IRAC Instrument Handbook - IRSA - California Institute of Technology

2.4.4 Subarray Mode
Instrument Description 15 Electronics
IRAC Instrument Handbook
In subarray mode, only one corner, 32×32 pixels offset by 8 pixels from the edges, is read out from one
array. Pixels (9:40, 9:40) of the array are read out. The subarray pixel size is the same as the full array
pixel size (~1.2”). Fowler sampling is performed as in full array mode, but a set of 64 subarray images are
generated and tiled into a single 256×256 image before data are sent from IRAC. In subarray mode,
Fowler sampling is performed at 0.01 sec intervals. Subarray mode is useful for observing very bright
sources and for obtaining high temporal resolution.
2.4.5 Calibration Lamps
IRAC contained two types of internal calibration lamps. The transmission calibrator lamps were designed
to illuminate all four arrays and provide an internal responsivity measurement. There were two
transmission calibrator spheres, each of which contains two lamp elements. To illuminate the arrays, the
shutter is closed, a transmission lamp is turned on, and the light from that lamp bounces off a mirror on
the back of the shutter. The flood calibrators individually illuminate each detector. The flood calibrators
could be controlled individually, and they could be used whether the shutter is open or closed. The flood
calibrators were operated at the end of each IRAC campaign and used as a consistency check. Calibration
of IRAC observations is described in Chapter 4.
2.4.6 Firmware
The IRAC firmware controls the focal plane assemblies, calibration electronics, and warm electronics
boards. Apart from autonomous fault protection, the IRAC firmware responds only to commands sent by
the Spitzer Command & Data Handling (C&DH) computer. The C&DH sends setup commands to
configure the electronics, requests for each telemetry packet, and integration commands to generate
images. IRAC responds to each command with an acknowledgment. In the case of a command that
requests telemetry, the acknowledgment consists of the telemetry packet, which is sent on the low-speed
connection between IRAC and the C&DH. There are two types of engineering data: special engineering
data, which are collected every 4 sec, and housekeeping data, which were collected every 30 seconds.
Special engineering data are used for onboard communication between IRAC and the C&DH, while
housekeeping data are used on the ground to monitor instrument performance. A command that generates
images from the arrays is acknowledged on the low-speed line, and when the frame is complete, the dataready
signal is sent on the high-speed line. The frames (together with their ancillary data) were then
transferred one at a time to the C&DH. The rate of transfer is 2 seconds per frame, which limited the data
collection rate to a maximum of four frames every 8 seconds for IRAC observations with all four arrays.
Autonomous fault protection ensures that none of the monitored voltages or currents entered into a red
limit. Fault protection is performed by a watchdog demon that is always running when the instrument is
on. The red limits are stored in IRAC memory. If a voltage in the focal plane assembly goes into a red
limit, IRAC will turn off the affected focal plane array. (The individual pixel values are not monitored, so
a bright astronomical source does not trigger a red limit.) The command sequences would continue to
execute; therefore, it was possible for normal completion of an IRAC observing campaign to occur with
only three of the four arrays returning data. If a second focal plane assembly had a telemetry datum go