help with elongated stars please

[andyo]

Hi for a while I have been getting elongated stars on my images. Given are skies it has taken a while to run through various tests in an attempt to narrow down the cause.

Polar alignment is fine it has been drift aligned and verified as good with alignmaster. 

spacing may not be perfect but i have tried adjusting it from 55mm to 56.4mm in 0.3mm increments using steel spacers bought specifically for the job. Little if any difference noticed

Guiding graphs have been excellent also.

Not field rotation as the stars are like this after only 20sec exposure times.

So i thought I would look at tilt as being the probable cause. I rotated the camera and the elongated stars shifted so the camera sensor can be ruled out.

I checked the focuser tube using a laser collimator and that appears to be as good as possible.

Maybe the weight of the camera and filter wheel are causing some droop on the focuser? The camera is screwed to the filter wheel which in turn is screwed to the custom built adapter this is screwed to the reducer which is screwed to the adapter from flo which is push fitted into the baader steeltrack focuser (not the diamond version so I had no choise in this).

The image below is what I am getting now the top two corners are ok (not perfect but acceptable) the bottom two are showing the elongated stars especially the bottom right corner.

I downloaded the trial version of ccd inspector and loaded 12 images into it then produced a curvature map and 3d plot of the images however I am not to sure how to interpret the results.

Any help is much appreciated and maybe someone experianced with ccd inspector could help explain what the graphs show thanks in advance

 

 

[alan4908]

The CCDInspector results are indicating that your camera is slightly tilted with respect to the objective in the direction indicted by the arrow.  The amount of curvature (20%) is an estimate of how effective the field flattener is performing.  If I compare this to some images taken with my ED 80 with the matched 0.85 FF/FR using a Trius 814 camera then I was typically getting a curvature of about 15% and a slightly lower tilt.  To me, this indicates that you are slightly out in terms of your optimum FF to camera spacing in addition to your camera tilt. I'd suggest that you concentrate on fixing the camera tilt and if you are still not happy with the image result, then vary the FF to camera spacing until the field curvature is minimized as a separate process. 

On the camera tilt front, I'd suggest taking some shots with the scope pointed vertically since this should reduce the impact of focuser droop - if you still see the tilt then you need to correct if by slightly tilting the orientation of the camera with respect the objective.  Certain camera's and focuser's have the ability to make slight tilt adjustments, if yours do not then you could buy one of these https://www.firstlightoptics.com/adapters/flo-m48-t-tilt-adjuster.html (check that these threads are OK for your setup). On the other hand, if the tilt disappears then it suggests you have focuser droop.  

As an aside, if you want to obtain the most accurate image results with CCDInspector - feed it the calibrated but unstreatched FITS files and average over a number of images. Never run the analysis on jpeg images or images that have been processed since you'll get misleading results. 

Alan

[andyo]

1 hour ago, alan4908 said:

The CCDInspector results are indicating that your camera is slightly tilted with respect to the objective in the direction indicted by the arrow.  The amount of curvature (20%) is an estimate of how effective the field flattener is performing.  If I compare this to some images taken with my ED 80 with the matched 0.85 FF/FR using a Trius 814 camera then I was typically getting a curvature of about 15% and a slightly lower tilt.  To me, this indicates that you are slightly out in terms of your optimum FF to camera spacing in addition to your camera tilt. I'd suggest that you concentrate on fixing the camera tilt and if you are still not happy with the image result, then vary the FF to camera spacing until the field curvature is minimized as a separate process. 

On the camera tilt front, I'd suggest taking some shots with the scope pointed vertically since this should reduce the impact of focuser droop - if you still see the tilt then you need to correct if by slightly tilting the orientation of the camera with respect the objective.  Certain camera's and focuser's have the ability to make slight tilt adjustments, if yours do not then you could buy one of these https://www.firstlightoptics.com/adapters/flo-m48-t-tilt-adjuster.html (check that these threads are OK for your setup). On the other hand, if the tilt disappears then it suggests you have focuser droop.  

As an aside, if you want to obtain the most accurate image results with CCDInspector - feed it the calibrated but unstreatched FITS files and average over a number of images. Never run the analysis on jpeg images or images that have been processed since you'll get misleading results. 

Alan

That is absolutely fantastic help Alan thanks a lot. There are adjusters on the baader focuser which I can try but first I will do the check to see if it is likely focuser droop. Do the ccd inspector results show if the spacing needs to be increased or decreased or can this not be elucidated from the results

[alan4908]

1 hour ago, andyo said:

That is absolutely fantastic help Alan thanks a lot. There are adjusters on the baader focuser which I can try but first I will do the check to see if it is likely focuser droop. Do the ccd inspector results show if the spacing needs to be increased or decreased or can this not be elucidated from the results

Glad to be of assistance.

The CCDInspector results do not indicate if you are too close or too near.  I suspect that you are close to the optimum FF/FR distance and I'd suggest that you remeasure the result after you have corrected the tilt. Personally, I suspect that you will get quite a good result once you have corrected the tilt.

Alan

[andyo]

29 minutes ago, alan4908 said:

Glad to be of assistance.

The CCDInspector results do not indicate if you are too close or too near.  I suspect that you are close to the optimum FF/FR distance and I'd suggest that you remeasure the result after you have corrected the tilt. Personally, I suspect that you will get quite a good result once you have corrected the tilt.

Alan

Thanks for your help Alan