I am having trouble with calibration of my mono Oiii data, in the form of my flats over-correcting. The issue does not arise when using my mono Ha filter.
All of my lights and calibration frames are perfectly time and temperature matched, but try as I might, I cannot fix this issue.
I have heard that there is a Pixelmath workaround in Pi, is there any way I can fix this in Siril?
Apparently, this appears to be a common issue with Oiii filters.
Yes I have tried a background extraction but to no avail. I know it is related to the flats as when I calibrate using flats and matching flatdarks from an earlier Ha integration, most of the overcorrection is gone. Obviously using Ha flats going forward is not a fix.
The original flats were sky flats (which I never usually do, as I have an artificial light box), but when I reshot them with my light box but the result was the same. Both were shot with a mean ADU of 20400.
OK. So your flat is really not good.
Either your light source is bad, really bad and not homogeneous, or … Actually I don’t see another option.
Why it is visible only in OIII? Because OIII needs longer exposure.
I tried reshooting my flats+darkflats with decreasing ADU down to 1000, with no success.
Tried blackening a surface of my step-down ring, on the objective end of my optics, to minimise internal reflections - didn’t work.
Last throw of the dice was to swap out the halogen bulb in my lightbox to a high power LED - success! Flat frames are much more evenly illuminated and mirror the optics.
Final stack is now how it should look. Here is the new masterflat.