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VIII. Post-Cryo Data Release

3. Data Processing

f. Quality Assurance (QA)

i. Judging the Data

As with all WISE processing, the objective of science data quality assurance was to assess data through each stage of processing - to identify and flag the (small amount of) data that did not meet WISE science requirements while verifying that the remainder were sound. Quality assurance (QA) for the Post-Cryo data was performed on the images and source lists from each frameset, a simultaneous group of frames in bands W1 and W2. Quality scores can be used by the scientific community to help characterize data products contained in the Post-Cryo Data Release.

The WSDS QA system was highly automated. This level of automation was vital so that the final arbiter of quality - the WISE QA scientist reviewing the data - could quickly assess and bless those data meeting project specifications, while spending most of his/her time on the small fraction of data most needing detailed scrutiny. The QA system collected summary reports from all of the data processing subsystems and compiled them into a single concise report to be reviewed by the QA scientist. These summaries consisted of software completion status reports, statistical analyses, and other tabular and graphical material on which data quality could be judged. The QA system collected parameters, compared them to concise metrics, and presented the results in a web-based interface. This interface was used by the QA scientists to efficiently analyze and troubleshoot issues.

An overview of the various steps in QA is given in IV.6, but for Post-Cryo data only ScanFrame processing and QA were run. The purpose of ScanFrame QA was to compare achieved performance to metrics based on mission science requirements and to otherwise scrutinize output of the processing pipeline. Using the automated reports, the QA scientist assessed the quality of the data, followed up anomalies or problems, and did random spot checking to assign quality scores to each scan and each frameset.

Automated QA consisted of a set of metrics judged against pre-assigned thresholds. The QA web interface contained other tabular and graphical info with which QA scientists could further judge the quality of the data. The automated scan-level metrics are described in the first subsection below, followed by the ancillary tabular and graphical data in the second subsection.

1. Scan-level metrics

Factors qs1, qs5, and qp were determined per scan and were the basis of the Scan Quality (SQ) score. SQ was the minimum of the qs1, qs5, and qp factors multiplied by ten. SQ was an integer of value 0 (poor quality), 5 (intermediate quality), or 10 (high quality). The thresholds for the scan-level metrics are the same as those used in processing for the WISE All-Sky (four-band) data.

qs1: This was the quality of scan synchronization as judged from the overall 1x1 PSF grid computed by the PSF Moments routine and evaluated based on the effective number of noise pixels (NoisePix). The overall value of qs1 was taken to be the lowest qs1 value of the four bands, computed as follows:

qs5: This was the quality of scan synchronization from the 5x5 PSF grid, also computed by the PSF Moments routine and evaluated based on the effective number of noise pixels (NoisePix). The overall value of qs5 was taken to be the lowest qs5 value of the four bands, computed as follows:

qp: This was the quality of the photometric zero-point offset computed using profile-fit photometry (pzp). The value of qp was 1.0 for a frameset passing the following criteria; otherwise the value was 0.0:

2. Frame-level metrics

Factors qc, qi, and qn were determined per-frameset and were the basis for the Frame Quality (FQ) score for each frameset. FQ was the minimum of the qc, qi, and qn factors multiplied by ten. FQ was an integer of value 0 (low quality), 5 (intermediate quality), or 10 (high quality). A threshold for one of these metrics was different for v5 Post-Cryo processing than for earlier data sets. Specifically, the failing threshold for qi_syn was changed from 14.5 NoisePix to 15.0 NoisePix. Thresholds for noise quality remained the same between Post-Cryo and earlier processing versions.

qc: This was the quality of completion. If the status value from the ScanFrame pipeline was equal to 0 (indicating a problem-free completion), then qc=1.0. Otherwise, qc=0.0 because the frame failed processing.

qi: This was the quality of image size and shape. This factor was the smaller of two subfactors qi_syn and qi_rat. The subfactor qi_syn was a check to look for possible image smearing using the NoisePix metric. The NoisePix value was checked for the shortest wavelength band (W1), and qi_syn was computed as follows:

The second subfactor, qi_rat, was a check for possible image smearing using the ellipticity of detected sources. The value of qi_rat = 1.0 if rbovera > 0.6; otherwise, qi_rat=0.0.

qn: This was the quality of the frame as judged by its background and noise characteristics. This factor was the smallest of three subfactors qn_bkg, qn_sgm, and qn_bdp. (The qn_rfp and qn_moc subfactors, used in first-pass processing as a way of monitoring possible cases of unanticipated electronic noise, were dropped as they were not needed for second-pass processing.) These were computed as follows:

ii. QA Scores in the Post-Cryo Data Release

QA scores can be found in the "WISE Post-Cryo Single Exposure (L1b) Frame Metadata Table" available through the Gator service at IRSA. Scan-level score factors qs1, qs5, and qp are listed there as qs1_fact, qs5_fact, and qp_fact, respectively; the overall scan grade, SQ, is presented in the qual_scan column. Frame-level score factors qc, qi, and qn are listed there as qc_fact, qi_fact, and qn_fact, respectively; the overall frame grade, FQ, is presented in the qual_frame column. (The astrometric quality factor qa, represented in the metadata table as qa_fact, was not used in Post-Cryo QA processing this time.) Users may examine these columns should a question arise regarding the quality of a particular set of data.

Last update: 2013 April 19

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