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Although I am no longer a beginner I am still in the foothills of the learning curve. For the first time this week, since I started imaging, images from two nights have suffered with what I believe to be dust. 3-4 darker circles appearing, but only visible, after stacking and extracting the background from my stacked subs. (I’m currently using an ASI294 OSC) I’ve cleaned the glass in the optical path but short of doing a full imaging session I wonder if there is any way of checking that the issue has ‘gone away’! Any suggestions would be welcome. Thanks.

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How about using flats as part of your imaging routine? That way you won't have to worry about odd dust spec here and there (still blow off any dust from sensor window from time to time just to prevent it from building up, same as filters and everything else exposed to it).

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24 minutes ago, vlaiv said:

How about using flats as part of your imaging routine? That way you won't have to worry about odd dust spec here and there (still blow off any dust from sensor window from time to time just to prevent it from building up, same as filters and everything else exposed to it).

Yes, thank you Vlaiv. I just wondered if there may be a simple way of checking which may provide an instant answer. I must admit to not using Flats, I only take Darks. Clearly I will have to develop my imaging routine. 

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Ah, yes, I did not offer any sound advice on question, so here it is - take a single flat :D

No, really - turn your scope to reflective surface like a wall and shine light on it (the wall not the scope) and do single exposure - you will not get perfect flat but dust shadows will show on it, so you can compare to existing images if you removed them all.

Edited by vlaiv
typo ...
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Thanks again both. That’s the approach I would have taken with a terrestrial camera/lens combo but it does not seem to work with my astro imaging set up. No bunnies show until the subs have been stacked. I thought perhaps there may be a quick software tweak that may show them up. However I will have another go! 

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