ESO Faint Object Spectrograph and Camera
EFOSC2 Documentation

Figure 3: Bad Pixel Mask for the 1x1 binning; click on the image to enlarge it.

The bad pixel mask (BPM) has been created in the usual way: a set of high-flux and a set of low-flux flat fields (FF) have been taken in the V band, and two bias-subtracted, normalized FFs have been generated. The ratio of low over high flux FFs has most pixels with value between 0.96 and 1.05, while pixels outside this range are those which are not linearly sensitive to flux.

The BPM was then created by taking the FF ratio and changing the value of its pixels this way:

if (min < px <= 0.96 ) then px = -1

if (0.96 < px < 1.05 ) then px = 1

if (1.05 <= px < max) then px = -1

The new image is the mask, which then has pixels with value 1 (good) and pixels with value -1 (bad). The image is shown in Fig. 3, and it can be downloaded at the following links:

BPM for 1x1 binning:

BPM for 2x2 binning

Figure 4: Example of usage of the BPM; the right image is an U flat-field before processing, while the left image is the same flat after applying the mask. Click on the image to enlarge it.

One way to use the BPM is the following, in Midas language:

filter/median raw_image median 7,1

replace/image raw_image fixed_imag badpix_1x1.fits/-1,-1=median.bdf

i.e. this will replace each bad pixel with the median of the 7 pixel surrounding the bad pixel in the same row. An example of an U flat-field fixed this way is shown in Fig. 4.

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Last modified: Wed Mar 15 02:32:38 CLT 2006
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