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QHY Master Dark Frame vs Single frame incongruence

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#1 astroboyabdi

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Posted 13 April 2021 - 08:18 AM

Hi all,

 

I was reviewing some dark subs recently and it occurred to me to review the dark frame masters vs a single dark frame.

 

QHY 168C, Dark frames captured with NINA (previously with APT), stacked in Pixinsight using default in the WBPP script.

 

30 frames ranging from 1.29sec to 600 seconds.

 

I was interested to know if the camera being CMOS showed variation in its dark current over the 2 years I have had it.

 

To my surprise my single dark frame subs were relatively even and clean when STF stretched.

 

However when viewing the master for each Dark subs there was a definite gradient left to right (left brighter).

 

I thought oh no I have a light leak or something. However I compared Darks taken over the space of the past 2 years with same camera but different imaging software and either in a cool dark room or just outside at night with lens cap on and no bright lights and in various other connections and environmental conditions.

 

I noticed the same or at least similar gradient in those dark frames when comparing the master, again the single subs were clean. The issue was more pronounced in shorter exposures.

 

To rule out an imaging train problem, I pulled the camera off and placed it in a completely dark , cool cupboard and retook all subs cooled to -20c (similar to before), Had the camera connected to its own power supply and it was the only USB3 device connected to the pc. The new library showed essentially identical characteristics to my my other darks taken over the past 2 years. That is the gradient.

 

Now my understanding was that this camera had no amp glow. My understanding is this would seem some sort of glow however it being worse on short exposures doesn't quite make sense.

 

I never really looked at master darks because I never saved them preferring to look at individual dark frames and if ok then just stacking them from fresh every-time (I know big waste of time).

 

My images appear to calibrate out ok with the workflow of flat/dark flats/darks/lights. 

 

Any idea if this expected behaviour or has their been an issue with the camera all along. It seems ok....

 

The only other conclusion is that somehow the WBPP settings maybe causing a problem, or maybe they might be showing me a problem with the camera.

 

Any ideas?



#2 Dynan

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Posted 13 April 2021 - 08:58 AM

A pic of your troubled frames might help here.

You can also try Warren Keller's recommendations for creating master darks and biases vs. WBPP and compare then to see if WBPP is doing something different.

 

The checklist I made for WK darks/biases is:

 

BIASES AND DARKS

 

PROCESS: IMAGE INTEGRATION

 

GLOBAL RESET -  ADD FILES

 

INTEGRATION DROPDOWN

 - AVERAGE COMBINATION - NO NORM - WEIGHTS 'DON'T CARE' - UNCHECK EVALUATE NOISE

 

PIXEL REJECTION 1 DROPDOWN

- NO NORM - PIXEL REJECT (1) - CHOOSE E.S.D FIT FOR 25+ / WINSORIZED SIGMA CLIPPING 10-25 / AV SIGMA 8-10 / PERCENTILE < 8

 

Then compare in Statistics and HistogramTransformation to see if there are any significant differences.

 

Just my penny.gif penny.gif


Edited by Dynan, 13 April 2021 - 08:59 AM.


#3 happylimpet

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Posted 13 April 2021 - 09:03 AM

look at them with the same absolute stretch. There will be a lot more noise in a single sub so an automated stretch will make it look flatter even if it has the same gradient as the master.


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#4 astroboyabdi

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Posted 13 April 2021 - 09:06 PM

I will try that and report back, much appreciated.

Also valid about the same stretch didn’t think about it but makes sense.

#5 Jared

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Posted 14 April 2021 - 01:17 AM

It’s just differences in the auto stretch routine. The stack will receive a much more aggressive stretch since it has better SNR. If you apply the stretch from the stack to the single frame you will likely see a bunch more noise with the same gradient.




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