Hi David,
My comments were not making a judgement, more an observation!
Sure, it was you who wrote in your first post:
So here's my question - the higher the threshold, the darker the background of the calibrated frames becomes. At 10 fer example the result seems much better.
'Better' is sort of a judgement, isn't it?
OK, let's stay at your first post:
I am ok with calibrating light frames using Superbias and a Master dark (without extracting bias) then using both in ''Image Calibration''. Sometimes however it warns me that the optimisation threshhold may be too high when using the default of 3. This seems to occur with luminance frames which have more saturated sky background - moving the threshold down to 2.0 gets rid of the warning message.
The warning has nothing to do with the light frames that you want to calibrate. It only depends on the MasterDark you are applying and on the inputted optimization threshold.
As described in the thread that I cited, optimization threshold has the following meaning:
PixInsight computes median and standard deviation of the MasterDark. From the input value "Optimization threshold" (in sigma units) the corresponding value of Td0 is evaluated (a floating point number, range 0 - 1) so that (Median + Td0) defines the lower bound for the set of dark frame pixels that are used for the dark frame optimization. Td0 is outputted to the console as well as the number of pixels and the the number of pixels in %. So for dark frame optimization PixInsight uses only the pixels with intensity between (Median + Td0) and 1.
So if you increase optimization threshold more and more, the pixel count (=
number or fraction of pixels of the MasterDark that is used for Dark frame optimization) is getting lower and lower. This value is outputted to the
process console only, and it has nothing to do with the pixel count that is indicated by ImageStatistics for the calibrated image. The statistical values (of the calibrated lights) that you gave in your second post are not meaningful in this respect. You need to observe (as a function of 'optimization threshold) the values 'count [%]' and 'k0' that are outputted to the process console.
The warning will appear, when this pixel count (process console!) has been fallen short of an PI-internal limit. So PI warns you that the fraction of pixels that is used to calculate k0 (the dark optimization factor) is far too low! Therefore one should
decrease the optimization threshold if the warning occurs - if you don't, you also might determine k0 by throwing the dice. My guess is that in your case increasing optimization threshold results in decreasing mean or median of the calibrated lights because k0 is calculated more and more imprecisely. Bottom line: it doesn't make sense to increase optimization threshold when PI outputs the warning - you have to decrease it. Calibrations results with an optimization threshold that lead to the warning are supposed to be incorrect. I fyou want to repeat your experiments (and take a note of count [%] and k0), begin with an optimization threshold = 3 and decrease it successively.
In my experiments with the Canon EOS 600D (= Rebel T3i) the warning never appeared, independent from the inputted optimization threshold. So obviously your and my camera behave differently, the difference being in the MasterDark. What camera are you using?
Bernd
