Collimation question

Hi, I have acquired an APO refractor used recently. At approx 300x, the inside and outside focus rings appear to be concentric to my eyes and centered in the middle- a sign of good collimation...

However, when used for astrophotography I have trouble getting absolute pin point stars- they appear to be very slightly blotted. See attached photo of M52 from center of frame. I'm using DSLRfocus with my 20D and I can't focus to pin point.

Any helpful ideas on how I can further my investigation are greatly appreciated. I'm not sure how good my eyes but the out of focus rings do not show signs of being skewed.

Thanks, Ivan

Attached Image:

Is this the entire frame? If so, that rules out
coma and field curvature.

Obvious focus problem (blobs) maybe with some
guiding error (faint tails).

Can you take a bunch of images stepping through
focus and see how that goes? Find some way to
accurately measure/record the focus position.
(Maybe some 1mm division graph paper wrapped around
the focus knob, and an cardboard indicator pointing
at it; ugly but works)

Ivan,

My photos often look like that. I hope you get a good answer that will cure my problems.

Alex

Ivan,

Do you use an electric focuser? How do you control it? What size steps do you take through the zone of focus?

Alex