The SPA Telescope

Plans now included

These plans show you how to make an astronomical telescope that will outperform many imported commercial telescopes at a fraction of the cost. It'll show you details on major planets, give you spectacular closeups of the Moon's craters and show you stars thousands of times fainter than you can see with the naked eye. And you can even use it as a powerful 500 mm telephoto lens on a single lens reflex camera.

If you decide to progress to a larger telescope, the money spent on this one won't be wasted. You can use it as a guide telescope when taking long-exposure photographs with the larger instrument, or use it for photography in its own right.

We've designed the telescope so that virtually anyone can make it, from easily obtainable materials. In its simplest form, construction should take only a few hours.

How the telescope works

The telescope is based around a 3.5 inch (90 mm) diameter main mirror and a small rectangular secondary mirror. The mirror set is made especially for this telescope by J D Greenwood, and is superior to the optics in many imported instruments.

The optical system is the Newtonian design, invented by Sir Isaac Newton in 1672, and used in a large number of reflecting telescopes, large and small. The main mirror has a concave spherical surface which reflects light from a distant object to a focus 20.5 inches (520 mm) in front of it. You can see this image on a screen such as a bit of paper -- but this is not convenient, because as soon as you try to get close to it you get in the way of the light falling on the mirror, and in any case the image is small.

The first problem is solved by using a small flat mirror -- called the flat or secondary -- to reflect the image to the side of the tube where you can look at it through an eyepiece or even photograph it.

The solution to the second problem, that of the image being small, is to use a magnifier. A simple magnifying glass would do, but it wouldn't enlarge the image much and would give poor results. Instead you need an eyepiece, which has two or more glass lenses to give higher magnification with good image quality. For a range of magnifications you need a range of eyepieces.

The image given by the basic optical system is upside down. You could get it the right way up by using extra lenses, but these cost more, absorb light and may introduce distortions, so astronomers put up with the upside down image and say that it doesn't matter anyway. This, and the design of reflecting telescopes, makes them unsuitable for daytime use, though they will produce rather washed-out images by day.

Rather than look at the image on a screen, it is better to use an eyepiece to look into the telescope. So the basic construction of the telescope has the mirror at the bottom of the tube, looking up; the flat mirror held at 45o at the centre of the tube in the right place; and an eyepiece looking in from the side of the tube so that all it sees is the light from the main mirror. Normally, the eyepiece is moved in and out to find the right focus position, but in this design the whole eyepiece and flat assembly moves up and down the tube.

How much does it magnify?

This is the usual question people ask when you show them your telescope (when they aren't asking how far can you see with it?' which is rather hard to answer). You get used to explaining patiently that it's the eyepiece that provides the magnification.

The main mirror's job is to collect light, and the more light it collects the fainter objects you can see. The distance from the mirror to the image it forms is called the focal length -- in the case of the 3.5 inch mirror this distance is 20.5 inches, or 520 mm.

The eyepiece, too, has a focal length. These days this is usually quoted in millimetres, and is shown on the eyepiece. The focal lengths readily available range from 4 mm up to 40 mm, with those in the middle of the range being the commonest and cheapest.

The magnification given by an eyepiece is simply the focal length of the mirror divided by that of the eyepiece. So a 20 mm eyepiece will give a magnification of 26 times (26 ') on this telescope. A practical limit, with a 4 mm eyepiece, is 130, but it is better to stick to lower powers. The more you magnify the image the fainter it gets. And a small mirror will only show a limited amount of detail, no matter what magnification you use.

Telescope mountings

When you are magnifying a view even as little as ten times, it's vital to keep the telescope as steady as possible. You also need to be able to follow objects slowly and smoothly through the sky as the Earth turns beneath you. At the same time, the telescope should not be too free to move or you'll knock it off target as soon as you put your eye to it.

There are many different types of telescope mounting, which are described in full in books on amateur astronomy. But the one chosen here is of the Dobsonian design, which has become very popular in recent years as it combines cheapness and simplicity with ease of use.

This kind of mounting is fine for simple viewing, but bear in mind that it's not suitable for taking long-exposure photographs of the stars. If you want to do that, you'll need a considerably more advanced equatorial mounting and a very smooth gearing. Consult the Book List for sources of information.

Construction outline

To keep these notes to a reasonable length we have had to assume that you have some skills in cutting and measuring. If not, seek help.

Materials

Use the list here to check off what to get. We've chosen 4-inch PVC drainpipe for the tube because it's easy to get -- in fact, you will probably have to buy more than you need. It comes in different lengths, depending on where you buy it -- sometimes 2 metres, sometimes 10 feet -- so shop around. You need about 3 feet, which you might be able to get as an offcut, but even the full lengths don't cost more than a few pounds.

Don't get very thin pipe -- it should be at least 1/8 inch (3 mm) thick. And 4 inches (100 mm) is the minimum inside diameter tube needed for a 3.5 inch mirror -- any narrower and you'll restrict its field of view.

It isn't essential that you use Melamine-faced chipboard for the cradle -- any similar sized wood will do. You could use wider material for added stability. The Melamine does, however, provide a smooth surface of reasonable friction qualities for the lower bearing, and it provides a ready-made finish. If you buy a 6 ft or 1.8 metre length you'll have a couple of feet left over.

The studding -- which is screw thread without a head -- is available from hardware shops. Brass is dearer than steel but won't rust.

The only other materials needed are a few woodscrews, glue, nuts and bolts and scraps of wood for the mirror mounting -- the sort of things you can scrounge from friends rather than buy if you haven't got them yourself.

Tools

You could use a hand drill but a power drill is much easier for use with a hole saw, which is a ring- shaped saw-blade attachment. A hole saw isn't essential, but you can buy one, suitable for wood and plastic, on market stalls for a few pounds, and they are very useful for making other astronomical bits and pieces.

Measurements

We give these mostly in mm, but the nearest Imperial equivalents will do. Some dimensions are quoted in feet or inches where these are more appropriate or familiar.

Buying the optics

These telescopes are designed around mirrors made specially for the SPA. The prices for these optics have been kept low for SPA members only. If you would like the printed plans for the SPA telescope, these are also only available to SPA members, by sending an A4 or A5 SAE to...

Chris Lord
Brayebrook Observatory
30 Harlton Road,
Little Eversden
Cambridge,
CB3 7HB.

Email: 106275.3222@compuserve.com

Notes
In addition to the mirror, you will need at least one eyepiece of 24.5 mm diameter fitting. These are sold by a number of suppliers. In practice, more than one eyepiece is desirable. An eyepiece with a focal length of about 20 mm will give good low-power views, while one of 7--10 mm will provide a useful higher magnification. If funds are short, your first eyepiece should be around 20 mm as this will enable you to find a wide variety of objects with no difficulty.

Note that the Huygenian eyepieces sold with small refracting telescopes are not very suitable for use with this telescope, as their optical design is intended for longer focal ratios, and nor are microscope eyepieces. But the eyepieces from binoculars will usually work very well for low powers.

Bear in mind that an eyepice which gives a magnification of 10 when used with binocular objectives will give a higher power when used with this mirror, which has a longer focal length. It would be worth buying cheap secondhand binoculars from jumble or car-boot sales for the eyepieces alone. It does not matter if one half of the binoculars is damaged.

Checklist

Materials
Tools

Plans

There are a number of diagrams, The plans have been split into separate pages so that they can be printed on A4 sheets, I suggest you do this by printing from your browser.

Taking photographs

Your telescope will perform well as a telephoto lens of 520 mm focal length at f/5.8. You can use it with any single lens reflex camera, from which you can remove the lens.

Rather than making or buying adapters, simply clamp the open breech of the camera looking into the open eyepiece hole, without the eyepiece. You will have to devise your own means of holding it firmly -- and, of course, it will seriously affect the balance position. Focus as you would with an eyepiece, by moving the whole plinth and camera up and down the tube. You will need to be much closer to tbe mirror than usual, which is one reason why the focusing system has been devised in this way. It's tricky, but possible!

If you don't have an SLR camera you may get acceptable results by simply pointing your camera lens, focused on infinity, through the eyepiece.

Care of the mirrors

Telescope mirrors are coated with a thin layer of aluminium on their front surfaces. There is an overcoating to prevent the aluminium from oxidizing rapidly, but this is still delicate. So NEVER touch the surfaces with your fingers. It's best to keep them covered whenever you're not using the telescope, since brushing dust off can cause fine scratches that degrade the image.

Book list

Please note that book availability and prices change rapidly, and some of the books listed here may be out of print or at a changed price.

General Astronomy
Star Maps
Photography
Telescope Making
Many of the above books are available through the SPA: see The SPA Book Scheme for more information.

The Society for Popular Astronomy

If you want to get the most out of using your telescope and aren't already a member, join the Society for Popular Astronomy. Its members include beginners of all ages, so you won't be out of your depth. The annual subscription of £12.00 includes four issues of Popular Astronomy, the most readable astronomy magazine, plus regular News Circulars.

To subscribe, send £12.00 to the Enrolment Secretary, Dept TL, 1 Milton Road, Addlestone, Weybridge, Surrey KT15 1JB. For more details of the SPA, write to 36 Fairway, Keyworth, Nottingham NG12 5DU.

What to observe

This telescope will show considerable detail on the Moon, features on Jupiter, the rings of Saturn and markings on Mars when it's close. You should see the phases of Venus and Mercury, and maybe even the disc of the remote planet Uranus.

You'll see clusters of stars and nebulae invisible to the naked eye -- in fact, under good conditions you should be able to see galaxies millions of light years away.

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 Maintained by Michael Oates Email: mike@ph.u-net.com
Last modified 16th Jan 1997