Some diagnostic help please

pmrid (Peter) · #1 21-10-2009, 11:07 AM

I'm trying to traqck down the cause of slight distortions in the stars I';m getting lately. The attached images are all of NGC2259. OK. I didn't intend to image that item. I was looking for 2359 but set the autopilot and went to bed for 4 hours and - well, you know what I mean.
Anyway, the problem is there and I'd appreciate help solving it.
The scope is an 8" Newt that has previously provided terrific images. It's on a pier and has been aligned and collimated to within an inch of it's life. Tracking was with an OAG and a QHY5 into the ST4 of the EQ6 mount. Tracking was spot on throughout. These images are from 4 hours with a QHY8 with 20 minute subs, darks subtracted and just stretched with PS. The QHY8 was fitted with a MPCC and a QHY8 spacer adapter for it (courtesty of Peter Tan).
The images are TL,TR,BL,BR,C as in top, bottom, left, right and centre. Also a full-frame and a crop of NGC2259 just to prove it was actually there.
The distortions are obvious enough.
Any suggestions?

Peter

Kal (Andrew) · #2 21-10-2009, 05:51 PM

It appears as though the biggest cause for distortion is tilt. The TL and BL images have stars that are certainly nowhere near as tight as the TR and BR corners.

pmrid (Peter) · #3 21-10-2009, 07:51 PM

Quote:

Originally Posted by Kal

It appears as though the biggest cause for distortion is tilt. The TL and BL images have stars that are certainly nowhere near as tight as the TR and BR corners.

So it's in the way the camera is sitting in it's mounting? Not sure how to interpret this. Does this suggest that the camera is not orthogonal with the moptical plane of the scope?
How to fix?
Peter

Merlin66 (Ken) · #4 21-10-2009, 08:32 PM

If everthing appears tight and screwed together, then it maybe the focuser is sagging. You'd need to confirm the orientation of the camera/ image to the telescope and the hour angle to see if the distortions match the possible effects of gravity!
I hang my spectroscope ( 2-3Kg) behind the scope and find that I need an extra metal support strap to make sure nothing can move ( or more importantly fall off the scope!!)

Kal (Andrew) · #5 21-10-2009, 08:45 PM

Quote:

Originally Posted by pmrid

So it's in the way the camera is sitting in it's mounting? Not sure how to interpret this. Does this suggest that the camera is not orthogonal with the moptical plane of the scope?
How to fix?
Peter

Yep, you are interpreting it correct. Try what Merlin66 suggests to work out if it is sag.

Moon (James) · #6 21-10-2009, 08:46 PM

Image 1 and 4 look a bit of out of focus to me - if so it could be making any tilt look worse.