I can't get flats right .. ideas?

Acquisition Calibration Deep Sky Help Needed Troubleshooting
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Andy Wray avatar
Andy Wray avatar
I'm struggling with getting my flats to work with my lights and was looking for ideas.  I'm using an LED flat panel and a white T-shirt cover on my OTA and when I look at the flats they look as I expect.  Unfortunately when I integrate my images the flats don't sort out my lights.  Please see below my master light and the corresponding master flat (all boosted auto stretch to exaggerate the issue).  Any ideas would be appreciated?


FWIW:  I used pixelmath to subtract 0.11 from each of the pixels in my flat and re-integrated and ended up with this for my master red light.  I'm thinking because my flat panel is so bright, I am probably imaging the flats in the non-linear area of my sensor.  I've ordered some A3 copier paper to dull down my panel.:


and in case it helps, here is an original sub on the left and the calibrated image on the right.  It seems to be under-corrected:
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andrea tasselli avatar
andrea tasselli avatar
Might have something to do with the other issue. Not fond of flat panel flats but if it used to work for you it should now.
jewzaam avatar
jewzaam avatar
Can you provide more info on how you're taking flats and calibrating the image?  If subtracting from the flat results in a better calibrated image I wonder about the whole process.

My recommendation is to target 40% to 50% max ADU for the flats with an exposure time above 1 second if you can.  And take matching darks for those flats.  Keep gain, offset, exposure, and temperature all the same.  If you want to modify gain, do not increase gain above what you are shooting lights with, the bit depth will be less on the flats and correction will be not as good (it still works, but just not as well as it could).
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Andy Wray avatar
Andy Wray avatar
Stupid me!  I worked out the issue as follows:

* I was using an old darks library from last November that was taken at an offset of 83. 
* I'm currently using an offset of 50.
* I modified the darks using pixelmath as follows "$T-(33/4096)" .... I have a 12 bit camera, hence the 4096

I re-ran WBPP and, hey presto, the flats worked.  Also a lot less noise in the images now as I'm not truncating.

I think it's time for me to create a new darks library ;)

Here's my image using the new modified darks





M51 with only 90 minutes of integration
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