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Help: Flats leaving perfect rings in calibrated image

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#1 Lead_Weight

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Posted 21 April 2021 - 12:09 PM

I've not experienced this before, and the very last subject I shot didn't produce this in the results, but for some reason this time these perfect concentric rings appeared where dust was corrected out.

 

So, from what I can tell the flats are perfectly aligned to the lights, but are overcorrecting somehow.

 

This is the ASI2600MM-Pro camera. Lights, Flats, Darks, Bias at -10 degC, gain 100, offset 5.

 

The fits headers match in terms of settings.

 

I shot the flats to a median ADU value of 32,500. 

 

All I can think is that I possibly need to use Flat Darks instead of bias, but like I said, my previous image using the same exact exposures and settings didn't produce this.

 

Any thoughts? Ignore the gradient, this was light pollution.

 

Screen Shot 2021-04-21 at 12.00.30 PM.jpg



#2 WadeH237

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Posted 21 April 2021 - 12:19 PM

My guess would be that there was a significant focus difference between the light frames and flat frames.

 

A softer focus for the flat frames would make larger dust shadows.  Larger dust shadows would cause overcorrection just past the edges of the dust shadows in the light frames.  That would give you bright circles around them.


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#3 Lead_Weight

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Posted 21 April 2021 - 12:26 PM

My guess would be that there was a significant focus difference between the light frames and flat frames.

 

A softer focus for the flat frames would make larger dust shadows.  Larger dust shadows would cause overcorrection just past the edges of the dust shadows in the light frames.  That would give you bright circles around them.

You're the man Wade! I ran outside to look at the equipment, and sure enough the focuser was racked all the way in. At some point during the night, clouds rolled in, and autofocus must have triggered moving the focuser all the way in before stopping. And I didn't notice this when I took the flats this morning.

 

I'll reset the focus, and shoot my flats again.


Edited by Lead_Weight, 21 April 2021 - 12:26 PM.

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#4 Lead_Weight

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Posted 21 April 2021 - 12:29 PM

I just read this in someone else's thread, but I'll say it again.  The collective experience in these forums is really what makes this hobby work. smile.gif


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#5 pyrasanth

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Posted 21 April 2021 - 12:51 PM

.......you can use selective rejection in PI to get rid of these artefacts if you have some subs that don't exhibit the bright ringwink.gif


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