For 70 µm imaging of a source up to ~ 2´ in diameter, the limited motion possible with the scan mirror requires that the source be referenced to sky in two ''halves,'' as illustrated in Figure 3.5. As above, 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 side A, and with the amplitudes of the offsets correspondingly scaled down in the cross-scan direction.
Figure 3.5: Photometry of a large source at 70 µm. The source is observed such that the coordinates are placed near the array center in the pattern shown. Spacecraft and scan mirror motions are made in such a way that the sky background is observed in two halves on both sides of the image in the in-scan direction. Note that for simplicity, the array is portrayed as a square; in-orbit realities mean that only half of the array is functional, and the pattern as portrayed here is centered on side A.
The scan mirror starts near one end of its travel within the large-scale 70 µm range, and the spacecraft is pointed to place the source 2.5 pixels below the center of the array and 0.5 pixel to one side of the center of side A. The scan mirror is set to have a throw of 2´, and the first pair of exposures is taken at the starting point (target image) and 2´ away in-scan (sky image). A second target - sky pair is obtained with the scan mirror advanced 2.5 pixels (24.6´´), and a third pair obtained after advancing another 2.5 pixels. Next, the spacecraft is maneuvered 1.25 pixels in cross-scan and 2´ in-scan, and the scan mirror is then used to obtain another three pairs of exposures, starting with a sky image, then chopping back to get the target. The overall sky coverage is illustrated in Figure 3.6.
Figure 3.6: Simulated source detections on the array during the large source photometry AOT shown in Figure 3.5. Time runs across the top row from left to right, then across the second row from left to right, and so forth. The observations start with an image in the sky position, then a stimulator flash, then the first source image. Three on source-off source pairs of images are taken, then a final stimulator flash. The spacecraft is then offset, there is a stimulator flash, three more on-off image pairs are taken, and the cycle is completed with a final stimulator flash. In repeat cycles of the AOT, frames 1 and 7 are omitted, so the AOT basically provides 6 source-sky image pairs per cycle. Note that the source stays on side A of the array.
Note that the viable part of the array is a 2.5´x5´ rectangle, but since the orientation of the field of view on the sky is a function of time and generally observers could not specify the time of their observation, the largest object that should have been observed using this mode is <2´.
Because of the smaller effective size of the 70 µm array compared to previous expectations, some users needed to efficiently map the approximately 5´x5´ (2.5´x2.5´ fine scale) area covered by the original large-field AOT. In order to do this, we suggested that the user create a cluster target with offsets of (0, +80´´), (0, -80´´) in array coordinates observing ''offsets only.'' This allowed recovery of the full area of the original AOT while saving the slew tax from mapping with 2 AORs.
Figure 3.7: Positions of the source relative to the 160 µm array during the compact source photometry AOT. The scan mirror positions and spacecraft offset (between frames 8 and 9) provide sampling of the image at ½ pixel offsets in both dimensions. A single cycle of the AOT provides only 2 complete images of the source because the array contains only 2 rows of pixels. Observers should request a minimum of 2 cycles to obtain at least minimal data redundancy. Frames 1 and 9 are omitted from repeat cycles of the AOT.