[allan gould]

This is an example of an SCT and miscollimation. Its particularly good with an SCT as the central x-hairs that Al has put there enable you to place that on the small white central dot you get with these scopes and then place the circles on the image from your camera and it shows you where the eccentricity lies.
One nice little improvement for the dyslexic among us would be to place three numbers (moveable on the circumferance of the outer circle) so that you could rotate this to line up with your Bob's knobs or the collimation screws so that you knew which one to turn to bring the airy disk back to centre. Just a thought, Al.
But as it is at the moment its a ripper.
Allan