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IV. NEOWISE Data Processing

IV.4. Quality Assurance

IV.4.a. Daily "Quicklook" Scan QA

In analogy to the "Quicklook QA" step in the WISE All-Sky Release (see section IV.6.i of that document), the purpose of this step was to check for key system performance parameters and to report any problems to the WISE Project as soon as possible. To this end, one scan from the ≈30 acquired each day was manually scrutinized.

In addition to the ScanFrame QA checks described below (see also ScanFrame QA Checks in the All-Sky release [section IV.6.ii]), a new plot (Figure 1) was created for this step. This plot shows "non-nominal spacecraft events" that occurred during a scan. "Non-nominal" events consist of any of the following housekeeping telemetry-reported instances: activation of any of the three torque rods; presence of high background signal in either or both star trackers; spacecraft rate in any of the X, Y, or Z axes that is more than 3 standard deviations from the median axis rate; bad attitude in any of the star trackers; bad rate estimate in any of the star trackers; outage of either or both star trackers. The plot is used to correlate poor image quality (streaking) with known spacecraft events, such as the temporary loss of tracking due to bright moonlight in the star trackers.

Figure 1 - Scan-level plot of spacecraft X-axis rates in scan 49594a, as a function of frame number, with non-nominal spacecraft events highlighted as color symbols: large green circles are framesets with deviant rates; purple tick marks are framesets with torque rod activations; cyan tick marks are framesets with high backgrounds in the star trackers; golden-rod tick marks are framesets with bad star tracker attitudes; orange tick marks are framesets with bad star tracker rate estimates, as reported by telemetry; sky-blue tick marks are framesets with star tracker outages. Note that multiple non-nominal events can be found in a single frameset. Additionally, framesets failing image quality are shown by small red circles. Most scans do not exhibit as many events as the one shown here; it was selected because it showed a wide variety of issues.

IV.4.b. ScanFrame QA

The purpose of this step was to assess achieved performance and to scrutinize output of the processing pipeline. Automated ScanFrame QA reports were generated. Using these reports, the QA scientist assessed the quality of the data, followed up on anomalies or problems, and assigned quality scores to each scan and each frameset.

One group of metrics was computed per scan and another group was computed per frameset. This division was necessary because some checks could be done on a detailed, frame-by-frame basis; others, such as tests of the photometric standards in the ecliptic polar fields, could only be performed on a per-scan basis.

IV.4.b.i. Scan-level QA checks

The checks described in section VIII.3.f of All-Sky Release were carried out here. The visual checks of all framesets section (see IV.6.a.ii of All-Sky Release) were also performed for all scans during ScanFrame QA.

IV.4.b.ii. Frame-level QA checks

Of the checks described in section VIII.3.f of the All-Sky Release, the primary one carried out to assess image smearing was scrutiny of the difference between aperture versus profile-fit magnitudes for well-detected (but unsaturated) sources; see Fig. 2 in section IV.6.a.iii.2 of the All-Sky Release. Plots of these differences were examined for all framesets that failed qi, and for all framesets in the SAA in scans before 19256r.

Starting with scan 19256r, due to network resource issues encountered during the 2020-2021 pandemic, the QA team no longer performed visual inspection of framesets graded with qi >= 0.5 and flagged as falling within the South Atlantic Anomaly (SAA). Previous reviewing of framesets prior to the pandemic indicated that a trivial percentage of qi >=0.5 SAA framesets were truly streaked (i.e., deserving of a qi = 0.0 score instead), so this modification to QA procedures has an insignificant impact on overall data quality characterization.

Background and noise metrics were also examined for framesets failing qn, as described in section VIII.3.f of All-Sky Release, and as illustrated in Fig. 3 of section 4.6.ii.2.2 of All-Sky Release.

IV.4.c. QA Scores in the NEOWISE Data Releases

Both the scan-level and frame-level ScanFrame scores can be found in the "NEOWISE Reactivation Single Exposure (L1b) Frame Metadata Table" available through the Search Catalog 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 as qc_fact, qi_fact, and qn_fact, with the overall frame score, FQ, as qual_frame. Users may examine these columns should a question arise regarding the quality of a particular set of data.

IV.4.d. Image Quality Behavior with Time

IV.4.e. Update to the Image Quality (qi) Metric

Last update: 14 March 2023

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