Its been a while, a very frustrating while. So much so that I had almost given up ever getting the collimation complete. Persistence is apparently the key.
I learnt two things.
1) the secondary mirror has 4 small hex bolts that adjust the shape of the stars. They can be set to distort the image. Moving these a small amount can bring the star into round. The problem with this is that if the primary mirror is not adjusted correctly as well its a move this, check, move that check back and forward. All of this on a defocused star (both sides of focus) smack in the middle of the image. Do no deviate even a little from the center, plus or minus 10 pixels in my case was enough to distort it.
2) Move the tip tilt plate out about 0.5mm all round and confirm the spacing with a feeler gauge. Make sure you know how the screws line up with the image, put your hand in front of the telescope so that its in the same relative position as the screw but at the front of the scope, take a picture and document where each screw lines up. Move the rotator so that the screws are aligned to with the image so that the triangle they form has one side lined up with a rectangle of the CCD image. When the stars look like horse shoes on the edges do up the screw, when they look like diamonds or triangles undo the screws. Movements are very small, 1/4 turns at a time. Start with the most out of shape stars.
Alignment of the tip tilt screws and the image.
The tricky bit is knowing when the primary mirror alignment stops and tip tilt should start. On my Proline 16803 image the horseshoe stars where only about (note the ABOUT) two/three times as long they where wide. 1/4 – 1/2 turn made these very close to round in the corners. Just refocus after each adjustment.
This is of course after the distortion correction above.
These scopes are a bugger to collimate and I will not be buying another one. Longer exposures will have to do.
Still the tech support form the Officina Stellare guys was very good and the final result is actually quite pleasing. Not perfect but close.
Final image, combined, stretched.