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First light for Atik 460EX.. odd-shaped stars?

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An unexpectedly short EAA session on the night of January 18 led to only one capture... it would have been longer but for the clouds, and I seem to have a problem with star shapes.

This is from my Quattro 8", successfully used before, but with a new (to me) camera and imaging train.  Here's a 20 second sub as captured on screen (Nebulosity) and then a post-processed image with 22 of them stacked (for a total of 7 minutes, 20 seconds)





The problem, though, is with the star shapes:



The image train is:

  • Coma Corrector (previously used and absolutely fine)
  • ZWO OAG (new for Christmas, but not in use yet, except as a spacer)
  • Baader variable spacer
  • Atik 460EX mono CCD cooled to -12C (this is 'Gina-cam', thanks @Gina!)

Everything is threaded connections, I don't think it's tilt, and it doesn't look like a classic CC spacing issue.  This is unguided, and certainly the PA wasn't perfect, but collimation was (I believe) good.  The OAG prism was way out of the way from the long-side of the chip, so my hope is that this might be caused by the high wind that night?  (Mount is an Avalon M-Uno.)

Any suggestions/advice?  I will, of course, be trying again in better conditions, if they ever arrive.



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I don't think the camera is at fault, I am wondering - pinched optics?  Can you even get pinched optics with a Newtonian?

However as you have used this scope previously with no problems, it will hopefully be the wind as you suggest, but am looking at that halo around the bright star just off the screen on the right and it does seem to be off centre.

Hopefully some-one with more technical savvy than me will come along soon. 


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1 hour ago, carastro said:

I am wondering - pinched optics?  Can you even get pinched optics with a Newtonian?

Thanks Carole.  Yes you can, but no it's not.  I had the mirror cell out the day before to finish blackening the bevel and the mirror edges.  It had rotated in the cell from when I first did it.  I left it loose enough to be able to do that again.  The mirror clips were actually clear of the mirror surface.  Good thought, though!

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1 hour ago, Thalestris24 said:

Looks suspiciously like thermal effects to me - did you do a decent cooldown before imaging?

Ah, an excellent suggestion!   I hadn't had the roof open for very long, it's true.  I'd not really meant to do anything serious at all, and was just initially looking to see if I had the spacing right.  It was also early in the evening and the sun had actually been out (gasp!) in the late afternoon. 

I'd say that was a prime candidate.  Let's say so, and put my mind at rest.  Next time out (with a longer cool-down) will tell!

Thanks indeed.

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