Hello Paul
Well, where do I start. I have been enjoying getting some good data over the past few months, and I have been happy with the results I have been getting from periodic with CCDInspector...but something has now occured, and I cant track it down.

I have elongated stars down the one side on my images...I use an STL11K with an RCOS Flat Field Corrector on a 12.5" RC.
Stars are now routinely out of round by about 15percent on that side.

I have dismantled the camera now and completely reasembled carefully.

What is interesting and puzzling, is that the results yield that my collimation has not really moved ...still hovering around 0.6", tilt is great at +/- 0.1 or 0.0...but my curvature has gone up markedly!! I am now seeing values of around 15-20%, whereas up until now, its been around 9-12%.

Phew....where could I start looking to track this down?

grateful for any pointers.

cheers
Martin

Hi Martin,

This sounds like something is tilted in the imaging train. Check to make sure nothing is misplaced, and everything screwed in well. This includes filters, FF corrector, the focuser itself, etc.

Do you have a rotator? Even if not, try to rotate the camera, including FF, 180 degrees. Does the tilt shown by CCDI change locations or stay in the same corner?

If it shifts with the camera, that means the tilt is between the whole camera/FF assembly and the optical system (very likely in the focuser).

If it doesn't move after the camera rotation, then the tilt is somewhere in between the FF and the camera. Then, try to narrow down where by eliminating one possibility at a time (image wihtout a filter, remove FF, etc.)

Good luck,

-Paul

Are you sure that when you put your system 'back together', you got the right number/length of spacing tubes present?. The field curvature would change, if the corrector is not the right distance from the CCD (as would the focus adjustment needed in the TCC). Also, the corrector itself screws into the mounting plate (inside the instrument rotator if you have this), and if it was not fully 'home', this would alter the curvature, but also could introduce a tilt in the 'correction', while not tilting the main optics.

Best Wishes

"..Also, the corrector itself screws into the mounting plate (inside the instrument rotator if you have this), and if it was not fully 'home', this would alter the curvature, but also could introduce a tilt in the 'correction', while not tilting the main optics."

Roger, could you elaborate on this a little bit on what you mean by the corrector being fully 'home'; do you mean threaded all the way in?

Sorry to have been so long replying. Been rather busy!.
Being 'fully home', implies somthing done up in it's target position. The screws being done up, doesn't automatically guarantee the object being 'home' (as was seen with the test setup used on the Hubble mirror, where a flake of paint, stopped one leg from going to the position it should take). So, an object, 'in place', with the screw donw up, and then possibly checked with a micrometer, for being parallel, and at the expected spacing, would be a good start.
I had trouble with my flattener, seeming to be tight, but actually not quite being properly 'home'.

Best Wishes