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Bad Pixel Detection and Removal
Before most of the reduction process can be conducted, bad pixel
values must be removed. This is usually achieved by flagging pixels
above or below some threshold value as bad, and replacing their values
by those of nearby, non-bad pixels. The LAMP ON or LAMP OFF flat field
images are a good place to start to define your bad-pixel map. To
determine the threshold values to use, you should examine the
statistics of the image and set thresholds 5 to 10 standard deviations
away from the mean in both positive and negative directions. The exact
thresholds for determination of bad pixels will depend on the details
of your flat field observations - filter, objective lens, integration
time and lamp voltage - so you should experiment with different values
until you obtain a satisfactory result.
Because IR arrays are sensitive to thermal cycling and to atmospheric
contamination, the bad pixel lists change over time. Recent lists are
available via the ESO WWW pages for comparison to bad pixel maps you
generate yourself, but are unlikely to perfectly match the list
derived from your own data.
In MIDAS, bad pixel detection is possible with the command MASK/IRAC2; the command CMASK/IRAC2 can be used for bad pixel
removal. Refer to the MIDAS manual for further documentation of these
commands.
Next: Construction of Flat Fields
Up: Off-line Reduction
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Petra Nass
1999-06-15