Installation:

These replacement hand-adjustable collimation screws are quite easy to fit but for best performance please follow these instructions:

It is important that the collimation adjustment screws be removed and replaced one at a time - never attempt to remove all three screws at once otherwise the secondary may fall out of it's holder causing damage to the telescope.

Unscrew and remove the first screw, use this to check that the threaded length of the replacement screws is similar to the existing screws (they won't be exactly the same, but should be within 1/16"). Screws for f/10 and f/6.3 scopes are of different lengths, those for the f/6.3 being much longer (about 3"). Make sure you have the correct ones for your scope before proceeding. Place the small brass seating washer over the screw if not already there, and screw it all the way into the hole from which the original was removed. Screw it in all the way until resistance is felt but don't screw it up too tightly. Repeat for each of the other two screws. With the new screws now supporting the secondary it's necessary to provide some latitude for adjustment to allow the optics to be collimated, so unscrew each of the three screws one full turn each (no more) from their fully 'in' position. Remember, that means when re-adjusting the collimation later you must NOT try to turn any of the screws in further than one full turn. The red dot on the face of the replacement screws will provide an indication of how far the screw can be turned.

Re-Collimating the Optics Using the Star Test:

Preparation:

Re-collimating the optics is a procedure that all SCT owners needs to feel comfortable with, you'll get the best out of your telescope only if you are able to make the necessary adjustments yourself. The task is not difficult once you know what to do, and neither is there need to buy any special equipment. The confidence gained by being able to perform the job yourself should encourage you to check collimation more often - every time you use the telescope in fact - and that way you will always get the best performance of which the telescope is capable. If you have a situation where you are 'frightened' of making these adjustments in case you make the collimation worse then you definitely need to practice! Believe me, it gets easier after the first few attempts. The following instructions describe the steps in practical rather than technical terms, my intention is to appeal to the more inexperienced owner.

Replacing the screws will certainly have caused the collimation to be grossly disturbed and so it will need to be re-set before you can use the telescope again. Whilst a laser device is handy for quickly setting the optics to be approximately collimated I much prefer to achieve final collimation using the star test. You should be familiar with the technique of using the star test even if you intend using one of the modern holographic laser collimators, the results from laser device may be accurate but they can also be erratic. The laser depends to a large extent on it's mechanical alignment within the visual back (the eyepiece holder) and any misaligment here will result in false readings. The star test on the other hand does not suffer from this problem, the image formed by the eyepiece is much more forgiving of whether the eyepiece is held exactly orthogonal or concentric with the optical axis (although it's not entirely immune as we shall see). A perfect star image observed at the center of the FOV of a high-power eyepiece is the final arbiter of good collimation, and you can check the readout of the laser collimator against this. If the laser tells you the optics are mis-collimated even when the star test is telling you the optics are collimated (or vice-versa) then it may be possible to adjust the laser collimator until it reads the same (look in the instructions that came with it).

To re-collimate using the star test you will need a high power eyepiece, of a focal length that provides something like the maximum usable magnification for your telescope. This is usually considered to be around 50x per inch of aperture so (for example) my 10" f/10 LX200 would need a 5mm EP providing approx 500x. This is for the final fine adjustment, for coarse adjustments I would start with an EP of longer focal length - say, 26mm - and this will surely be needed if you have replaced the collimation screws. The ideal conditions are a night of very steady atmospheric conditions, a night when the diffraction rings of an in-focus star are normally visible with your high-power eyepiece. Where I live there might be a half-dozen such nights in the year, so it's fortunate that less favourable conditions will suffice! You will need to select a medium-bright star (perhaps Mag 3 or 4) which is situated near the meridian and about 45 degrees elevation, this will avoid diffraction effects - a star at high altitude will be observed through a thinner layer of atmosphere. You might think a star at the zenith would be best, but unfortunately this is not at a convenient angle for an SCT because you don't want to use a star diagonal (in case it introduces it's own errors). For convenience, your telescope should be polar-aligned and the drive engaged so that it is tracking well, this will give you more time to observe the effects of any adjustments you make. Remember, all adjustments should be made with the star image in the center of the Field of View (FOV) of the eyepiece, as you make adjustments the image will tend to move off to one side or another so you will need to use the hand-paddle to bring it back to the center each time. Large adjustments may move the star entirely out of the FOV.

These instructions apply equally well even if making the adjustments using the standard screws supplied with the telescope, so have all the necessary tools to hand - including the small Allen wrench if you need it. Prior to making any adjustments allow the scope to stand outside for at least 1 hour to achieve thermal equillibrium.

Making the Adjustments:

If you elect to use the star test for both coarse and fine adjustments (as opposed to using a laser first) then use the medium-power eyepiece first and switch to a high power for fine adjustment later. If the 'seeing' is poor you can still attain reasonably close collimation but you will need to perform additional fine adjustments on a subsequent night when conditions are better.

Defocus the star image a little to reveal about a half dozen rings, your primary objective is to get this pattern of rings exactly concentric with the central dark circle. For coarse adjustment at low power discrete rings may not be observed but the position of the secondary shadow (the dark circle), surrounded by a bright donut, will be quite obvious and initial adjustments should be aimed at getting that black circle centered.

These 3 images show the appearance of a de-focussed star image (left column) and in-focus star image (right column) when the optics are aligned (top), misaligned by 3 minutes of arc (middle), and misaligned by 6 minutes of arc (bottom). (from Suiter's 'Star Testing Astronomical Telescopes' ISBN 0-943396-44-1)

To select which screw to adjust I find it easiest to place a finger in front of the corrector plate whilst looking through the scope (wear gloves if it's cold outside otherwise the heat from your hand will cause excessive distortions). This will have a visible impact on the defocussed star image observed through the eyepiece, there will be an intrusion at some point around the edge of the donut which is the result of your finger being in the light-path. By moving your finger around the circumference of the corrector the intrusion will also be observed to move around the image of the de-focussed star. Adjust the position of your finger such that the intrusion lies close to the place where the rings are most compressed (or, for coarse adjustment where the black circle is closest to the bright donut). Where your finger is now pointing indicates the position of the screw you need to adjust (or, if it falls between two screws then it's the one on the opposite side - but in that case it needs be turned in the opposite direction).

The appearance of the de-focussed star image when an obstruction (such as a finger) is introduced into the lightpath by placing it in front of the corrector plate. On a night of 'average' seeing the image is likely to be far more 'spikey' than this. The procedure is to move the obstruction around the circumference of the corrector plate until the intrusion is aligned at the position where the rings are closest together - as shown here.

When the intrusion appears to be in the right place then look at the front of the telescope to see which screw your finger was pointing at. In this diagram the finger is shown (A) pointing at screw number 2 and this is the one to adjust. Had the finger been pointing mid-way between two screws (as in B) then it's the screw on the opposite side that needs adjusting i.e., screw number 1, but in this case it needs to be turned in the opposite direction to achieve the desired effect.

Look through your telescope and try to memorise the overall shape and position of the de-focussed star image, imagine a clock face and use this to determine where the black circle is most off-center, you will be comparing changes to this image. In the image above (as an example) it should be clear that the rings are most compressed at the 3-O'Clock position.

Now we know which screw to turn - but how far the screw needs turning depends on the degree to which the optics are mis-collimated. For now, try turning the indicated screw about a quater turn inwards and observe the effect through the eyepiece. This procedure will have a gross effect but it will establish for certain the direction you will need to turn the screw. The problem with making cautious (tiny) adjustments at this stage is that you may be left in some doubt about whether you are turning it in the right direction, and whether it's the right screw, simply because the effects will be small. So, it's best to get this very clear to start with.

Most likely, the star will have moved well out of the field of view so bring it back to the center again by moving the telescope using the buttons on the hand controller. It should also have significantly altered the image of the out of focus star. You need to establish whether the pattern improved or degraded - is it more concentric than before? Watch that the black circle hasn't actually moved all the way from being close to one side (for example, at the 3-O'clock position as in the picture above) to being close to the opposite side (the 9-O'clock position). This is probably the cause of most confusion because it may appear at first glance that the image hasn't altered much.

If the image looks even less concentric and the dark circle has moved even closer to the donut you will hen know the screw needed to be turned in the opposite direction. Remember you can only go one full turn inwards with any one screw, and if that's insufficient then you will need to judiciously back off both of the other two screws half a turn and try again. You may want to turn that screw back again at this stage and repeat with a smaller adjustment (say, 1/8 turn). Use the red indicator mark on the knob as reference to reverse the action.

It's imperative you don't just keep unscrewing and tightening the screws at random hoping to get it right, the job has to be done logically and using progressively smaller increments. It's important not to over-tighten the screws in an attempt to get that last bit of movement of the secondary - if you feel the slightest resistance then back off the other 2 screws. The ideal situation is to have the optics collimated whilst the screws are almost all the way in, that way they offer the most support for the secondary holder and therefore it's less likely to get knocked out of collimation in response to vibration as you transport it.

Further than that, the job really is an iterative process, each adjustment should get you a little closer each time. Familiarity with exactly how much adjustment is required to correct a particular amount of misalignment comes only with practice. To get collimation close to perfect will require very small final adjustments, perhaps only a degree or two rotation of the screws - something not easy with the stock screws (impossible actually). As you get closer to good collimation then have the test star nearer to being in-focus, best collimation will actually be achieved by having the star truely at critical focus (at, say, 500x) and using the diffraction rings as a guide. Look again at the picture above showing 3-arc minute misalignment, the effects on the in-focus image are more noticeable thant those on the out of focus image. However, tweaking with in-focus star images requires a night of very steady seeing as air turbulence will soon destroy the delicate diffraction pattern, but don't let it worry you too much because you can get real close using an out of focus image.

If this is your first attempt at collimating then please take your time, give the job an hour if necessary, and don't get frustrated or panic if it won't seem to do what you want! A little experimentation is what is needed to understand how much even very small adjustments affect collimation. When collimated, you can try purposefully taking the optics out of collimation by turning one of the screws 1/8 of a turn, observing the result, and then turning it back again. Because you know which screw to turn - and in which direction - it's an easy way of establishing how much the screws need be turned to get the desirable effect. Use the red indicator marks as a reference to put the knob back in the same position. With the experience gained you will find you can look at the star image, and then quickly tweak the correct screw to get perfect collimation in seconds.

With close-to-perfect collimation you will be rewarded with significantly improved performance compared to a scope with optics even slightly mis-collimated. This improvement will be observed as improved contrast, more detail on planetary images, and stars that snap into focus more easily.

©Chris Heapy 2000.

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