Flat issues

[whzzz28]

This one has me scratching my noggin'


First night out, did some LRGB. No issues with flats at all.
Second night out, switched to HA. No issues with flats at all.
Third night out I switchted to S2 filter. And from here onwards I have been having problems with flats - with all filters.

I normally aim for 10-15k ADU for flats.
In my testing i tried flats with ADU's ranging from 2000 up to 40000.
The results were always the same. So i dont think my ADU values are a problem.


The problem appears to be that the flats are over-flattening?
I notice a dust mote in the flats, and visible in the lights as a dark spot , becomes a light spot after flats are applied rather than neutral.
I can also see lighter corners on the flattened lights, compared to the raw lights.



First image attached is the flat calibrated light (left) and the raw on the right. Same STF stretch on both.
Image 1: https://drive.google.com/file/d/1Nse1RYIF6EnwVbdD4vuW3ZpvxMgvc-ZI/view

The second image is S2 filter (stacked). Its horribly bright around the edges, and the white dot is clearly visible.
Image 2: https://drive.google.com/file/d/1so_UCABUMqPtA7mhCsJO_5CbbQkx9rsm/view (https://drive.google.com/file/d/1so_UCABUMqPtA7mhCsJO_5CbbQkx9rsm/view?usp=sharing)

The third image shows the bad single calibrated frame (left) vs the good calibrated frame (the 2nd, when i first started doing HA captures, those flats worked fine)
Image 3: https://drive.google.com/file/d/1zAXMnO6rsulH7b7mm5TSx54_TJ10VBoN/view (https://drive.google.com/file/d/1zAXMnO6rsulH7b7mm5TSx54_TJ10VBoN/view?usp=sharing)

The fourth image is a good flat (left) and the "bad" flat (right) - both streched.
They look almost the same, and have almost the same ADU.
Image 4: https://drive.google.com/file/d/1G_l09tYwaIjzJwVb0AhjVnL2DSglaXuN/view (https://drive.google.com/file/d/1G_l09tYwaIjzJwVb0AhjVnL2DSglaXuN/view?usp=sharing)



My pixinsight flow is the same between all dates. In fact I have it documented, to make sure i dont forget an option etc ;).
I also checked online to see what process flow others were using, and it was the same/similar to mine.
I also tried using Batch Preprocessing tool, to see what results it would provide. But its the same. The white blob from the dust mote is clearly visible.


So I am not sure what is going on. Are my flat frames somehow bad? Or is there something wrong with my processing when creating the master flat? dark frame issue? or maybe even a light frame problem?


This is my pixinsight flow for flats.

1) Process -> ImageCalibration. Add flat files. Select an output directory. Select the master Bias and Master Dark. Select "Calibrate" under master dark. Unselect Master Flat. Process then repeat for all other channels.
2) Process -> ImageIntegration. Select flat files that were calibrated above. Combination=average. Normalization=multiplicative. Weight = dont care. Turn off "Evaluate noise".
Pixel Rejection (1): rejection algorithm=sigma clipping. Normalization: equalize fluxes.
Pixel Rejection (2): Sigma low: 3. Sigma high: 3


Cheers,
Nathan.

[phomer]

Nathan,
It would be useful to look at the flat as FITS format.
There are a number of things to be aware of when doing flats.
- You should stack multiple to reduce noise.
- They should be taken with the camera and telescope at the same focus and rotation as the image (or near to).
- There should be no light leakage from outside.


I would also advise taking darks for your flats.


Paul

[Atmos]

Over/under correcting in flats in my experience is usually caused by other calibration frames not correctly correcting the flats; most notably dark/bias calibration.

The other simpler one can be having the "Optimize" ticked under "Master Dark" during image calibration. Optimize shouldn't be selected by the way.

Given your calibration process, I'd just simply ditch the bias frames and try again. Remember, as long as your dark frames as the same length as your flats then they contain both the bias and dark signal so bias frames become useless. The only time that bias frames are actually useful is if you want to scale your dark frames which doesn't work with 95% of CMOS sensors as they have a non-linear progression of dark current.

[whzzz28]

Thanks all,


Issue has been found.
At some point I must have changed the offset on the camera (ASI1600) from 21 to 100.
My dark library is all set for an offset of 21, not 100.



Changing offset back to 21 worked and images are 'flattening' again without issue.
Time to take some offset 100 darks and get my data working.


Cheers.