IRAC Instrument Handbook - IRSA - California Institute of Technology

The linearization solution to the above quadratic model is:
where
2n+
w n
α ⎡⎛
2
L = ⎢⎜
∑ i − ∑i
2
n(
w + n)
⎣⎝
n+
w+
1 1
IRAC Instrument Handbook
−1
+ 1−
4LαDN
obs
DN = (5.17)
2Lα
2
⎞ t ⎤
d
⎟ − 2(
1−
) n(
n + w)
⎥ , (5.18)
⎠ tc
⎦
A
and α = , n is the Fowler number, and w is the wait period. The above expression for L is the
2
m
correction required to account for multi-sampling. This is required because the multi-sampling results in
the apparent time spent integrating not actually being equal to the real time spent collecting photons. Note
that td is the time between the reset and the 1st readout of the pixel. For channel 3, we use a cubic
linearization model:
For the cubic model, the solution is derived via a numerical inversion.
5.1.14 BGMODEL (zodiacal background estimation)
DNobs = Ckt 3 + Akt 2 + kmt (5.19)
For this module, a spacecraft-centric model of the celestial background was developed. For each image,
the zodiacal background will be estimated (a constant for the entire frame) based on the pointing and time
that the data were taken. This value is written to the header keyword ZODY_EST in units of MJy/sr. The
zodiacal background is also estimated for the subtracted skydark (see next module) and placed in the
header keyword SKYDRKZB.
5.1.15 Dark Subtraction II: SKYDARKSUB (sky “delta-dark” subtraction)
This module, the second part of dark subtraction, strongly resembles traditional ground-based data
reduction techniques for infrared data. Since IRAC did not use the photon-shutter for its dark
measurement, a pre-selected region of low zodiacal background in the north ecliptic cap is observed in
order to create a “skydark". At least twice during each campaign a library of skydarks of all Fowler
numbers and frame times were observed, reduced, and created by the calibration pipeline. The skydarks
have had the appropriate labdark subtracted in their DARKCAL pipeline and are therefore a “delta-dark.”
Pipeline Processing 83 Level 1 (BCD) Pipeline