How to collimate a telescope

First you need to be sure that your telescope needs collimation by testing it. Then you have to adjust the mirrors in the correct order, working from the eyepiece end downwards. What is collimation? Collimation means accurately aligning the optical components of the telescope to give the best image possible. This is usually only needed with reflecting telescopes, such as Newtonians and catadioptrics. Refractors do need to be collimated, but they have simpler designs and the collimation usually stays fixed once they are made. Because the mirrors of reflectors need to be recoated from time to time, it’s usually necessary to provide them with adjustments so they can be brought back into line after recoating. There is no reason why a well-made reflector should not stay properly aligned for a long time, but unfortunately many commercial instruments are out of collimation even when new, and may need to be collimated to give good images. People are often scared that they might make things worse, and are reluctant to meddle, but as even Celestron’s manual points out, the rewards of correcting bad collimation are well worth the effort. And judging by some of the questions I have been sent, collimation is easier to do than to spell! It’s colli-mation, not coll-mination. Is it necessary? Testing your telescope The easiest way to test your telescope for collimation and other defects is to observe a bright star. Do this outside, not through a window, and allow the telescope to cool down to night-time temperature or thermal effects will undoubtedly affect the image quality. With an eyepiece in place, and the star central within the field of view, slowly defocus the star image. As it goes out of focus it will get larger until you see a disc of light, probably containing brighter zones. If you are using a reflector, you will see a circular shadow within the bright disc. This is the shadow of the secondary mirror. In a well-collimated telescope, this should be dead central within the disc (but only when the star itself is centred in the field of view, remember). In extreme cases, the defocused disc is some kind of weird crescent shape, in which case the telescope is badly misaligned. If the circular shadow is not dead central, you will need to collimate the telescope. This is possible while looking at the star, but doing this makes a difficult job much more tricky because as soon as you start to adjust the mirrors, the star moves away from the centre of the field of view. So it is best to carry out the task during daylight, or in a brightly lit area. While you are looking at the defocused star, however, you can check other things. Is the outline of the bright disc completely even and circular? A common fault is to find that there are permanent blobs or streaks at the edge, often arranged at 120º to each other. This is a sign that the mirror is too tight in its cell. It should be held just tightly enough that it won’t move, without putting any strain on it. You may also see other shadows. The legs of the spider assembly will also be visible, and there is not much you can do about these as they are part of the structure of the telescope. There may be a rectangular shadow jutting into the edge of the field of view. This is probably the focusing tube of the eyepiece, and is a sign that the telescope has not been well designed. For every departure from a plain, perfect disc of light, you are losing performance. Check the shape of the disc on either side of focus. If it shows a radically different structure on either side, the mirror is probably badly made. If, as you go through the focus position the disc is slightly elongated in one direction, then the other, there is a fault known as astigmatism somewhere in the system. Neither of these faults can be corrected. Many telescopes I have seen show slight astigmatism, and as long as you can get a good star image it is not too serious. You may find that it is worse with low-power eyepieces than at higher powers. Collimation In a nutshell, to collimate your telescope you start from the eyepiece end and work your way down to the main mirror. You first adjust the secondary mirror so that you can see the main mirror centrally within it, then adjust the main mirror so that you can see the top of the tube and the secondary centrally within that. What follows is this procedure in detail, for the faint-hearted. Step 1 Before you start to collimate your telescope, you need one essential accessory – a dummy eyepiece of sorts, with a central hole. This could be nothing more than a piece of card with a hole punched in it, stuck over the eyepiece draw tube with the hole dead centre, or you could make something a bit more elaborate if you want. 35mm film cannisters are excellent raw materials for this. The size of the central hole is not critical. Its purpose is just to make sure you look right down the centre of the drawtube, so about 5 to 7 mm is fine. Step 2 In conditions bright enough that you can see inside the telescope tube, look through this hole and work out what you can see. When collimating, the key thing is to adjust everything from the outside inwards. The outside edge of your view is the end of the draw tube, and beyond that is the secondary mirror. To start with, ignore anything that the secondary reflects. Its outline should be centrally placed within the outline of the eyepiece tube. If it is not, serious adjustment of the secondary position is called for. In the case of catadioptrics, it is not usually possible to adjust the position of the secondary within the tube, as this is fixed in place. In my experience, even in a Newtonian the secondary is not often grossly misaligned, except perhaps in home-built telescopes, so I will not dwell on the possibility now. Get the hang of the other adjustments first. Step 3 Now look at the reflection in the secondary. You should see the outline of the main mirror somewhere within the outline of the secondary. Ideally it should be central and symmetrical, but if not you will have to realign the secondary. This is possible in both Newtonians and catadioptrics. There should be a central screw or rod that moves it in and out, and there will probably be three screws for adjusting its alignment. On a cat these screws may be hidden by a blanking plate. You may need to slacken off the central screw slightly, which also holds the mirror in position, before you can adjust the outer three. Your aim is to be able to see the outer main mirror dead centre within the outline of the secondary. Make tiny adjustments so you can see which way each screw moves the mirror, and keep adjusting the other two screws so as to keep the mirror fairly well held. If you loosen one, tighten the other two, and vice versa – you don’t want it so loose that it can slop around and ruin your adjustment. Step 4 Once the secondary is properly adjusted you can now look even more centrally, still through your dummy eyepiece. What can you see reflected in the main mirror? Ideally, it should be the top end of the tube, with the secondary central within that. If you can see more of one end of one side of the tube than the other, with the secondary also off-centre, you now need to turn your attention to the three adjusters at the back of the main mirror. On catadioptrics there are no such adjusters available to you, and adjusting the secondary is as far as you can go. On most Newtonians you will find three pairs of screws. One of each pair is a locking screw, while the other is the adjuster. Slightly loosen the locking screws, and by trial and error (made much easier by a helper doing the adjustment while you look through the dummy eyepiece) adjust the other screws to get the secondary central. Again, take up the slack with the other two screws as you adjust one. Eventually you should get everything in line and you will be able to see your own eye looking up at you from the dead centre of the main mirror. Tighten the locking screws, but not to battleship standards. You don’t want to break the casting of the mirror cell. Now everything should be lined up and tight. Check by swinging the telescope around a bit and make sure the alignment is good in all tube positions. If not, one of the mirrors could be loose in its cell which is another matter. In most cases, however, your telescope should now be properly collimated. These instructions are rather long, to allow for most possibilities, but in practice the job is quite simple and should take only a matter of minutes with a bit of practice. The shorter the focal ratio (eg f/4), the harder it is, but the more essential. Tip You may find it hard to judge the centre of the main mirror when looking through the eyepiece. If you can get to the mirror there is a simple trick that might help. Get a small blob of Blu-tack, and by measurement with a ruler stick it dead centre. This sounds drastic, and you might worry about it affecting telescope performance, but the central part of the mirror should always be in the shadow of the secondary anyway and never contributes to the image. An alternative is to stretch two pieces of thread from one side of the tube to the other, close to the main mirror, so as to define the centre of the tube where they cross. Contribution by Robin Scagell

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.