I've just tried this myself on my Skywatcher 200PDS, and (pending a clear night) managed to get what I think is a very good collimation.
I'd already spent a considerable amount of time using a collimation cap, but what I was struggling with was the secondary mirror position and alignment. Judging concentric circles while looking down a dark tube isn't easy (though I was using pieces of coloured card in the main tube for contrast), and each adjustment tended to throw out previous adjustments. In addition, I don't think that my focuser was quite aligned, as I could not get the secondary mirror centred laterally simply by adjusting the three adjustment screws - or at least if I did that, then the centralised position was lost when I had to readjust to get the primary mirror centred.
My first task was to get the draw tube, the secondary mirror, and the reflection of the primary mirror all concentric. Using the multiple reticule tool, I placed a coloured circle on each of the three objects, and I could see from the crosses at the centre point that they were all slightly off, although not too bad, I think, from just using a collimation cap and judging concentricity by eye.
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I then moved the centre point of the primary and secondary reticules so that they were all concentric with the draw tube.
Over 2-3 iterations, I adjusted (where needed):
* axial position of the secondary with the middle screw
* lateral position of the secondary using the three adjustment screws on the Crayford focuser
* reflection of the primary using the three adjustment screws on the secondary
until the secondary, and reflection of the primary, were all aligned with the appropriate circle reticule.
After this, I followed the procedure described by Robin earlier in this thread. As it turned out, the adjustments needed after my concentric circle alignment were minuscule and, after this, the camera showed no noticeable movement when rotated 180deg. The two photos show the two rotations after I'd finished. Although not obvious on these photos, the very short reticule line remained centred on the camera reflection.
Hopefully, with the secondary now accurately aligned, it will just be an occasional check of the primary collimation with a star test.
So if you happen to have a camera with a lens (I used an ASI120MM with the all sky lens that came with it), this seems to be a good and, once you've done it a couple of times, fairly straightforward procedure.
Geoff
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