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IV. NEOWISE Data Processing

IV.2. Scan/Frame Pipeline Updates

IV.2.b. Source Extraction

IV.2.b.ii. Astrometry

IV.2.b.ii.1. 2MASS + UCAC Proper Motions

NEOWISE (and previously released WISE) image and extracted source products are astrometrically tied to a bright, clean subset of the 2MASS PSC referred to as 2MRef (see IV.4.d.i in the WISE All-Sky Release Explanatory Supplement). Because there is a 10-25 year difference between the 2MASS and WISE/NEOWISE observation epochs, source motion must be accounted for if the resulting NEOWISE reference frame is to be inertial. WISE primary mission Single-exposure and Atlas release products, in fact, display minor reference frame distortion caused by solar apex motion (see section VI.4 in the All-Sky Explanatory Supplement).

The motion of the astrometric reference frame stars was compensated for in the AllWISE data processing with an on-the-fly proper motion correction during the coadd process section (see II.5 in the AllWISE Explanatory Supplement). However, source extractions from the underlying Single-exposures did not benefit from this correction.

For NEOWISE, we account for astrometric reference star motions by associating 2MRef sources to the UCAC4 catalog (Zacharias, N. et al. 2013) and use the UCAC4 proper motions to bring the 2MRef sources and NEOWISE extractions to a common epoch, tying NEOWISE Single-exposures to an inertial reference frame.

NEOWISE positions do not need to be corrected for the motion of the reference frame. Because this correction was not applied during position reconstruction from the Single-exposure detections from the primary WISE mission, you may see small systematic offsets between NEOWISE positions and those from the earlier data sets.

IV.2.b.ii.2. Updated Band-to-band Offsets and Distortion Maps

For NEOWISE, the stable offset between the W1 and W2 detectors was re-evaluated and new band-to-band corrections were employed in frame position reconstruction.

The distortion properties of each detector were also re-evaluated and new distortion maps were computed. For the WISE primary mission 3-Band cryo and Post-cryo periods, the detector temperatures were changing so rapidly that time-dependent distortion maps were applied. Temperature changes during the NEOWISE survey are relatively small, and changes in distortion even at the beginning of the survey were correspondingly small. Therefore, for NEOWISE only a single distortion map for each band is used.

Last update: 29 January 2023

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