IRAC: AOT Description: Readout Modes

Readout Modes and Frame Times

It is possible that bright sources will saturate the array at the frame time you choose to meet your sensitivity goal. To allow sensitive observations without losing dynamic range, we provide a high dynamic range (HDR) option. When this option is selected, the IRAC AOT will take extra frames, with frame times shorter than your selected frame time. The HDR frame times are given in the SOM. You can use the shorter frames to measure the brightness of the brighter sources and the longer frames to reach your sensitivity goal. No spacecraft repositioning is done between frames, and the frames always go from shortest to longest. If you elect to perform repeats using the "Number of Frames" entry box on the AOT form, then only the longest frame time is repeated. However, if you elect to perform dithers, then the entire frame set is repeated at each dither position. Independent of the HDR option, taking frame repeats is recommended only in the case of time series measurements. This is because the first frame effect is worse with repeats, and image artifact rejection is best accomplished with dithers. The repeated frames will need additional processing to remove residual bias patterns.

Stellar photometry mode is available for observations of objects much brighter in channels 1 and 2 than in 3 and 4 (typically stars). This mode takes short exposures in channels 1 and 2, and long exposures in channels 3 and 4. Originally developed for engineering observations for taking calibration stars, this mode is now available for all observers. Three framesets are available. The shortest set takes a single 0.4 sec frame in channels 1 and 2, and a 2 sec frame in channels 3 and 4. The next set takes two undithered 2 sec frames in channels 1 and 2, and one 12 sec frame in channels 3 and 4. The longest frame time combination available takes two undithered 12 sec frames in channels 1 and 2, and a 30 sec frame in channels 3 and 4. The sensitivities of each frame are identical to those in full array mode. Dithering and mapping are also available in this mode.

For very bright sources, which would saturate the array in full-array readout mode, you can select subarray mode. In this mode, only a small 32x32 pixel portion of the array is read out, so the field of view is only 383x383. Further, the telescope must be repositioned for each detector s subarray region to be pointed at the same piece of sky. Mapping is not allowed in subarray mode. However, small maps can be made using a cluster target. Therefore, use subarray mode only for observations of single, very bright targets. Advantages of the subarray mode, in addition to the ability to observe sources that would saturate full-array readouts, are that sets of 64 frames are taken back-to-back, enabling better temporal resolution and decreasing the data volume (which means that rapid sampling can continue for longer periods). Also, because of the more rapid (0.01 sec) sampling, more Fowler samples are taken, resulting in lower readout noise per frame compared to full array mode for the same frame time. In subarray readout mode, there are 3 selectable frame times: 0.02, 0.1, and 0.4 sec. For one commanded image in subarray mode, a set of 64 Fowler-sampled frames are taken in succession, so that each time an image is commanded in subarray mode, a cube of 64x32x32 pixel images is generated. This means that the durations of a single repeat at each of the three subarray frame times are 1.28, 6.4, or 25.6 sec, respectively. The IRAC AOT moves the telescope to point to the subarray region of each requested channel at the target in turn. For the 0.02 s frame time, data rate limitations allow only data in the channel actually pointing at the target to be taken. For the 0.1 s and 0.4 s frame times, data are taken in all four channels at each pointing position, although only one channel at a time will point at the target.

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