The calibration models for the sky mapper (SM) and astro field (AF) CCDs must
be sufficiently detailed and flexible to allow obtaining optimal image
parameters for astrometry and band photometry, yet not so complex that it
becomes unrealistic to carry out the calibrations. These devices are physically
almost identical, but, from the way they are operated, they fall in three
The SM CCDs provide large ( pixel; Table 1.1) images around each source, with
the purpose to eventually map the surroundings. The SM CCDs are read in full
image mode and with a pixel binning. They have therefore a high
readout noise, and are in reality under-sampled. For bright sources (
mag), images saturate, while for the fainter sources ( mag), a further
binning is applied before sending the data to ground (Table 1.1). Image
parameters for SM therefore have only little or no weight in astrometry
and photometry, but the devices must be calibrated to facilitate their future
The first AF CCD, AF1, serves the purpose of confirming the detections from SM,
in order to censor detections caused by for example cosmic rays. All windows are
therefore read with 2D resolution ( pixel; Table 1.1), at the price of a higher
readout noise due to the larger number of samples. To save telemetry, the
windows sent to ground for fainter sources ( mag) have their samples
co-added in each line, and lose again their AC resolution (Table 1.1).
The following AF devices, AF2–9, are the workhorses of astrometry and band photometry. They are the CCDs providing the highest potential and
therefore the ones with the more demanding calibration requirements.
A limitation of the calibration models in place at the present stage of the
mission is that they are focused on the treatment of isolated point sources,
and they may not be fully applicable to more complex sources.