At 160 µm, the array width is less than the FWHM of the Airy pattern and is unfilled (2x20 pixel rows with a one-pixel-wide blank row between), so a series of frames must be taken to build up an image of the source. As a result of the on-orbit realities, we made changes to our pre-launch plans for this mode; it resembles the pre-launch AOT, but shifted to the side away from the inoperative readout and with the amplitudes of the offsets correspondingly scaled down in the cross-scan direction.
The sequence currently implemented is illustrated in Figure 3.7. At the beginning of the observation, the scan mirror is near the center of its range, and the telescope is used to center the source halfway between the two rows of the array and offset to one side in the cross-scan direction (pixel position ([-6.5, 0.0]). An image is taken, and is followed by a stimulator flash image. The scan mirror is then used to build up a filled image by moving the source to pixel positions [-6.5, -2.0], [-6.5, +1.0], [-6.5, -1.0], and [-6.5, 2.0]. A final image is taken at the starting position, and is followed by a stimulator flash image.
The telescope then nods to place the source on the other half of the best side of the array, and offset by ½ pixel in the in-scan direction (pixel position [-3.5, 0.5]). The above sequence of images and scan mirror motions is then repeated, completing one cycle of the AOT. The relative positions of these images are chosen to provide ½ pixel sampling of the PSF in both the scan and cross-scan directions. If multiple AOT cycles are needed, several sets of frames will be taken at one nod position before switching to the other position - this is transparent to the observer. The basic AOT cycle provides only two complete images of the target when all dither positions are mosaicked together, with an extra pair of sub-images on the first (and perhaps a few later) cycles.
Figure 3.8: Simulated visualization of individual frames in the first cycle of the 160 µm small source photometry AOT shown in Figure 3.7 (upper panels), and a full mosaic of those frames (lower panel) with the 2 source images coadded. Note that pixels in the lower panel are shown twice as large as they are in the upper panels. Numbering along the sides shows the positions of the rows of the array during the execution of the AOT. Note that the source is offset in the final mosaic as a result of the AOT modifications to accommodate the dead readout (not shown).
It was strongly recommended that observers specify at least two cycles of the 160 µm small field photometry AOT (i.e., 4 complete images) in order to provide the minimum level of redundancy. Observers were also encouraged to use the new enhanced mode (see below) which became available in Cycle 4, particularly for point source photometry.
Figure 3.8 illustrates the individual pointings for a complete cycle, and a coadded mosaic (with the two individual images coadded in the central region of the mosaic). As can be seen in the figure, the AOT provides in the end an image of the target that is offset, i.e., more of the background sky is observed on one side of the target than the other.