There seems to be a bit of minor quibbling over the desired target level for taking Sky flats.
- The PixInsight forum suggested a Target ADU = 40,000 somewhere.
- The Wodaski book uses a formula
ADU = FullWell / eGain * bin * %brightness
For 8300,
FullWell = 25,500 (spec)
eGain = 0.53 (from FITS header)
%Brightness = 0.3 – 0.5 assume 0.4 ===> Gives Target ADU = 38000
- The sample ACP AutoFlatConfig file has a default value of 20000 +/- 4000
I had been using 38000 +/- 50000. I noticed that sometimes exposures were “overdone”, where the vignetting effect of the flat results in an “anti-vignetted” image. Dark in the center, lighter around the outsides. I think this is particularly happening with HAlpha shots, perhaps with shorter test shots. I tried making the HAlpha flats with lower targets (20000) thinking that the values were too high, causing too much offset in the image or something. However, I still noticed occasional anti-vignetting.
So, what is the effect of using different ADU targets?
- I created 3 different Master HAlpha flats with target levels of 20,000, 30K, and 40K target values. 25 frames of each, combined in Maxim DL6 using the SD Mask combination method.
- I took a single 10 minute test Halpha frame from an STF8300 with HAlpha filter.
- I calibrated the test frame with each of the 3 Halpha Flat masters. Each calibration used the same Dark and Bias frame masters; only the Flat masters were changed.
Conclusion
A big disappointment – it doesn’t seem to matter which Flat master I use! Visually the 3 calibrated frames look identical. No anti-vignetting, I can’t tell the difference between the frames. Pixel Math shows the resulting frames have the same values within +/-2 ADUs, which I assume is due to very slight variations in the Flat frames.
So, I guess I’ll go back to the 38000 range again. The odd frames must be due to some other effect. At least I don’t see the effect in regular images, just in short duration test shots. I think.