COS Data Quality Flags

Every photon event in a COS corrtag file has a Data Quality (DQ) flag (Table 2.3). Each flagged condition sets a specific bit in the 16-bit DQ word, thus allowing each event to be flagged with multiple conditions using the bitwise logical OR operation. The DQ flag values are listed in the table below. These flags can be divided into four types:

• Detector flaws (DQ = 2), which mark unusual features seen in COS exposures.
• Detector shadows (4) include the locations of the grid wires for the FUV detector, and the vignetted region on the NUV detector.
• Detector edges (8) are areas near the edge of the active area which may be suspect.
• Dead spots (16) are regions on the detector where the response is very low.
• Hot spots (32) are regions on the detector where the background count rate has been observed to be higher than the surrounding region and/or unstable.
• The Pixel out of bounds (128) flag marks regions outside of the active area of the detector. This includes any area not in a subarray.
• Background features (4096) are regions in which unusual features have been identified in dark images. These features may have an effect when the count rate is very low or the background is higher than normal.
• Low gain areas (8192) are regions on the FUV detector where the gain is unusually low.

Temporal flags mark photons that occur during time spans in which the data quality is suspect. Events flagged in this way will be discarded by calcos at the extraction step, and the exposure time will be adjusted accordingly. Two types of temporal flags are used:

• FUV event bursts (64), which are flagged by the BRSTCORR module of calcos. As of this writing, no bursts have been seen on orbit, so the BRSTCORR step has been set to OMIT by default. If bursts are seen at some point, it is likely that the parameters in the BRSTTAB reference table will have to be adjusted before using BRSTCORR.
• Other Bad Time Intervals (2048) can be defined in the BADTTAB reference file, for time ranges that are known to be problematic. At present, STScI has not defined any bad time intervals, but users running calcos on their own may wish to define their own intervals in order to exclude times with high background, etc.

Event flags are set by calcos if a photon event falls outside defined thresholds. Currently, two flags fall into this category:

• FUV Pulse Height too low Flag (512) - All FUV events with pulse heights falling above or below the range specified in the PHATAB reference file will have this flag set, and the data will be excluded by the DQICORR module. This flag is not used for NUV data.

Lost Data flags occur if data is missing or corrupt for some reason, such as errors in transmitting the data from the instrument to the ground. Data marked with these flags is always excluded from the final products. There are two flags in this category:

• Reed-Solomon errors (1)
• Fill Data (256)

COS Data Quality Flags

Flag Value	Quality Condition	Type	FUV or NUV
0	No anomalous condition noted	N/A	Both
1	Reed-Solomon error	Lost Data	Both
2	Detector flaw	Spatial	Both
4	Detector Shadow	Spatial	Both
8	Detector Edge	Spatial	Both
16	Dead spot	Spatial	Both
32	Hot spot	Spatial	Both
64	Burst	Temporal	FUV
128	Pixel out of bounds	Spatial	Both
256	Fill Data	Lost Data	Both
512	Pulse height out of bounds	Event	FUV
1024	[Currently unused]	N/A	N/A
2048	Bad time interval	Temporal	Both
4096	Background feature	Spatial	Both
8192	Low gain area	Spatial	FUV
16384	[Currently unused]	N/A	N/A