IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
Figure 4.12. Noise as a functi on of exposure time (number of frames) in channel 2. The results from the warm
mission data are shown with x’s and the expected behavior with the solid line. The results from the cryogenic
mission are shown with open s quares and the expected behavior with the dashed line.
These plots show that background noise in IRAC channels 1 and 2 does decrease roughly as expected
with exposure time. The slight deviation at larger exposure times is likely caused by the first frame effect
and by residua l source wings.
4.12 Pointing Performance
Pointing is controlled by Spitzer's Pointing Control System (PCS). This uses a combination of a star
tracker and gyros to locate and control the attitude of the spacecraft. Absolute pointing is controlled by
the star tracker, through a filter (known as the "observer") which smooths the raw star tracker output.
Slews under control of the observer take ~ 10 seconds to settle, so only the initial slew and cluster slews
in celestial coordinates are carried out using the observer. Once the observatory has taken the initial frame
at the starting position, attitude control is handed over to the gyros. Mapping and dithering slews are
made under gyro control with a shorter (~ 5 sec) settle time. The price to be paid for the shorter settle time
is that the spacecraft attitude will slowly drift with respect to the observer attitude, at a rate ~ 1 mas/sec.
Calibration 64 Pointing Performance