Quote:
Originally Posted by jase
Peter,
I'm not sure where you are going with the pedestal. Can you please elaborate?
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Jase....this is my understanding of Bias frames...which may well be wrong...but it works for me.
Short explanation:
Separate Bias exposures: you simply don’t need them
Long explanation:
A bias frame is the same as a dark frame of zero seconds exposure. But taking a zero second exposure can result in negative pixel values. This is why a small and constant positive value, ie the pedestal is added to all exposures by the camera’s readout software.
It ensures the bias values are all positive, and while it could be a very large number, only needs to be large enough to make the negative values zero or positive. Hence my statement the bias is “contained within the pedestal”.
In post processing, a non zero second dark contains the zero second exposure (ie bias) plus the normal dark (ie long time) exposure. Thus the one step “dark frame subtraction” takes care of both.
Cryogenically cooled cameras (ie professional CCD’s) have effectively no dark signal, but the underlying bias structure will still be present, so typically only that is subtracted out in these instruments (ie no need to wast time with a non zero dark frame)