VI. Analysis of Release Products

4. Photometric Properties

d. Characteristic Source Brightness for the Onset of Saturation

Summary

i. Saturated Source Characterization

Although WPHOT returns a flux measurement saturated sources, the accuracy of the measurements may be degraded. Photometric uncertainties are elevated at the onset of saturation, and other biases lead to systematic errors in the estimated flux levels as saturation intensifies.

During the measurement of each source, WPHOT determines the fraction of pixels that are saturated within the fitting region, and reports the values in the w?sat fields of the source Catalog record. Saturated pixels are identified by WPHOT using the single-exposure bit-masks that capture the DEB saturated-pixel encoding provided in the raw payload image data. A pixel may saturate because of the presence of a bright astrophysical source, or from the strike of an energetic cosmic ray. We make use of these saturated pixel statistics to determine the characteristic brightness at which point sources reach saturation and at which WPHOT is unable to return a useful magnitude.

ii. Saturation as a Function of Brightness

1. Source Catalog

Figures 1-4 show differential source counts near the saturations levels from the Source Catalog. In each of these diagrams, differential source counts are plotted as a function of profile-fit magnitude, w?mpro. The black curves represent all sources, the green curves are sources that have w?sat=0 and the red curves are sources that have w?sat>0. W?sat values are computed in the Multiframe Pipeline version of WPHOT (IV.5.c) using the pixels in all of the frames in the "stack" that contribute to a source measurement in a particular band.

The onset of saturation for point sources is indicated by the transition between the curves for saturated and non-saturated sources. The saturation boundary is well-defined but not perfectly sharp because the sub-pixel variations in the location of a source near the saturation brightness may affect whether or not it saturates a pixel. Approximately 1% of sources fainter than the nominal saturation boundaries have w?sat>0 because of charged particle strikes within the fitting area of a source.

The characteristic saturation level for point sources in each band can be defined as the point at which 50% of sources have a non-zero fraction of saturated sources. Figure 5 shows for each band in the Source Catalog the differential fraction of saturated sources as a function of profile-fit magnitude in that band. In Table 1 are listed the magnitudes at which 50% of the sources have non-zero saturated pixel fractions for each band, along with the implied flux density assuming the values for absolute flux calibration given in the notes to the Table.

Figure 1 - W1 Source Catalog differential source counts	Figure 2 - W2 Source Catalog differential source counts
Figure 3 - W3 Source Catalog differential source counts	Figure 4 - W4 Source Catalog differential source counts
Black curves represent total differential source counts for each band from the Catalog. Green curves represent source counts for sources that do not have any saturated pixels in the profile-fitting region. Red curves represent source counts for sources with one or more saturated pixels in the profile-fitting region.

Figure 5 - Fraction of Catalog sources with one or more saturated pixels in the profile-fitting region, plotted as a function of profile-fit brightness for multiframe processing. W1, W2, W3 and W4 are represented by the black, blue, green and red curves, respectively. The horizontal magenta line marks the 50% saturated source fraction.

2. Single-Exposure Source Database

Figures 6-9 illustrate the differential source counts near the expected saturation brightness levels from the Preliminary Release Single-exposure Source Database. The brightness range for the transition from non-saturated to saturated is broader in the Single-exposure source measurements than the Catalog because the source sub-pixel centers are not "homogenized" when using only one frame in the measurement.

Figure 10 shows for each band the differential fraction of saturated sources in the Single-exposure Source Database as a function of profile-fit magnitude in that band. In Table 2 are listed the approximate magnitudes of 50% saturation fraction for each band, along with the implied flux density assuming the values for absolute flux calibration given in the notes to the Table. As seen in the distributions in Figure 6-9, the observed saturation levels span a range as much as +/- 1 magnitude around these values.

Figure 6 - W1 Single-exposure Source Database differential source counts	Figure 7 - W2 Single-exposure Source Database differential source counts
Figure 8 - W3 Single-exposure Source Database differential source counts	Figure 9 - W4 Single-exposure Source Database differential source counts
Black curves represent total differential source counts for each band from the Level 1b source Working Database. Green curves represent source counts for sources that do not have any saturated pixels in the profile-fitting region. Red curves represent source counts for sources with one or more saturated pixels in the profile-fitting region.

Figure 10 - Fraction of sources with one or more saturated pixels in the profile-fitting region, plotted as a function of profile-fit brightness from scan-frame processing. W1, W2, W3 and W4 are represented by the black, blue, green and red curves, respectively. The horizontal magenta line marks the 50% saturated source fraction.

3. Bright Source Photometry Limits

The source count distributions in Figures 1-3 exhibit a cut-off at the bright-end where profile-fitting photometry in first-pass processing becomes unable to extract a measurement because too few non-saturated pixels are available in the source profile wings. These cut-offs occur near 1.0, 0.0 and -2.0 mag (123, 172 and 200 Jy) in W1, W2 and W3, respectively. There does not appear to be an obvious bright-end cut-off in the W4 counts, although the curve may be steepening brighter than approximately -6 mag (2100 Jy).

Estimates of the number of objects on the sky that will fail to have a magnitude reported in the WISE Source Catalog and Single-exposure Source Database can be obtained using the IRAS and 2MASS Point Source Catalogs. There are 256 and 24 sources in the full IRAS PSC that have F₁₂>200 Jy and F₂₅>2100, Jy, respectively. There are 791 and 236 sources in the full 2MASS PSC that have K_s= +1 and 0 mag, respectively, assuming the bluest objects will have K_s-W1 and K_s-W2 colors of approximately zero mag.

Last update: 2011 April 28