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: http://www.stsci.edu/documents/dhb/web/c23_stisdataV2.fm5.html
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Note that both the error and data quality image extensions may be represented with a null array (i.e., NAXIS=0 following STScI conventions) if all the values are identically zero (see Table 20.8). See also STIS ISR 95-006.
20.5.1 The Error Array
The error array contains an estimate of the statistical error at each pixel. In the raw file, the error array is empty. The first step of calstis is to calculate the error array for the input data. This raw data error is simply given as:
The raw data quality files will be filled only when there is missing (datalost) or dubious (softerr) data. If no such errors were taken, generic conversion will produce an empty data quality file whose header has NAXIS=0.
These flags are set and used during the course of calibration, and may likewise be interpreted and used by downstream analysis applications.
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The most significant bit is on the left in this representation.
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