Flats and mottling pattern in final stacked image

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ippiu
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Flats and mottling pattern in final stacked image

Post by ippiu » Wed Mar 27, 2019 2:25 pm

Last time i used Sharpcap Pro i did a normal live stacking session, i took a 20 flat frames to have the final master flat: then i started live session with flat subtraction enabled, and my final stacked image was this, horrible, mottling pattern
stacked_flat1.jpg
stacked_flat1.jpg (123.51 KiB) Viewed 2021 times
stacked_flat.jpg
stacked_flat.jpg (219.31 KiB) Viewed 2021 times
So i decided to try a live stacking without flat frame: the final stacked image was perfect, with no artifacts (apart noise caused by having stretched the histogram)
stacked_no_flat.jpg
stacked_no_flat.jpg (118.82 KiB) Viewed 2021 times
I took my flats at: bias enabled, monochrome, 0 gain, 0,5 sec, 30.000 ADU (45-50% peak white histogram), brightness 10, white R and B at 50.

Here my master flat created by Sharpcap with bias enabled and monochrome: https://drive.google.com/open?id=1OTOdU ... CmAmJtX433

Because 0 gain has the maximum level of dark current, is this the problem? Do i have to take flats for example at a minimum of 120 unity gain or upper?

Cheers

Andrea
IOptron Cem40 + Apo 115 f7 + Asi533 Pro + Asiair Pro + Optolong L-eNhance + L-Pro 2"

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Re: Flats and mottling pattern in final stacked image

Post by admin » Wed Mar 27, 2019 6:46 pm

Hi,

Actually that pattern looks more like it might have come from a noisy dark frame than from flat frame correction. You use darks subtraction as well while capturing those images and if so how smooth is the dark frame when you stretch it to have a look? Remember that the dark frame is subtracted from every frame that goes into the stack so if you have noise in the master dark frame that you use no amount of stacking will ever get rid of it.

Cheers, Robin

ippiu
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Re: Flats and mottling pattern in final stacked image

Post by ippiu » Wed Mar 27, 2019 9:07 pm

Yes i used a dark frame.
Here you can find the master dark (30 dark frame): https://drive.google.com/open?id=1oi97R ... 4Dwe98kylV

Same thing happens here: https://www.cloudynights.com/topic/6553 ... pro-flats/

And when i wanted to leave only flat frame correction (so uncheck dark frame correction) suddenly everything become dark and almost completely black...Seems that just flat frame correction caused an error in correcting every frame...It wasn't possible to use just flat frame correction...
IOptron Cem40 + Apo 115 f7 + Asi533 Pro + Asiair Pro + Optolong L-eNhance + L-Pro 2"

BlackWikkett
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Re: Flats and mottling pattern in final stacked image

Post by BlackWikkett » Wed Mar 27, 2019 9:14 pm

I've had a bit of experience with SharpCap, Live stacking and flats. See this post: viewtopic.php?f=5&t=1282

I'm assuming you're using a CMOS camera since you mention 0 gain. Not sure which camera you have but would suggest using a gain of unity + to get away from read noise. You'll of course wan to lower your exposure to get the correct histogram without going into over saturation.

Hope that helps

ippiu
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Re: Flats and mottling pattern in final stacked image

Post by ippiu » Thu Mar 28, 2019 8:16 am

I've read all on the web about flats and how to do them properly, also your topic Blacwikkett, before posting here...
You can read all my stuff in my signature..
I'm saying that the problem maybe flats taken at 0 gain because when i try to experiment 0 gain on my CMOS i obtained this final stacked image (no dark, no flat correction):
0gain.jpg
0gain.jpg (172.01 KiB) Viewed 1998 times
The above image, at 0 gain, is very similar to this one (with dark applied, flat subtracted taken at 0 gain):
stacked_flat1.jpg
stacked_flat1.jpg (123.51 KiB) Viewed 1998 times
The common feature about these 2 images is that both have the distinct mottling pattern of 0 gain but they are completely different in the way they are taken and corrected...
Anonimo.jpg
Anonimo.jpg (96.31 KiB) Viewed 1995 times
So the single common aspect present in both images is 0 gain, even if in the left is about frame gain, in the right is about flat gain...
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Re: Flats and mottling pattern in final stacked image

Post by admin » Thu Mar 28, 2019 6:40 pm

Hi,

Looking in more detail at your shared flat frame, it's pretty clear that the mottled pattern is present in the flat frame for some reason which means it's no surprise that applying that flat frame leads to the sort of problems you are seeing. So it looks like the problem to solve is why is your flat frame behaving weirdly and showing a mottled pattern.

Here's a screenshot of a zoom on your flat frame in fits Liberator showing the pattern which is something I have not seen before.
Capture.PNG
Capture.PNG (140.19 KiB) Viewed 1977 times
I would be tempted to run some experiments on capturing flats with different gains, exposures et cetera and inspect each master flat until you can work out what it is that is causing the patterning in the flat frame. Once you've eliminated that, everything should be back to normal hopefully in your next imaging run. Fortunately you can experiment with flat frames without needing to wait for a clear night.

Cheers, Robin

ippiu
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Re: Flats and mottling pattern in final stacked image

Post by ippiu » Thu Mar 28, 2019 11:06 pm

Thanks Robin for your useful tips.
Before reading your message, i was out to try different live stacking with and without flat frame.
I took flat frame at gain 200...and everything was perfect.
Live stacking went smooth, flat applied correctly, and final stacked image was perfect.

No more problems. So the problem was in the gain at 0: under unity gain, that is 120, Asi294 shows this mottling awful pattern.

You can close the topic! Solved...:)

Thanks Robin for your cooperation and help
IOptron Cem40 + Apo 115 f7 + Asi533 Pro + Asiair Pro + Optolong L-eNhance + L-Pro 2"

jeff2011
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Re: Flats and mottling pattern in final stacked image

Post by jeff2011 » Wed May 20, 2020 6:17 pm

I was running into this issue and a friend pointed me to this thread. 200 gain flats look promising but I still noticed a bit of mottling in the flat. I have also found that longer flat exposures also seem to reduce mottling. A 200 gain flat at 2 seconds looks indistinguishable from a zero gain flat at 25 seconds. I have also discovered a way to correct the mottling in the flat (pdf attached). I have ordered a neutral density gel sheet to reduce the brightness in my flat panel and also a minus green gel sheet to reduce green since it overpowers the other channels. I hope to get those in a few days. I will then take my future flats at 200 gain with those gel sheets and then apply the technique in my PDF to reduce any minor mottling if needed.
Attachments
ZWOASI294_FlatCorrection.pdf
How to correct a Mottled Flat with PixInsight
(572.88 KiB) Downloaded 17 times

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Re: Flats and mottling pattern in final stacked image

Post by admin » Wed May 20, 2020 7:38 pm

Hi,

Thanks for sharing the info on how to correct this issue. If anyone works out the actual initial cause of that mottled pattern in the flat images, it would be very interesting to know about it. I just can't see any good reason for that sort of pattern to appear.


Cheers, Robin

jeff2011
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Re: Flats and mottling pattern in final stacked image

Post by jeff2011 » Thu May 21, 2020 3:23 am

I believe it is an inherent issue with that sensor Robin. It was designed as a low light sensor and does not react well to the bright light of flats with short exposures. Lots of issues reported on Cloudy Nights taking flats with that sensor, both the QHY and ZWO models. ZWO markets it more as an EAA camera. Some have resorted to night sky flats or synthetic flats. Here is an example: https://m.youtube.com/watch?v=kYaApzvfepk

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