Instrumental characterization data is acquired during nearly every night of 2MASS operations. These data include series of dark measurements (frames acquired with a cold shutter obscuring the detectors), and relative pixel responsivity measurements (flat-fields) made of the rapidly dimming or brightening twilight sky.
Nightly bias correction images in each band are generated in the pipeline processing by combining all of the dark sequence frames. Responsivity images (multiplicative gain corrections) are derived from the measurements of the twilight sky by charting the relative change in intensity seen in every pixel in response to the changing illumination level of the twilight sky. The resulting pixel-by-pixel responsivity images are normalized to have a median of unity. Each nightly responsivity image in each band is compared to a running mean of the responsivity maps from the previous five nights. If the nightly flat-field is in good agreement with the running "canonical" flats, the new measurements are averaged in to generate new "canonicals". If the nightly responsivity measurement deviates from the running average, such as might occur if clouds contaminate the twilight measurements, the nightly measurements are rejected and not added to the "canonical" responsivity images.
The nightly dark and responsivity measurements allow the 2MASS detector systems stability and performance to be monitored with unprecedented accuracy. Deviations as small as a few percent from long-term mean dark and response "canonical" are easily detected.
For each "Read 1" (R1) and "Read 2-Read 1" (R2-R1) data frame in a scan, the appropriate nightly dark frame is subtracted, and the corresponding average "canonical" responsivity image is divided into it. Within each scan, a series of additive sky illumination corrections are derived by creating -trimmed averages for blocks of at least 42 dark-subtracted, flat-fielded sky frames. The trimmed averaging rejects any sources within the frames and yields a measurement of residual dark-sky illumination patterns on the detectors within each block. This so-called "sky offset" frame is then subtracted from each input frame, resulting in a data frame ready for source detection and combination into the final survey Atlas Images. The background levels of the final instrumentally calibrated frames correspond to the original sky levels.