Imaging Data Quality Assessment

 

The quality of each field of data from the 2.5-meter imaging camera is assessed based on a number of criteria. A quality is assigned for each criterion, and an overall quality is assigned based on the worst value of these. The possible values for quality in the class Field (Table 16) are:

• AR_DQ_GOOD: Data meet requirements
• AR_DQ_ACCEPTABLE: Data do not meet requirements but are usable for target selection; replace with new data if time permits.
• AR_DQ_HOLE: Data do not meet requirements; the area of sky is small enough to not warrant replacing with new data.
• AR_DQ_BAD: Data do not meet requirements.
• AR_DQ_MISSING: Data are missing; the area of sky is small enough to not warrant obtaining new data.

For the EDR, all fields are in one of the first three categories. The quality for each field is indicated in Figure 8.

A set of flags is maintained for each field in which the quality for each of the criteria is recorded. These flags are 32-bit masks; there are 4 masks, one each for GOOD, ACCEPTABLE, HOLE, and BAD, with bits assigned for each criterion used to judge quality. For example, if the seeing is judged acceptable, then the bit assigned to seeing in the ``acceptable'' mask is set. Additionally, an overall quality is stored separately.

The following are the criteria used to assess data quality:

• PSF (Seeing): The median point-spread function (PSF) full-width at half maximum (FWHM) over all 30 (photometric) CCDs must be less than 1.5 arcsec for GOOD data. Because seeing is a function of wavelength, all measures of FWHM are corrected to the value that would be measured in the r band by scaling as . The actual algorithm also smoothes over time, so the quality of a field depends on more than just the instantaneous seeing. If the seeing fluctuates rapidly from field to field, quality is set to AR_DQ_HOLE. Additionally, for fields where the FWHM is greater than 1.5 arcsec and the largest FWHM for any CCD is less than 1.8 arcsec, the quality is AR_DQ_ACCEPTABLE.
• Tracking: The telescope must track to within 8 arcsec in the direction perpendicular to nominal path for GOOD data and 18 arcsec for ACCEPTABLE data. This is necessary to not leave gaps between adjacent scanlines.
• PSF Model: The postage stamp pipeline (§ 4.3) outputs a status flag that indicates how accurately it was able to model the variable PSF in a field. This flag is converted to a quality of GOOD, ACCEPTABLE, or HOLE.
• Timeout: For some fields, the frames pipeline requires an inordinate amount of time to process, particularly if there are complex objects such as star clusters. In such cases the pipeline processing times out and no object file is created. For such fields a HOLE is created.
• Photometric: Data must meet certain conditions on photometry. At present this quality assignment is not automated due to the complexity involved in creating a robust test. Non-photometric data are excised by hand.
• Manual: In some cases it is desirable to override the overall automated QA assignment.

The parameter badMask in Table 16 encodes reasons why data were not declared GOOD.

The SKY and AMP bits are not used for the EDR. They are used to mask imaging data because of high sky values or bad amplifier noise, respectively.

A substantial portion of fields in the EDR have had quality set manually, overriding the PSF or tracking quality in most cases. A list of all fields that have had the quality overridden are listed in Table 25. They have the AR_FQ_MANUAL flag bit set.