Corrector coma correction problem?

[ChristerS]

Hi,
 
I have an issue with my 3KORRR / 3 inch Reducer factor x0.73 Corrector (on my ASA12N), I start to suspect it could be defect, I always get star trails in the top-left direction in the image after a lot of calibration atempts with glatter laser and Catseye tools.

See attached image
Left image: corrector at my default mount position[/font][/size]
Right image: corrector rotated about 45 degree while the CCD camera is held in the same position
(the images showing the top-left part of the whole image – exposure time 3 sec)

Anyone else have hade similar problems? I awaits answer from ASA support
 
Regards,
//Christer

[MarkS]

Hi Christer,

 

I have had a similar issue, but it was not due to a fault in the corrector. It was due to a combination of the back-focus distance being wrong by 0.3mm and focus not being spot on.

 

There may also have been a very slight tilt on the image plane. In any event, it was all fixed by getting the back-focus exact, and careful attention to focus.

 

The telescope set-up at f/2.8 is very critical on tilt as the focus depth is very shallow.

 

I notice from your images that the one on the right appears to have not only a rounder big star, but different focus: the diffraction cross is different, and the little star below and to the right is more blurred. This could be due to seeing, but it may also be contributed to by some alignment inaccuracy similar to that described above.

 

Hope this helps.

 

Regards,

Mark

[ChristerS]

Hi Mark,

Thanks for the tips. My backfocus is not optimal at least 0.5mm off (ASA support told med it was not a problem), I will modify it a wait for darknes as white nights are starting now where I live in north  of Sweden.

 

Best Regards,

//Christer