focussed image of the outside world (NOT a sharply focussed image of the window frame!).

Measure the distance from the lens to the wall. That distance is the focal length (approximately) of the lens.

Important point:
An important point to remember about objective lenses is that it does matter which way the light passes through them – they are not reversible. When you examine an objective lens closely you will notice that one of the faces of the lens is more highly curved than is the other. The most highly curved surface is the front surface of the lens and it should face towards the distant object that is being viewed. The less curved surface is the rear surface of the lens and that surface should be inside the telescope i.e. facing towards the eyepiece.

How to find the magnification of a telescope

The magnification of the telescope depends upon the focal lengths of both the objective lens and the eyepiece. The magnification is calculated by DIVIDING the focal length of the objective lens by the focal length of the eyepiece.

Magnification = Focal length of objective lens divided by Focal length of eyepiece.

As an example, we can imagine a telescope in which the focal length of the objective lens is 800mm, while the focal length of the eyepiece is 25mm.

Magnification = 800mm divided by 25mm = 32
So such a telescope would produce an overall magnification of x32.

Another method:
Sometimes you need a quick and handy way of measuring the magnification of a telescope that does not involve taking it to pieces. This is especially true if you are in a shop, buying a pair of binoculars.

Here is what you do:
(1) Measure the diameter of the objective lens.
(2) Hold the telescope or binoculars a few centimeters from your face, with the instrument pointed towards the bright sky (NOT the SUN!). Look at the eyepiece(s) and measure the diameter of the bright circle of light that you see in the centre of the eyepiece(s).
(3) Divide the diameter of the objective lens by the diameter of the circle of light in the eyepiece. The answer will be the magnification of the instrument.

Example:
The objective lens of a certain pair of binoculars has a diameter of 50mm, while the circle of light seen in the eyepieces when the binoculars are held away from the eyes is about 7mm

The magnification produced by the pair of binoculars will be 50mm divided by 7mm, which is approximately 7. So this pair of binoculars will give a magnification of x7.

Easy, huh? There's more information and diagrams in our Making Telescopes section.

Do feel free to use any information that has been given here or anywhere else on this website. All we ask is that you mention the source of your information.

If you need any further advice please ‘phone – we are glad to help.

Enjoy your lenses.

Links

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British Astronomical
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