Bad Pixel Mask
The badpixel mask presented here includes:
They are computed from the histograms of flats taken during Sofi Calibration Plan : the distribution of the pixels values is fitted with a Gaussian (mean, FWHM), and we select the dead pixels as the ones whose value lays below mean-4*FWHM.
They are computed from the histograms of darks taken during Sofi Calibration Plan : the distribution of the pixels values is fitted with a Gaussian (mean, FWHM), and we select the hot pixels as the ones whose values lays above mean-4*FWHM.
They are computed from the histograms of the sigma image obtained when averaging the flats and the darks of the previous two steps.
The mask also include as bad those pixels that are typically masked by the focal plane field mask. For all intents and purposes these are undistinguishable from the dead pixels.
Statistics of bad pixels by type over the recent years is available here.