Testing your telescope - Part 3

The previous test involved a bright star. This next test uses the same bright star, but is much more difficult to carry out. If you have a small refractor on a fiddly mounting it may defeat you at first, but keep trying! Knife edge test This is a very similar test to the one which amateur telescope makers use when making mirrors - the Foucault test. In this case, though, you need no elaborate equipment, just a simple knife edge. If doesn't have to be very sharp (indeed, that would be dangerous, particularly in the dark); all that is necessary is that it be straight, with a well defined edge. A small table knife is perfectly adequate. Keep tracking your bright star, but now remove the eyepiece and look into the eyepiece hole. Put your eye very close up and you should see the main mirror or lens filled with light from the star. If you are too far away you will simply see a shaving mirror type image of the star, so you will need to rack the eyepiece tube in somewhat. Having done this, bring your knife edge across the front of your eye, fairly close to it (now you know why it shouldn't be too sharp!). You will see a shadow move across the mirror or lens as it cuts off the light from the star. If you are finding this too difficult, on account of the star moving out of the field of view all the time, practice on a distant streetlight. But the streetlight is not a point source, so it won't be good enough for a proper test, and you must go back to the star once you have practiced cutting off the image. If the knife edge shadow appears to move in the same direction as the knife edge itself, then you are holding it at a point inside the focal point of the telescope. But if the knife edge is outside the focal point, its shadow will actually appear to move in the opposite direction. You must establish where the focus point is by chopping the beam in different places so as to get the shadow moving in one direction, then the other. The first diagram shows why this happens. The light from the star comes to a focus, forming an image, which you normally look at through the eyepiece to magnify it. In order to simplify the diagram, I have just put a straight line for the objective - it could be either a mirror of a lens. If it is a mirror, then of course you will have a secondary mirror as well, but I have left this out for the sake of simplicity. If you chop the beam near to the mirror at point (a), then obviously the shadow will seem to move in the same direction as the knife edge. But if you chop the beam beyond the focus point, it will first cut the rays coming from the opposite side of the mirror, as shown at (b). When you chop the beam at the focus point itself, the shadow should not come from any direction - it should cover the mirror instantly, as it is slicing into the point image of the star and not spread out beam. This is the crucial point for your test - when the knife edge is precisely on the image of the star. If the optics are perfect, practically all the light from the star will be focused at that point, so all the light you can see is cut off. But if there is an imperfection, resulting in some of the light being diverted to one side of the point of light, the knife edge may not cut off all the light, and you will see a bright area remaining after the rest of the light has gone. Alternatively, that bit might be cut off first by the knife edge, and will go dark before the rest. So with the knife edge just at that critical point you get a view of all the hills and valleys on the mirror that shouldn't be there - deviations, that is, from the perfect paraboloidal figure. The second diagram shows how a shadow zone can be caused by a defect in a mirror throwing some of the light to one side of the image instead of into the focal point. This sort of thing can be caused by the mirror being left too long on a polishing machine. The effect has been exaggerated in the diagram. In carrying out this test, you may discover three apparent strain marks evenly spaced around the rim of the mirror. This is almost certainly due to the mirror being gripped too tightly by its clips. A mirror should not be held tightly, but should just be able to move very slightly. Another effect you may see is turbulence moving across the objective. This is very local, and could be caused by, for example, a chimney pot just below the line of sight to the star. Or it could be the result of your own body heat, or the telescope itself not being at air temperature. Test the effect by putting your hand into the line of sight - you'll probably see the heat rising from it. This setup is the basis of the schlieren system used in wind tunnels to reveal air density variations. If you can see turbulence, you must wait until it has subsided before you can test the optics. To analyze the appearance of the mirror further, you'll have to consult a book on mirror making, for the test is very similar to the Foucault test. But in that test you use a pinhole close to the mirror, and aim to produce a certain appearance of shadows on the mirror, whereas when using a star for a test object, you want to see no shadows at all. If you have a mirror which shows a bright rim on once edge using this test, it has what is called a 'turned down edge' which may reduce the contrast of the image. The best way to deal with this, if it is not serious, is simply to mask off the offending bits of the mirror. If you suspect that the secondary mirror of a reflector is at fault, the thing to do is to allow the star image to drift so that you are looking at the main mirror through different bits of the secondary. This again calls for care and practice, so you know what you are looking at. Having done all this, you should now have a good idea of the performance of the telescope. not only will you have tested the optics - but in trying to keep the star in the centre of the field all the time, you'll know just how steady it is mechanically as well! Contribution by Robin Scagell.

Testing your telescope - Part 2

Our previous article described simple checks you can make, including viewing the bright Moon, to show that your telescope's optics are in order. This next test involves pointing at a star. Bright star test Find a good bright star reasonably high up. A first magnitude star is ideal, particularly a white one such as Vega. Train your telescope on it and focus it as carefully as you can: you should be able to get it to a single very tiny disc of light (assuming that you haven't chosen a double star!). Use a moderate magnification, of the same order as the aperture in millimetres (that is, 150 for a 15 cm - 6 inch - instrument). You may have read that a good telescope will show what are called diffraction rings around star images. This is not always the case: it depends on the aperture and the magnification as well as the seeing. A small telescope (say 50mm) will show these delicate rings surrounding star images quite readily if it is any good. However, a larger instrument may not show them unless you see a fairly high magnification and look at a fairly faint star. But if the seeing is bad all you will see is an animated blur, even if the telescope is perfect. So diffraction rings are not a good guide to performance. Once you have found the best focus point, move the eyepiece slightly outwards and inwards, equal amounts on either side of the focus point, and compare the defocused star images. What should happen is that the point of light opens out to a disc, getting larger and fainter as you go further from the focus point. This disc should be evenly illuminated and perfectly circular. In the case of a reflector which has a secondary mirror, you will see the image of this as a shadow in the centre of the disc. The defocused disc should be identical on either side of the focus point. If it isn't there is something wrong. You may find, for instance, that on one side of the focus the disc has a bright edge, which indicates a fault in the mirror or lens. Small deviations from the ideal should not affect the images too much, however. A very common fault which the bright star test will reveal is astigmatism - where the optics have a slightly different focal length in one direction. You will find in this case that the star image is never perfectly round, and that as you move very slightly inside focus it elongates into a short line. The other side of the focus, it turns into a line at right angles to this one instead. If you detect this fault, it's worth checking that the eyepiece isn't the cause. Rotate the eyepiece but not the telescope: if the short line moves as you turn the eyepiece, then it is at fault. Your eyes, too, can produce the same effect and you may have to try rotating your head to check this (it's not easy!). But if the effect is not in the eyepiece or your eye, then the problem lies with the rest of the optics. Incidentally, as you move outside focus you are actually focusing on comparatively nearby points in space - the atmosphere a few hundred metres away from you. You may well see streams of turbulence against the defocused disc, indicating that this may be the cause of the bad seeing. The next test will use the same bright star, but is much more difficult to carry out. Contribution by Robin Scagell.

Testing your telescope - Part 1

Stargazing guides often recommend that you get an expert to check a telescope before you buy it. This may be excellent advice but it can be difficult to do in practice. If you are just starting out, you may not know anyone else interested in astronomy. And even if you do, the chances are that they know little more about telescopes than you do! Then of course there's the biggest problem of all - how do you get your expert to the telescope on a clear night before you've bought it? So here is another way: a series of simple checks which you can carry out for yourself on a clear night, and which should at least tell you how good your instrument is, once you've bought it, plus where any faults lie. No complex test equipment is needed, just plenty of time, including allowing the telescope to settle down to night-time temperatures. Don't take it straight out of a warm room. In the shop Unfortunately, there is no quick way to test an astronomical telescope during the day in the shop. All you can do is to look through it, but there are so many other factors which can affect the image that this tells you little. It may seem poor, yet be ideal for astronomy, or it may seem to give a bright, sharp image by day and dismal images at night. Only if you can see strong, false colour around the edges of objects silhouetted against a bright sky (such as TV aerials or chimney pots) can you be sure that the fault will be worse on astronomical objects. A small amount of false colour is inevitable anyway in all but the most expensive refracting telescopes. A lunar check-up The Moon is an excellent object to begin your checks with, especially when it is high up and bright. The bright lunar limb should appear perfectly sharp and free from false colour under low powers. If you can detect ghost images of the limb, or cannot get it sharp, then there is something definitely wrong. Increased magnification will start to show up any faults, but you can easily be misled by atmospheric turbulence - seeing - which has the effect of blurring the image and sometimes even producing its own double images as air cells act as lenses in their own right. If your instrument fails the Moon test, you will want to find out why. If it passes, then you will still want to test it further. So in the next guide, we will show you how to test on a bright star. Contribution by Robin Scagell.