Odd shaped stars

[rubecula]

The other night I had a first chance to try out my recently purchased used Atik 383L+ and EFW2.  The images showed some star elongation.  The elongation was all in the same direction and on the first frame of the run the stars were circular in the bottom left corner and most elonghated in the top right. The first image is a 200% crop off the first luminance sub.  By the end of the session the stars were showing a sickle shape. This was seen all across the image but worst in the top right. The second image is the last taken in the session and the scickle shape is most pronounced in the feinter stars on the left side of the crop.

I don't think this is a giuding error as PHD2 was recording error of 0.44 px in RA and 0.41 px in DEC.  Oh, and the second image was taken after a meridian flip.

Could this be a miss alignment of the camera and filter wheel or a scope colimation issue?

Thanks

 

[ollypenrice]

The key thing to know is the orientation of RA and Dec on the image. Can you give us this information? (I always try to work with the camera orthogonal to RA and Dec, either landscape or portrait, for various reasons.) The elongation might align with one or other of the axes in which case the prime suspect would be backlash in that axis.

If your guidestar is well off axis and you have poor polar alignment you might also see something like this arising from field rotation.

However, the stars seem to have a bigger end and a smaller end, so the speak, the smaller end being at a little before 12 o'clock. This suggests to me that it may be backlash with the mount oscillating between two positions but spending more time in the lower position than the higher. This may be making too much of the available information, though.

Olly

 

[rubecula]

Only, thanks for your response. I’m not exactly sure of the camera orientation. I need to check, but at the moment I can’t get into the observatory. It’s a bit wintry here today and the door is frozen shut. I don’t want to risk damage by trying to force it. 

I was wondering if it was some droop in the imaging train. I’ve tried to keep the connections  fairly tight but not excessively so as I’ve previously had problems taking things apart.

I also may have issues with some of the filters I bought, but that may be the subject of another post.

thanks,