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20.5 Error and Data Quality Arrays

 The STIS pipeline propagates both statistical errors and data quality flags throughout the calibration process, combining, appropriately, the statistical errors and data quality flags from both the science data and the reference file data so as to produce triplets of science, error and data quality in the calibrated data and extracted spectra.

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:

Eq. 20.1

where

20.5.2 Data Quality Flagging

 Data quality flags are assigned to each pixel in the data quality extension. Each flag has a true (set) or false (unset) state. Flagged conditions are set as specific bits in a 16-bit integer word; in this way up to 15 data quality conditions can be flagged simultaneously for a single pixel, using the bitwise logical OR operation. Note that the data quality flags cannot be interpreted simply as integers but must be converted to base 2 and interpreted as flags. Table 2.4 gives the specific conditions which are flagged by the different bits being on or off.

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.


STIS Data Quality Flags

FLAG Value

Bit Setting1

Quality Condition Indicated
1

 0000 0000 0000 0001

 Reed solomon decoding error.

2

 0000 0000 0000 0010

 Lost data replaced by fill values.

4

 0000 0000 0000 0100

 Bad detector pixel (e.g., bad column or row, mixed science and bias for overscan, or beyond aperture).

8

 0000 0000 0000 1000

 Data masked by occulting bar.

16

 0000 0000 0001 0000

 Pixel having dark rate > 5 sigma times the median dark level.

32

 0000 0000 0010 0000

 Large blemish, depth > 40% of the normalized P-flat.

64

 0000 0000 0100 0000

 Reserved.

128

 0000 0000 1000 0000

 Reserved.

256

 0000 0001 0000 0000

 Saturated pixel, count rate at 90% of max possible-local non-linearity turns over and is multivalued; pixels within 10% of turnover and all pixels within 4 pixels of that pixel are flagged

512

 0000 0010 0000 0000

 Bad pixel in reference file.

1024

 0000 0100 0000 0000

 Small blemish, depth between 0.4 and 0.7 of the normalized flat. Applies only to MAMA p-flats at present.

2048

 0000 1000 0000 0000

 >30% of background pixels rejected by sigma-clip, or flagged, during 1-D spectral extraction.

4096

 0001 0000 0000 0000

 Extracted flux affected by bad input data.

8192

 0010 0000 0000 0000

 Data rejected in input pixel during image combination for cosmic ray rejection.

16384

 0100 0000 0000 0000

 Reserved.

1 The most significant bit is on the left in this representation.


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