IRAC Instrument Handbook - IRSA - California Institute of Technology

3 Operating Modes
IRAC Instrument Handbook
The IRAC Astronomical Observation Template (AOT) consists of an (optional) dither pattern superposed
on an (optional) rectangular-grid raster.
3.1 Readout Modes and Frame Times During Cryogenic Operations
In full-array readout mode, there were four selectable frame times: 2, 12, 30, and 100 sec (and a fifth, 200
sec, during the early mission). To allow sensitive observations without losing dynamic range, there was a
high dynamic range (HDR) option. When this option was selected, the IRAC AOT took extra frames,
with frame times shorter than the selected frame time.The HDR frame times are given in Table 2.6. No
spacecraft repositioning was done between frames, and the frames always were taken from shortest to
longest. If dithers were selected, then the entire frame set was repeated at each dither position.
Stellar photometry mode was available for observations of objects much brighter in channels 1 and 2 than
in 3 and 4 (typically stars). This mode took short exposures in channels 1 and 2, and long exposures in
channels 3 and 4. Originally developed as engineering observations for taking calibration stars, this mode
was available for all observers. Three framesets were available. The shortest set took a single 0.4 sec
frame in channels 1 and 2, and a 2 sec frame in channels 3 and 4. The next set took two undithered 2 sec
frames in channels 1 and 2, and a 12 sec frame in channels 3 and 4. The longest frame time combination
took 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 were also
available in this mode.
For very bright sources, a subarray mode was available. In this mode, only a small 32×32 pixel portion of
the array was read out, so the field of view was only 38”×38”. Mapping was not allowed in subarray
mode. However, small maps could be made using a cluster target. In subarray readout mode, there were
three 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 were taken in succession, so that each time an image was commanded in
subarray mode, a cube of 64×32×32 pixel images was generated. This means that the durations of a
single repeat at each of the three subarray frame times were 1.28, 6.4, or 25.6 sec, respectively. The IRAC
AOT moved the telescope to point to the subarray region of each requested channel at the target in turn.
For the 0.02 sec frame time, data rate limitations allowed only data in the channel actually pointing at the
target to be taken. For the 0.1 sec and 0.4 sec frame times, data were taken in all four channels at each
pointing position, although only one channel at a time pointed at the target.
Operating Modes 26 Readout Modes and Frame
Times During Cryogenic
Operations