IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
Figure 5.4: Application of IRACWRAPCORR to Channel 1 data. The many apparently “hot”
pixels are actually wrapped negative values, which are detecte d on the basis of the ir vastly
exceeding the physical saturation value for the detectors, and corrected by subtracting the
appropriate value. Real hot pixels do not exceed the physical saturation value, and hence are not
changed.
Figure 5.5: Illustration of bit truncation used by IRAC for ground transmission, necessitating
IRACNORM. The internally stored 24-bit word in truncated to 16 bits, with a sliding window set
by the barrel shift value. Illustrated is the case for ABARREL=4.
5.1.8 SNESTIMATOR (initial estimate of uncertainty)
The module SNESTIMATOR calculates the uncertainty of each pixel based on the input image (here, the
input image is the output of IRACNORM). The uncertainty for each pixel is estimated as the Poisson
noise in electrons and the readout noise added in quadrature. The formula for the calculation is as follows:
2 2
σ σ readnoise +
= σ
(5.7)
2
poisson
Pipeline Processing 75 Level 1 (BCD) Pipeline