A cold pixel map is an image of so-called cold pixels in the CCD detector. Cold pixels are broken pixels which report low or zero counts even when illuminated. These pixels are determined from flat-field exposures because those have high counts (relative to the night sky background) across the image. In Astro-WISE the ColdPixelMap is derived from the DomeFlatFrame. Cold pixels destroy the value of all pixels behind the broken pixel as charge is moved through it during the read-out process of the CCD. The result is a bad column (see figure ).
The ColdPixelMap is a mask image: good pixels have a value of 1 and bad pixels a value of 0.
In Astro-WISE all pixels that deviate substantially from the other pixels in the (dome) flat-field are considered ``cold'' even though brighter pixels are also detected.
To derive a ColdPixelMap, type in the following at the AWE prompt:
awe> # Example using distributed processing awe> dpu.run('ColdPixels', i='WFI', d='2000-04-28', f='#842', C=1)or
awe> # Example using a Task (single CCD) awe> task = ColdPixelsTask(instrument='WFI', date='2000-04-28', chip='ccd50', filter='#842', commit=1) awe> task.execute()