Local sky background levels for AllWISE source flux measurements are estimated in each band using the pixels in annular regions with inner and outer radii of 50 and 70 arcsec around each detected source, as was done for background estimation in the All-Sky Data Release. However, the mode background estimation algorithm that was used for the All-Sky Release processing was found to not be optimal in bands W2, W3 and W4 after the reprocessing of the Single-exposure images to correct the pixel histogram "spike", and after other AllWISE source extraction modifications.
Specifically, the mode estimation technique resulted in consistently underestimating the background in W2, W3 and W4 AllWISE measurements, which led to systematic source flux overestimation that increased towards fainter flux levels. Several alternative statistical techniques were tested by comparing the resulting AllWISE flux measurements with measurements from the Spitzer Enhanced Imaging Source Lists. It was found that a robust trimmed median estimator produced the best agreement over all flux levels between the AllWISE and Spitzer Source List photometry in W2, W3 and W4. "Robust" here means stable against the presence of stars in the background annulus. The original mode estimation continued to produce the best agreement between AllWISE and Spitzer in W1.
The background estimation algorithm selected for AllWISE source extraction was therefore the original mode in W1, and the robust trimmed median in W2, W3 and W4.
The uncertainty in the background estimation and the confusion noise components for the AllWISE photometric error model are computed the same way as they were for the All-Sky Release process, as described in section IV.4.c.ii.3 of the All-Sky Release Explanatory Supplement.
The local background estimation measurements, uncertainties and confusion noise estimates in each band carried in the AllWISE Catalog and Reject table entries are:
where ?=1,2,3,4 for W1, W2, W3 and W4.
Last update: 12 November 2013