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IRAC: AOT Examples |
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In this section, we present some completely worked examples of IRAC
observations. For more complete information, consult the Spitzer Observer's Manual. Further
detailed examples are found in the Spitzer Observation Planning Cookbook.
Example: Shallow SurveyThe goal of a shallow survey is to cover sky rapidly, while maintaining al of a shallow survey is to cover sky rapidly, while maintaining some redundancy in order to reject cosmic rays and reduce effects of pixel-to-pixel gain variations. For this survey, we request coverage of our target region with all 4 IRAC bands. The survey will be conducted with a rectangular grid, with the grid steps aligned with the focal plane array in order to make coverage uniform. We will step by about 95% of the array width for each grid step; specifically we will use a step size of 292.8 arcsec =244 pixels. At each map grid point we will observe 3 dither positions using the cycling dither table. Because we used only a small overlap between map grid positions, we want to constrain the dithers to keep the coverage as uniform as possible, but we also want to separate large-scale photometric variations from sky variations, so we choose the medium dither pattern scale factor as a compromise. The frame time at each position is 30 sec.The observation described here could be used as a "tile" for a survey of a larger area. Suppose you want approximately 9 times the area, so that your survey region could be broken into a 3x3 set of these "tiles." The entire observation could not be done in a single AOR, because it would take longer than the 8 hr maximum duration of an AOR. To implement this large survey, you would generate 9 identical AORs and constrain them to occur within a reasonably short period of time (in order to keep the relative roll angle between the AORs small). Further, you would specify offsets for the center of each AOR. You will need to specify, in addition to the AOR parameters already described, the array coordinate offsets for each AOR that would place them onto the desired grid. Thus if the two array coordinate axes are called (Y, Z) and the desired spacing between map grids is G, then the map center offsets would be (G, G), (G, 0), (G, -G), (0, G), (0, 0), (0, -G), (-G, G), (-G, 0), (-G, -G) for the nine AORs, respectively. Using the constraints editor, you would constrain that the observations all occur within a reasonable time (typically <1 week) of each other, but you should not specify the exact date.
Example: Deep ImageSuppose we want to make a sensitive image at 8 microns of an object that is less than 4 arcmin in size. In this case, we will not map, but instead will perform many dither steps in order to minimize the effect of pixel-to-pixel gain variations. For this example, we will use the 36-position Reuleaux triangle dither pattern, medium size, and 100 sec frames. The 5-sigma point source sensitivity (medium background) is 9.5 microJy at 8 microns, and the per-pixel surface brightness sensitivity is 0.05 MJy/sr at 8 microns. The same field will also be observed at 4.5 microns, and a neighboring field (not overlapping with the target field) will be observed at 3.6 and 5.8 microns.Example: Imaging an Elongated ObjectWe wish to make an image of an elongated galaxy oriented 60 degrees E of N. The galaxy has an optical size of 13.5 by 2.5 arcmin, and we want to cover about 17 arcmin with both fields of view. The galaxy is very bright, so we need to use a short frame time (12 sec will work) to avoid saturating or operating exclusively at the high end of the linearity curve. To get the desired sensitivity and source confirmation, we take 5 frames using the small scale cycling dither pattern. In this example, we will perform the observation two different ways, to compare the results.
Array coordinatesIn order to cover the desired area regardless of schedule date, we will need to make the map much larger than the galaxy size. Using the equations above, to cover 17 arcmin, we need 5 columns and 4 rows. We select both fields of view to center the image on the nucleus. The duration of the observation is approximately 2775 sec. The map grid size is equal to the array size. It is possible to make a smaller map, tailored to the size of the target, if we fix the observing date. For a very large observation, fixing the date may be the best solution. But for a small observation such as this one, a better solution is given in the next subsection.
Celestial coordinatesAnother way to observe the elongated galaxy that is both efficient and independent of scheduling constraints is to observe a map grid in celestial coordinates. To get the desired depth of coverage at each position, we could use many different options, but there is one method that will optimize the sky coverage and provide the desired redundancy with minimal overhead. We make a finely-spaced map grid that avoids holes due to the (unknown a priori) roll angle on the scheduled date and yields the desired 5 observations of each sky position. To do this, we use a map grid spacing of A/5=61.4 arcsec. The number of rows is 13. The position angle is 60 degrees E of N. We select the cycling dither pattern (small scale) with a depth of one. Making a 13 by 1 map in celestial coordinates covers the desired long axis of the galaxy, but the perpendicular coverage with the desired depth is only for a relatively narrow strip about the same size as the optical disk.
 Sky coverage for an oversampled, celestial-coordinate map of an edge-on galaxy. The map was made with a position angle 60 degrees E of N. The scale bar gives the number of times each sky position is observed.
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