VIII. Post-Cryo Data Release

3. Data Processing

The 2013 NEOWISE Post-Cryo Release products were generated from complete second-pass data processing using version 5.0 of the WISE Science Data System (WSDS). WSDS v5.0 was adapted from WSDS v4.5 that was used to process WISE 3-Band Cryo data. Development of WSDS v5.0 was supported by a grant from the NASA ROSES Near Earth Object Observation Program. WSDS v5 includes all pipeline updates and improvements implemented for the second-pass reductions of the cryogenic missions data, and it corrects known deficiencies in the first-pass Post-Cryo processing, and employs calibrations and reduction algorithms that are optimized specifically for the post-cryogenic survey data. The reprocessed Post-Cryo products provide the best individual flux measurements for solar system objects that were identified by the WISE Moving Object Pipeline during the Post-Cryo survey, and they enable consistent follow-up of inertial sources found in the WISE All-Sky Release Catalog to investigate time-dependent properties such as flux variability and proper motion.

The NEOWISE Post-Cryo Preliminary Data Release (July 21, 2012) is superseded by the 2013 Post-Cryo Release. Post-Cryo Preliminary Release products were generated during first-pass data processing using WSDS v3.5 that utilized calibrations that were not yet optimized for the changing characteristics of the warming telescope and detectors. You should always defer to the newer 2013 Post-Cryo Release products. Please review the limitations and known features that are present in the both the 2013 Post-Cryo and Post-Cryo Preliminary Releases described in the Cautionary Notes section of this Explanatory Supplement.

Because the WISE Post-Cryo operations were intended primarily to support investigation of moving solar system objects, the Post-Cryo data were only run through the Scan/Frame and WISE Moving Object Pipelines. The Post-Cryo data did not undergo Multiframe Pipeline processing. Therefore, the Post-Cryo Release products do not include coadded Atlas Images and deep source detection Database/Catalog like those in the All-Sky and 3-Band Cryo Releases.

The basic design, functionality and algorithms in each WSDS pipeline module are described in section IV of this Explanatory Supplement. Sections VIII.3.a.i and VIII.3.a.ii contain summaries of the pipeline and calibration changes that were made for second-pass and first-pass Post-Cryo processing, respectively. These modifications deal primarily with eliminating W3 and W4 from all aspects of the processing and accommodating the changes to and time-dependence of the performance of the W1 and W2 detectors during the Post-Cryo survey phase. Changes to the individual modules, as well as known limitations in the processing and features of the Post-Cryo Release data products are described in more detail in sections VIII.3.b-d. Modules that are not highlighted in the following subsections are largely unchanged from WSDS v4.

a. Summary of Pipeline Modifications for Post-Cryo Processing

i. WSDS v5.0 Updates for Second-Pass Post-Cryo Processing

• All functionality that relies on W3 and W4 data was removed from the pipeline science modules and pipeline wrappers. Some 4-character string quality flags, where one character corresponds to each of the four WISE-bands, were left in the extracted source record outputs (e.g. the contamination and confusion flag cc_flags). The third and fourth character in these strings is always set to the null equivalent, usually "zero".


• Infrastructure for handling time-dependent parameter file input for all science modules was introduced. This was generally handled by defining discrete calibration intervals during which calibration products and module parameters are fixed. Different modules utilized different intervals.


• New and time-dependent calibrations were developed and utilized in the Single-Exposure Image Calibration (VIII.3.b), Position Reconstruction (VIII.3.c), and Photometric Calibration (VIII.3.e). These better responded to the changing payload characteristics as the telescope and detectors warmed during the Post-Cryo survey phase.


• The dynamic sky-offset correction algorithm in the Single-exposure instrumental calibration module was updated to eliminate the spike in pixel values that resulted in systematically high background measurements. The overestimated backgrounds induced part of the systematic source flux underestimation bias that is present in the All-Sky and 3-Band Cryo source Catalog and Databases. Therefore, this bias is largely absent in the 2013 Post-Cryo Release Single-exposure Source Database photometry. See VIII.3.b.vii for details.


• Saturated pixel tagging was recovered for bright point sources using prior knowledge of the position and flux from the All-Sky Catalog and external sources. This methodology was developed for second-pass processing for the WISE 3-Band Cryo data and is described in VII.3.b.iv.1.


• The threshold for source detection (IV.4.b.i) in the Scan/Frame Pipeline was lowered to SNR>3.0 to improve the detectability of asteroids and other moving solar system objects following the loss of the W3 and W4 bands.


• Recursive background fitting was introduced in profile-fit photometry source extraction module to provide more robust background estimation in crowded regions. This also mitigated part of the systematic source flux underestimation bias that is present in the the second-pass processing that generated the All-Sky and 3-Band Cryo source Catalog and Databases that produces the source density-dependent bias.


• Artifact flagging parameters were updated to correspond better to the detector performance during the Post-Cryo survey phase. See VIII.3.d.


• The lists of orbital elements used in the known solar system object association module were updated to be comprehensive at the time of the start of second-pass Post-Cryo processing. The lists included 333,273 numbered asteroids, 182,744 unnumbered asteroids, 579 periodic comets, 2,543 non-periodic comets, the 5 planets observed during the post-cryo survey phase, and 167 planetary satellites.


• Selected Quality Assurance scoring threshold parameters were adjusted to account the missing W3 and W4 data, and for the elevated number of noisy pixels present in the Post-Cryo data (VIII.3.f).

ii. WSDS v3.5 Updates for First-Pass Post-Cryo Processing

• All functionality that relies on and W4 data was removed from the pipeline science modules and pipeline wrappers. Some 4-character string quality flags, where one character corresponds to each of the four WISE-bands, were left in the extracted source record outputs (e.g. the contamination and confusion flag cc_flags). The third and fourth character in these strings is always set to the null equivalent, usually "zero".


• Updates were made to those pipeline and wrapper operating parameters necessary to have Post-Cryo successfully complete processing. These updates affected primarily data handling and accounting and not actual algorithm parameters.


• The basic W1 and W2 image calibration products, including dark and flat-field responsivity images, and pixel bit masks, were updated approximately five weeks after the start of Post-Cryo first-pass processing (VIII.3.b). Because the reductions of the first five weeks of image data were done using calibrations carried from the end of the 3-Band Cryo survey period, there is a discontinuity in the quality of the calibrations beginning with the data in scan 09760a taken on 11/5/2010 UTC. This discontinuity can be seen clearly in the scatter of standard star photometry residuals illustrated in Figures 5 and 6 in VIII.3.e.iv.


• The threshold for source detection (IV.4.b.i) in the Scan/Frame Pipeline was lowered to SNR>4.0 in first-pass processing to improve the detectability of asteroids following the loss of the W3 and W4 bands for the Moving Object Pipeline that was run during first-pass processing while WISE was still collecting data.


• Selected Quality Assurance scoring threshold parameters were adjusted to account the missing W3 and W4 data, and for the elevated number of noisy pixels present in the Post-Cryo data (VIII.3.f).

Last Update: 2013 June 5