Newtonian Astrograph Collimation Help!

[gustavo_sanchez]

Hello to all,

Last time that I was taking pictures with my Orion 8" F/4 Astrograph and my CCD camera, I noticed a strange defect on my star spikes. It looks like one of the spikes is much wider than the others, and I don't know why. I usually use laser collimator to get the scope collimated, and it has worked so far. My guess is that this is not related directly to the primary and secondary collimation knobs... what do you think? I attached some crops of my images. Please help me out!

[ollypenrice]

Diff spikes split when you are out of focus. Here you seem to be out of focus on one plane and not on the other. Could this be a tilted chip, either tilted in-camera or tilted by focuser droop?

What is the pattern of the defect over the entire chip?

Fast systems are ultra sensitive to tilt because the depth of field is so shallow.

Olly

[andrew s]

It might also be that the spider vanes are not fully aligned with each other or one is twisted. You only get 4 clean spikes if they are at 90deg to each other. Each vane produces two spikes i.e. 8 in total and they have to be exactly aligned to get only 4 visible.

Regards Andrew