[QUOTE=Peter Ward;1358746]Thanks Greg,
As you know squaring on a 16803 can be a challenge.
1) If you have mechanical sag in the secondary or focuser this needs to be eradicated as you'll be chasing your tail until that is fixed. (fortunately the Alluna has none that I've been able to measure).
Collimate your telescope to the limits of your ability to get everything centred. I use a Takahashi Collimating scope with a M100 adapter made by Precise Parts.
Lock everything down, and re-check the collimation to make sure there has been no movement by torquing up the locking bolts
2) Calculate your focuser's step travel per step. (travel distance/ number of steps)
3) Use CCDinspector to get the orientation of the focal plane tilt.
4) The tilt in my system was along the X-axis. I focused at the bottom of the CCD, then again using the stars at the top edge. Looked at the step difference and got a number. From that got a distance.
Then I used some precision plastic shim stock (coloured so you can easily pick the right thickness) trimmed a small strip and placed it under the edge of the nosepiece that needed raising.
Took another test shot (or ten). Round stars to the edge.
Poured myself a single malt.
Thanks for the procedure. I have mine trimmed up but last time it was more painful than I remembered and I lost a night over it.
Greg.
...I now have pristine stars to the very edge of the 16803)
You think? 
