One cycle of the basic 24 micron photometry observation acquires 14 separate images of a source. The locations of these 14 images are illustrated in Figure 3.1 and listed in Table 3.1. To start the observation sequence, the source is centered to the left of the array center by 25 pixels (65'') and the scanning mirror is chopped in a nearly symmetrical fashion, above and below the array center, to obtain a column of 7 images. The spacecraft is then slewed to position the source 25.5 pixels (66.3'') to the right of center and another column of 7 images is obtained. Table 3.1 contains the resulting approximate image positions in units of pixels, with the origin taken to be the center of the array. Array distortion and the twist of the scanning mirror with respect to the array columns have not been taken into account in this table, but the positions are good enough for observation planning purposes.
The observer specifies only one parameter, the number of observing cycles. If the observer specifies N cycles, the sequence repeats the first column N times before the spacecraft moves to take N sets of data in the second column. There are two complications to this simple scenario: (1) two extra exposures are obtained at positions 1 and 8 for each AOR, and (2) exposures 1 and 8 of the first cycle (and a few subsequent cycles in long AORs) are one second shorter than the specified exposure time. Spot takes this all into account when it calculates the total integration time, but it is approximately ((Nx14)+2) x (Exposure Time)-2 seconds. The 2 sec reduction comes about because the first second of frames 1 and 8 are used for obtaining calibration data, not science data.
Figure 3.1: Photometry/Super Resolution for compact sources with the 24 micron array. The positions of images in a single cycle of a 24 micron photometry observation are shown relative to the central 64x64 pixels (shown schematically by the grid) of the full 128x128 array. The circle diameters are the FWHM of the Airy disk. The first set of 7 images are dithered using the scan mirror, then the spacecraft is offset by a half-integer number of pixels, and the second 7 images are acquired. Frames 1 and 8 of the first cycle of an AOR (and a few subsequent cycles in long AORs) are 1 sec shorter than the observer-requested exposure time; the other 12 frames of such cycles are taken with the full observer-specified exposure time. Not shown are 2 additional frames, taken at the positions 1 and 8, obtained for each AOR.
Note that half of the images are obtained at each spacecraft pointing. The telescope relative offset accuracy allows meaningful fractional pixel offsets. Moreover, in practice, alignment tolerances make the chop motion slightly non-parallel to the array columns. In any case, the implemented pattern provides a well-sampled Airy disk. Although for simple photometry, the images can be combined by integer shifts, careful processing will be required to take full advantage of the oversampling implicit in these images.
Table 3.1: Source positions for 24 μm compact source photometry (in units of pixels, the origin being the center of the array).
Frame #
X position
Y position
1
-25.0
16.7
2
-25.0
-30.5
3
-25.0
21.3
4
-25.0
-25.9
5
-25.0
25.9
6
-25.0
-21.3
7
-25.0
30.5
8
25.5
16.7
9
25.5
-30.5
10
25.5
21.3
11
25.5
-25.9
12
25.5
25.9
13
25.5
-21.3
14
25.5
30.5
Note also that the final highest-S/N region of the combined mosaic is not a full FOV, but rather a ~3' square area.
