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M15 from 21/09/2020- focus out of whack


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The attached image is from Sunday night- 20 lights, 10 darks all 30 second exposures and 10 bias at 0.1s? Scope C6, camera ASI290MC.

Focus was pin sharp according to my Bhatinov- difration pattern very evenly distributed around Vega. The resulting stack looks very soft however.

Any thoughts on what I should be doing to sharpen things up?



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Is that definitely M15?  It doesn't look very familiar...

Anyway, M15 is quite an angular distance from Vega and I wonder if that involved a meridian flip?  Either way I am wondering if the focus issue is a result of mirror flop?  Or the camera slipping a little if you have it secured in an eyepiece holder rather than via a threaded connection?

The other potential culprits would be dropping out of focus as the temperature fell and the OTA contracted (unlikely to result in the image going quite that soft I think, but easy to check by looking to see if the subs get progressively more out of focus) or dew on the corrector plate (it was quite humid here on Sunday) or thin high cloud.

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I am sure it is M15- target set in Syscan and it was just on the edge of the FOV when it finished. No flip involved.

I had my camera installed via a nosepiece into the back of the scope- 2 x thumbscrews to secure. next time I will screw directly into the scope.

Possibly high haze. No dew evident anywhere on the scope surfaces.

Thanks for the ideas @x6gas


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