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Chris Icough Memorial

I am sad to announce that Chris Icough died 11th April (Simone Icough)

A website has been created in his memory to raise money to send Chris' ashes to the surface of the moon and we hope this will be early 2012. Chris Icough Memorial


 
 

Collimation

Collimation is the alignment of the optics. On an SCT you do this by adjusting the three screws on the front of the corrector plate and tilting the mirror behind it to the desired place.

Collimation is at first a daunting task, and many new users are afraid to do it, just in case they make it worse and cannot get it back. I was also like this at the beginning, but after doing it a few times, I have learned a really good trick. First of all I would suggest getting a set of ‘Bobs Knobs’ or equivalent. These are thumbscrews rather than allen screws and are a good investment. If by mistake in the dark you miss the screw and leave a fingerprint on the corrector plate then that is infinitely better than missing with an allen key and scratching the corrector.

There is a trick to installing Bobs Knobs or thumbscrews. If you remove all three screws then the secondary mirror will fall into the telescope, and if you remove them one at a time and replace with the thumbscrews without checking each one then you will end up miles out of collimation and from this far out, you can spends hours trying to get it back. This tecnique is NOT a suggestion but a MUST when replacing your allen bolts for thumbscrews.

During the daytime, in Land mode (no motors running) focus you telescope on a distant object. Remove one of the allen bolts and replace it with a thumbscrew. Without moving the telescope adjust the thumbscrew until the object is back centred in the field of view – now the thumbscrew is in exactly the same position as the allen bolt was. Now repeat with the second until the object is perfectly centred again, and then lastly with the third screw.

This way your collimation will be very close to as it was before switching the screws.

Checking Collimation


You can check your collimation with an eyepiece, or with a CCD camera. Most people say that you must use as high a power eyepiece as the seeing allows to check and adjust collimation. Whilst I agree to this for visual viewing, I have found that with the CCD it is easier to see miscollimation as you can enlarge the image and therefore mimic a high power eyepiece, but more importantly I have found that when the collimation is perfect through the eyepiece, after checking with the CCD camera I have found on occasion that it is slightly out. Now this is either my eyes, the alignment of the eyepiece (not exactly perpendicular to the telescope optics), or the CCD camera not exactly perpendicular. Either way I now prefer to check and adjust collimation with the CCD camera (I am in the minority here, so please by all means do this by visual or CCD, whichever you feel the most comfortable).

1) Centre a bright star and defocus, you will see a ‘donut’ of light. If the telescope optics are aligned (collimated) then you will see a perfectly concentric circle of rings like the diagram. Colim1

This is a computer generated example so you can see it perfectly. In actual practice if you are looking through the eyepiece you will have to deal with seeing and therefore the image of the star will jump around a bit and you have to use your judgement as to the actual concentricness of the rings. If it looks good then change focus to the other side of focus and recheck, use different sized ‘donuts’ (focus) to get the best view. If you are happy then increase magnification and re-check. There will be a seeing limit on how much magnification you can usefully use. If using a CCD camera then use a star that allows 2 second exposures, that way you will average out the seeing. You can also average together a few images to combat seeing conditions. With an image from a CCD camera you can enlarge the image and play with the back and white point sliders to get the best contrast on the rings to check for concentricity. You can also measure the pixel distance from the centre point to the edge of the donut in each direction to confirm your alignment (if your camera like my MX7C does not have square pixels then you will need to resize before measuring).

If the donut looks like the one below (which is exaggerated) then you need to adjust the collimation (alignment) by adjusting the three screws on the centre front of the corrector.

From this example you can easily tell that the optics are misaligned.Colim2

This next part of the collimation technique is for SCT’s only as I believe it to be the reverse if you have a Newtonian.

1) Using the hand controller, move the defocused star in the direction of the ‘fat’ side of the donut. In this example you would need to move the star in the eyepiece field of view or the CCD image to the right and down. Do not move it very far until you get a bit of experience and can ‘guess’ how far to move it. The above example would need to be moved a few fields of view, but don’t move if further than half a field of view at the moment.

2) Move one of the adjusting screws (by a very small amount) on the front corrector and check where the star has moved to. What you are trying to achieve here is by moving the screws on the front corrector, you want to re-centre the star. This is why you want to move the screws in very small increments so if you move the wrong screw and the star moves in the opposite direction, you won’t lose it in the field of view. When you lose the star, it is very difficult to find it again, because by changing the alignment of the optics you will be unaligning the finderscope, which then becomes useless to you. When loosening one screw, tighten the other two, and in the same way if you need to tighten a screw, the other two need loosening slightly first. This way you always keep the secondary tight (but not too tight as to stress the mirror – another reason to use thumbscrews rather that allen screws).

3) Once the star is re-centred go back to step 1 and check the collimation.

4) Repeat until the stars ring are concentric.

Tips For Collimation


Ø When adjusting the collimation screws for the first time, write down the direction each screw makes the star move.

Ø Always re-centre the star to check the collimation. Most telescopes fields of view are curved, and therefore the flattest part of the system is the dead centre.

Ø Sometimes the centre of a CCD image is not the centre of the optics, if you can - without moving the telescope, turn your camera around by 180º and take an image of the star – as long as the telescope didn’t move (impossible with my LX200) then you can work out the dead centre of the optics as the point exactly in between the two star images.

Ø Without perfect collimation you cannot achieve perfect focus, as one side of the star will be in focus but the other won’t resulting in comet shaped stars.

Ø Collimation will not be time consuming, I check my collimation about once a month as my scope does not get bumped around much, and normally I don’t make any adjustments, and when I do it usually takes less than 10 minutes as I now have a ‘feel’ for how far I need to move each screw.

Ø Always move one screw at a time – by saying this I mean if you loosen screw a by an 1/8th of a turn then tighten the other 2 by a 16th of a turn. The secondary mirror is held on a pivot so you can adjust it as necessary, but if you don’t keep all three finger tight then the secondary will move and you will lose your collimation (usually when the telescope moves from one side of the sky to the other). A simple diagram shows this:Colim3

Now you can easily see what we are trying to achieve, we want the light path to converge into a focal point exactly in the centre of the optical path, and we want the CCD chip to be at the exact focus point.

That is collimation – remember if you have a Newtonian reflector then instead of moving the star in the direction of the fat side of the ‘donut’, move it in the direction of the thin side, and you can then follow the exact same procedure. Don’t ask me about Mak’s and RC’s because I wouldn’t know!

This image is an actual image I took of a defocused star to check my collimation.Colim4