Photometric calibration of the 3-Band Cryo measurements followed the same basic approach that was used for the full cryogenic survey data described in IV.4.h. The most significant change introduced for calibration of the 3-Band Cryo data is the time-dependent Single-exposure instrumental zero points that adapt to the changing system throughput as the telescope and focal planes warmed. A constant Single-exposure instrumental zero point magnitude in each band was sufficient for the full cryogenic survey data because of the stability of the system during that period (IV.4.h.iv). Derivation of the time-dependent instrumental zero points is described in VII.3.g.ii, and the zero points are illustrated in VII.3.g.iii.
As with the full cryogenic survey phase data in the All-Sky Release Source Catalog, Reject Table, and Atlas Images, 3-Band Cryo Source Database and Atlas Image photometry is calibrated using a constant instrumental zero point in each band. The Single-exposure images are scaled to a common gain in the throughput matching stage during Atlas Image generation in the Multiframe Pipeline processing. The same instrumental zero points are used for the 3-Band Source Database and Atlas that were used for the All-Sky Release Source Catalog and Atlas Images. The values are 20.5, 19.5 and 18.0 mag in W1, W2 and W3, respectively.
Instrumental zero points were derived for the Single-exposure data in the full cryogenic survey phase using the measurements of calibration sources observed in each survey scan. For the 3-Band Cryo survey phase, photometric calibration was tied directly to the full cryogenic phase by computing the trimmed average difference between the 3-Band Cryo measurements of all high SNR sources in each scan and their associated measuremetns in the All-Sky Release Catalog.
The time-dependence of the 3-Band Cryo Single-exposure zero points was modeled using a subset of scans that spanned the 3-Band Cryo phase. Instrumental zero points were determined for the subset of scans, then linearly interpolated in time to determine the zero points for the remaining scans. Zero point changes were relatively small and smooth in W1 and W2. The W3 zero point changed dramatically during the 3-Band Cryo period, and the changes were further complicated by the exposure time changes used for that band. Throughput changes due to the exposure time changes were accounted for in the W3 instrumental zero points rather than by adjusting the measured fluxes.
The 3-Band Cryo Single-exposure instrumental zero point magnitudes are plotted as a function of time in Figures 1a-c.
The changes in W3 exposure time are marked in Figure 1c with vertical colored lines. Red marks the start of the 3-Band Cryo phase, blue marks the start of the 4.4s exposure time, green marks the start of 2.2s exposure time and cyan marks the start of 1.1s exposure time.
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| Figure 1a - W1 | Figure 1b - W2 | Figure 1c - W3 |
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The trimmed average photmetric residuals between the All-Sky Catalog and 3-Band Cryo Single-exposure source photometry in each scan, plotted as a function of time are shown in Figures 2a-c. The high frequency structure in those diagrams illustrate the periods when the throughput was changes faster than the time resolution of the model could track.
CAUTION: The 3-Band Cryo Single-exposure Images and Source Database photometry will exhibit these biases with respect to the All-Sky Atlas and Catalog. However, compensation for the residual bias were made in the 3-Band Cryo Multiframe Pipeline Atlas Image generation (VII.3.e.iii), so the residual bias in the 3-Band Cryo Atlas and Source Working Database will be smaller.
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| Figure 2a - W1 | Figure 2b - W2 | Figure 2c - W3 |
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Last update: 2012 August 21