[JohnH]

I know there are already many posts on this subject - I found most of them somewhat helpful but none perfect for my purposes. I have been doing AP for a few years with moderate results and modest gear and have used a few bits of kit - a meade ETX90, a Vixen VMC200l and Sphinx, an ED127 and my current scope a GSO RC250cf with a G11. I have never had to dive into collimation before but my current scope got quite dirty (pollen etc on the promary after 3 yrs of regular use) and was in need of a clean - but before I did that I wanted to be comfortable with collimation hence this thread.

I have a cheshire I got with the scope and could tweak the secondary to get the dot in the middle but I always had a corner where the stars had always been little comets (not really bad and could be cropped out but still they should not really be there with my ST2000xm which gives a small fov even when using a FR).

So I read up and came to the conclusion I must have tilt, I bought the collimation ring and a laser collimator to help fix that.

I had a go and though I did not fully understand what was occurring got close to collimation on the bench but could not get a really good result into the corners and my final "star test" tweak attempts would make it worse not better and send me back to the bench. I got some ok results but I was intending to clean it anyway so, having had some success with collimation I got up my courage, took the scope apart and washed the primary and seconday. I got it back to rough alignment as before but my star shapes were not good.

After many iterations of adjusting and testing approaches here is what works for me, I hope it helps someone else "get it".

1. Align the focuser (you do have that collimation ring right? If not stop here now and go get one) to get your laser to hit the secondary centre spot. Do this with the draw tube in the "camera at focus" position (not all the way in or out).

2. Remove the internal baffle tube (you have to remove the secondary ring assembly, then the baffle tube and then reinstall the ring, this is easy to do because its all one piece. Use some tape to mark ring orientation before removal so you can put is back the same way).

Note : If you have the secondary a long way off from previous "adjustment" you will be able to see it now. Please the ring on the bench, 2ndry holder side down on a level surface and put a bubble level on the top, adjust to get the bubble in the centre to get it back to a neutral position.

3. With the secondary re-mounted use the cheshire to line up the vanes by adjusting the primary. This is not possible with the baffle tube in place but is an easy adjustment with it removed. I check alignment with the Howie Glatter laser and concentric ring attachment projected onto a wall I use an A4 sheet and mark the centre and each ring position and make sure the ring spacings are symmetric.

4. Next use the cheshire to adjust the secondary and put the dot in the circle. The laser can confirm this as the beam should now return to the lasers aperture - if it does not you will see a double bright spot.

5. Reinstall the baffle tube.

6. Check everything still looks ok with the Cheshire and laser. You will now be close but not perfect (well you might be if you are lucky). This is where I got stuck. I figured at this point only a tweaks to the secondary would be required to get a good result but I was wrong. I believe the reason there may still be an issue is that, up to this point, I have used mechanical references (centre spot, vanes, etc) to align on and the optical axes of the mirrors do not have to be coincident with the physical, in fact they certainly are not with my scope. Also I really want the ccd centred with and orthogonal to the optical axis not the cheshire or laser and there may be differences in the adapters spacers etc.

7. Next install the camera and perform a star test (remove all filters/reducers first). Use an open cluster and as short an exposure as you can (to eliminate tracking problems but still give a useable image - binning helps), defocus (inside) to get small donuts on your screen and look at the shapes, adjust the secondary (use very small tweaks) to get a good central star shape (concentric rings, Al's collimation tool helps here!). Now look at the rest of the field. If there is still distortion in the shapes it should be even, that is good in the centre with some Radial or Tangential distortion increasing as you get to the corners, other patterns or uneven amounts mean tilt. Tilt is nasty as it can mean a very confusing field.

Unfortunately the most likely outcome seems to be compound after a mechanical collimation, that is we are Ok in the middle and one, two or even 3 corners are ok but one is bad. That was the situation I kept finding myself in. And I thought I had everything in good order and just needed to tweak the seconday a bit. This is NOT true. In my case the residual TILT had to be fixed first. To do that I tweaked the collimation ring to move the best star shapes towards the centre of the field. After that I then check the centre star again, tweak the secondary slightly if required. Keep doing this until you have good shapes all over the field. Then focus and do a 5 min test exposure. If all is ok then reinstall all the kit, reducers, AO, FW, OAG etc and perform final testing, try a few scope orientations, if the results vary you may have flex in the optical train that will need fixing not good news but at least you now know it in NOT a collimation issue.

My current result is attached, a full res shot of NGC3324 using my ST4k at 0.74" per pixel. I think this is ok or even good (the collimation - not the image which has nasty compression to get under 200k!), there may be a little improvement possible but I am happy with this.