II. AllWISE Source Catalog and Reject Table

II.3. Photometric Properties

II.3.b. Photometric Accuracy

Unlike the All-Sky data release, there is no formal requirement for photometric accuracy in the ALLWISE data; nevertheless, the overall results are outstanding and would in fact satisfy the All-Sky requirement.

We will be assessing the photometric accuracy of the WPRO (profile-fitting) photometry of bright stars using the internal repeatability of WPRO measurements. However, in order to draw the sample to study, we will be using the S/N as given by the standard aperture photometry. The reason is that the WPRO S/N includes a contribution due to the uncertainty in the Point Spread Function (PSF), essentially flooring all S/N values to be less than S/N=60 or so; whereas, the aperture photometry provides a reasonable estimate of source's S/N in a given band.

II.3.b.1. Highlighted Sample

For detailed study (Figures 1-4, presented below), we will first draw a clean sample of bright starsfrom high galactic latitudes, thus minimizing the effects of stellar confusion. Bulge. We then draw samples from different latitudes, stepping all the way down to the Galactic Plane. The results are presented below.

Aperture photometry magnitude uncertainties: w?sigm < 0.01086 mag (S/N > 100)
Avoid saturation, artifacts, blending, etc
Avoid upper limits
Apply w?rchi2 threshold to avoid extended sources and poor WPRO photometry
WISE Pass 2 3-Band Cryo Multiframe Source Table

For example, the SQL command for W1 (band-1) extractions is as follows:

WHERE (cat = 1 and glat > 60 and glat < 75 and (wx >= 36.4 and wx <= 4058.6 and wy >= 36.4 and wy <= 4058.6) and use_src = 1 and nb = 1 and w1frtr = 0 and w1rchi2 <= 2 and w1sat = 0 and cc_flags[1] not matches '[DHOP]' and w1sigmpro < 0.02171 and w1sigm < 0.01086) ' The extracted photometry for the unsaturated and bright sources is presented below.

II.3.b.2. Internal Repeatability

The WISE photometric accuracy is assessed using the internal repeatability of WPRO measurements. For a given source found on M individual frames (typically ~12), WPRO estimates the flux for each one and the distribution statistics are part of the extracted information (per band) for the source. The reported statistics include weighted mean, the unbiased weighted sample variance (re: population variance) and the standard error of the mean. Internal repeatability is a proxy for photometric accuracy, although it is not sensitive to confusion noise. For our purposes here, confusion is not an important contributor to the flux uncertainty.

The W1 repeatability results for the bright S/N > 100 sample are shown in Figure 1. The panels show, per band, the standard deviation in the population (parameter w?sigP1), representing the RMS of WPRO measurements for one frame. The results show that the photometric accuracy for each band, as indicated by the repeatability RMS (w?sigP1), is ~2 to 3%, well within the ALLSky 4-band Cryo requirement of 7% for bright S/N > 100 sources.

Figure 1: High Galactic Latitude Sky: 60 > GLAT < 75 deg

Figure 1 - WPRO internal repeatability RMS for low source densities: W1 (upper left), W2 (upper right), W3 (lower left), W4 (lower right). The red dashed line demarks the 4-Band Cryo (All Sky) Photometric Accuracy requirement, and the blue dashed horizontal line the achieved accuracy (mean for all bright sources).

Figure 2: Moderate Galactic Latitude Sky: 18 < Glat < 23 deg

Figure 2 - WPRO internal repeatability RMS for moderate source densities.

Figure 3: Low Galactic Latitude Sky: 9 < Glat < 12 deg

Figure 3 - WPRO internal repeatability RMS for high source densities.

Figure 4: Galactic Plane: Glat = 0.5 deg

Figure 4 - WPRO internal repeatability RMS for high source confusion.

**Table 1** – Summary of Photometric Repeatability for Large Regions of the Sky

II.3.b.3. Summary

Observed WPRO repeatability satisfies the nominal ALLSKY relative photometric accuracy requirement.

Comparison of WISE W1 and W2 with Spitzer photometry (S/N > 100) is consistent with "accurate" WISE photometry, exceeding the Level-1 requirement.

Last update: 22 November 2013