Reply To: Bias Frames for CMOS

Forums Photometry Bias Frames for CMOS Reply To: Bias Frames for CMOS

Mr Ian David Sharp

Hi Kevin,

The first thing to bear in mind is that bias signal always needs to be subtracted during calibration. However, the thing to also remember is that *all* images contain the bias signal and this includes your darks and flats.

The traditional CCD workflow is to take separate bias, dark, and flat frames, then explicitly subtract the bias from darks, flats, and
lights. This works well with CCD’s because they are very well behaved in terms of their linearity.

CMOS cameras, however, have certain non-linearities with short exposures and dark frames (amp glow). (a lot of new CCD cameras seem to have all but eliminated amp glow).

To calibrate CMOS images, we don’t need to take separate bias frames, but rather keep the bias in the darks and flats and the bias subtraction is done when calibrating the flats with dark flats and the lights with their darks. So, it is not that bias is not used, but rather how it is ultimately subtracted.

If you use PixInsight and the excellent WBPP (Weighted Batch Pre Processing) script, then all the settings are nicely set up for you. You just need to feed it with your darks, flats and flat-darks (yes, take dark frames to match the exposure of your flats as well as your lights). The only time that PixInsight needs Bias frames is in the situation where you have not taken dark frames to match your light frames. In this situation, the bias frames are used to scale your darks to match. So, for example, if you have 300s darks in your library and you have taken some 360s lights, the program will scale the 300s master dark to produces a scaled-dark. This is not ideal but works very well.

In summary, take darks for all the exposures and temperature combinations you are likely to use. Take flats for all of your filters and also take flat-darks to match the flat exposures. I’m assuming here that you have chosen your Gain and Offset values. These must be kept the same for all lights and calibration frames.

Adam Block explains all in great depth here:

Hope that helps!