Changes between Initial Version and Version 1 of FAQ/aplmask


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Timestamp:
Oct 18, 2011 10:18:39 AM (8 years ago)
Author:
benj
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  • FAQ/aplmask

    v1 v1  
     1=== Q: What do the values in the mask file delivered with my data represent? ===
     2
     3'''A:'''[[BR]]
     4There is a description of this data at the bottom of the accompanying
     5.hdr file for the mask files. The values are:
     6
     7|| Value || Description ||
     8|| 0     || Good data   ||
     9|| 1     || Underflows  ||
     10|| 2     || Overflows   ||
     11|| 4     || Bad pixels in the CCD detector ||
     12|| 8     || Pixel affected by uncorrected smear ||
     13|| 16    || Dropped scans ||
     14
     15If there is more than one issue with a pixel it will have a value equal
     16to the sum of the corresponding values. For example, a bad pixel which
     17has also overflowed would have DN=6 (2+4).
     18
     19The bad pixels in the CCD detector (i.e. the pixels marked 4 in the mask
     20file and in the separate ASCII file) come from a list supplied
     21by the manufacturers resulting from the Hawk instrument calibration.
     22These pixels are supposedly not recording accurately and are not to be
     23trusted. We know this list to contain certain inaccuracies and hope to
     24generate a more representative list in the future.
     25
     26The masks have been generated as a by product of applying the
     27calibration to the raw data. So if during calibration the value of a
     28pixel has underflowed (gone below 0) then we give it a value of
     291 in the mask file.  Similarly, if the data has overflowed (> sensor
     30maximum) we will apply a value of 2 to the mask.
     31
     32In the level 1 data, underflowing pixels are set to 0 (the minimum
     33possible for an unsigned integer).  An overflowed level 0 pixel may
     34result in a valid but wrong level 1 value, which is kept if it is less
     35than 65535 (the maximum allowed level 1 data value) or otherwise forced
     36to 65535.  We retain the computed value mostly in order to allow people
     37to quickview the image more easily without scaling problems, but it
     38should be masked when actually used.
     39
     40[wiki:FAQ Back to FAQ]