HOW-TO Create a ColdPixelMap

What is a cold pixel map?

Figure 1: DomeFlatFrame containing a bad column of cold pixels.

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 1).

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.

Making a ColdPixelMap

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()