The relation between the rows and columns for the four CCDs is shown in Figure 1.1, where the short arrows on each CCD are placed near pixel (1,1) and point in the -Y (readout) direction. Each CCD's axes are defined by a 90° rotation from the adjacent CCD. If a 4-CCD image is taken and then each subimage is displayed with rows in the "X" direction and columns in the "Y" direction, each successive display would appear rotated by 90° from its predecessor.
Figure 1.1 also illustrates the projected orientation of the WFPC2 CCDs onto the sky. The beam is split between the four cameras by a pyramid-shaped mirror in the aberrated HST focal plane. In an effort to insure images from the four CCDs can be reassembled into a single image without gaps, there is a small overlap region on the sky between each CCD and its neighbors (see also Figure 3.12). On the CCDs this region appears as a blank "shadow" region along the X~0 and Y~0 edges of each CCD; the exact limits of this region are given in Table 2.5 for each CCD. Because the OTA beam is aberrated at the pyramid mirror, the edges of the shadow region are not sharp, but instead there is a gradual transition from zero to full illumination on each CCD. The width of this vignetted region is essentially that of the aberrated OTA beam (~5"). Table 2.5 gives approximate limits of this vignetted region on each CCD. Note that astronomical sources in the vignetted region are imaged onto two or more CCDs.
The WFPC2 has two readout formats: full single pixel resolution (FULL Mode), and 2x2 pixel summation (AREA Mode which is obtained by specifying the optional parameter SUM=2x2 as described in the Proposal Instructions). Each line of science data is started with two words of engineering data, followed by 800 (FULL) or 400 (AREA) 16-bit positive numbers as read from the CCDs (with 12 significant bits). In FULL Mode the CCD pixels are followed by 11 "bias" words ("over-clocked" pixels), yielding a total of 813 words per line for 800 lines. In AREA Mode, there are 14 bias words giving a total of 416 words per line for 400 lines. Either pixel format may be used to read out the WFC or PC. These outputs are reformatted into the science image and extracted engineering (over-clocked) data files during processing in the HST ground system prior to delivery to the observer. Note that calibration support for AREA Mode data has been curtailed since Cycle 10, since this mode is very seldom used.
The advantage of the AREA Mode (2x2) on-chip pixel summation is that readout noise is maintained at 5 e- RMS for the summed (i.e., larger) pixels. This pixel summation is useful for some photometric observations of extended sources particularly in the UV. Note, however, that cosmic ray removal is more difficult in AREA Mode.
The readout direction along the columns of each CCD is indicated by the small arrows near the center of each camera field in Figure 1.1 (see also Figure 3.12). Columns and rows are parallel and orthogonal to the arrow, respectively. Each CCD is read out from the corner nearest the center of the diagram, with column (pixel) and row (line) numbers increasing from the diagram center. In a saturated exposure, blooming will occur almost exclusively along the columns because of the MPP operating mode of the CCDs. Diffraction spikes caused by the Optical Telescope Assembly and by the internal Cassegrain optics of the WFPC2 are at 45° to the edges of the CCDs. Unless specified otherwise in the Phase II proposal, the default pointing position when all 4 CCDs are used is on WF3, approximately 10" along each axis from the origin (WFALL aperture, see Table 3.14).
Observations which require only the field-of-view of a single CCD are best made with the target placed near the center of a single CCD rather than near the center of the 4 CCD mosaic. This results in a marginally better point spread function, and avoids photometric, astrometric, and cosmetic problems in the vicinity of the target caused by the overlap of the cameras. Even so, for such observations the default operational mode is to read out all four CCDs. This policy has resulted in serendipitous discoveries, and sometimes the recovery of useful observations despite pointing or coordinate errors.
On the other hand, any combination of 1, 2 or 3 CCDs may be read out in numerical order (as specified in the Proposal Instructions). This partial readout capability is not generally available to GOs, although it can be used if data volume constraints mandate it, after discussion with the WFPC2 instrument scientists. It does not result in a decrease in the readout overhead time but does conserve space on the HST on-board science data recorders. This was especially useful with the initial science tape recorder, which had a capacity slightly over 7 full (4-CCD) WFPC2 observations or 18 single CCD WFPC2 observations on a single tape recorder side (of two sides). Readout of only a subset of the WFPC2 CCDs, or use of AREA mode, was advantageous when many frames needed to be obtained in rapid succession. However, the new Solid State Recorders installed during the 1997 and 1999 servicing missions are capable of holding well over one hundred 4-CCD WFPC2 images. This capability was phased in during Cycle 7, and has lead to relaxation of the above data rate restrictions.
Multiple exposures may be obtained with or without interleaved spacecraft repointings and filter changes without reading the CCDs (READ=NO). These would then be followed by a readout (READ=YES). Note that WFPC2 must be read out at least once per orbit.
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