IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
called “the first-frame effect". Figure 5.7 shows how the DC levels of darks change as a function of
delay-time.
Figure 5.6: Correction of cable-induced bandwidth error by IRACEBWC. The illustrated data
show a cosmic ray hit.
Due to the decision not to use the photon-shutter on the IRAC for dark and flat measurements, we have a
somewhat sophisticated dark subtraction procedure. There will be two steps for the dark subtraction, one
using a dark from the ground-based laboratory measurements (called, “lab darks"), and another using a
delta dark which is the difference between the lab dark and the sky dark measured at the low zodiacal
light region.
In the first step of dark subtraction, we subtract a calibrated lab dark from the data at this point in the
processing. This lab dark subtraction occurs before the linearization of the array, so that we can linearize
the data as well as possible. The labdark subtraction will be handled by a combination of modules
including LABDARKSUB and FFCORR depending on which kind of labdark data is needed. In some
observing modes (subarray mode, shortest frames within the HDR mode and the first frame of an
observation or AOR), not enough data are available to construct delay-time dependent darks. In such
cases, a single mean dark has been computed using 30 sec as a delay time, and it is used as a labdark. The
LABDARKSUB module subtracts this mean labdark. The correction of the first-frame effect for all other
frames is handled by the FFCORR module, which interpolates the library of labdarks taken at different
exposure times with different delay times, and creates a labdark corresponding to the particular delay time
of the frame being calibrated. These delay-time dependent darks are then subtracted from the
IRACEBWC-processed frame. Therefore, FFCORR requires a number of different labdarks taken with
different delay times to calibrate properly. These were taken pre-launch and have been loaded into the
calibration database. The IRAC pipeline determines the delay time (header keyword INTRFRDLY), and
the lab dark file (header keyword LBDRKFLE) that was subtracted is placed within the header keywords
of the BCD.
Pipeline Processing 78 Level 1 (BCD) Pipeline