Calibration : Bias
What are Bias Frames ?
Bias Frames are taken in absolute darkness with the shortest possible shutterspeed. As this changes for each gain, offset setting and camera mode (QHY) that you use, they should be taken for each combination that you use.
As these are taken in absolute darkness, and with the shortest possible shutterspeed, they contain the image that is produced by the electronics of the camera (also called read noise), plus the given offset (if CMOS camera).
It's the base layer that is present in each photo taken with your camera, at that gain and offset.
With a Formula :
Bias = Offset + Read Noise
Why more than one bias frame ???
The last term in the formula is a Noise term. Noise that is always present when photons or electrons are counted (quantum mechanics at play). As that noise is totally random it can be lowered by averaging a number N of photo's, which lowers the noise by a factor : √ N.
That effect is visible in the next histogram. Left a histogram of one single bias photo. The width of the curve indicates the noise level. Right the same histogram of a master bias averaged out of 25 bias photo's.
How to proceed in AstroPie ?
Make your series of bias frames (e.g. 25) and place them in a directory. Really pay attention that they are taken in absolute darkness, and with the shortest possible exposure time.
In my example the directory is : C:\AstroPie\master_mode1_20240214\BIAS
The default value for the bias directory is to be found in the astropie.ini file, parameter : "biasFolder="
In this case the bias directory is different as the default one from the .ini file, thus it has been changed via the button "Change Bias input directory"
The default output directory for the masters is found also in the astropie.ini file, parameter ; "masterFolder="
In this case it has been changed via the button "Change Output Directory for Bias master".
Result
