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!