IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
source effective gain is slightly different from the point source effective gain. IRAC point source
photometry then requires a correction for the effective gain change between extended and point sources.
Second, the spectrum of the zodiacal background peaks redward of the IRAC filters. The vast majority of
objects seen by IRAC are not like this. Many have spectral energy distributions in the IRAC filters more
closely resembling stars. Stars (and many galaxies) have color temperatures that are fairly high, and peak
blueward of the IRAC filters. Generally speaking, for these objects the IRAC filters are well on the
Rayleigh-Jeans side of the blackbody spectrum. IRAC photometry of warmer sources then requires a
correction for the spectral slope change between the zodiacal light and Rayleigh-Jeans spectra.
Lastly, there is a variation in the effective filter bandpass of IRAC as a function of angle of incidence,
which in turn depends on the exact position of an object on the array (Quijada et al. 2004, [21]). As a
result of this, all objects in the IRAC field of view need to be corrected based on their location on the
array.
All three of these effects can be directly measured and a correction derived. Stars (Rayleigh-Jeans, point
sources) were sampled at many different locations on the array, and their flux was measured from the
(C)BCD images (see Chapter 6 for the definition of the various types of data, including BCD and CBCD).
The systematic variations in their measured fluxes were used to derive the corrections. The amplitude of
this effect is sizeable. It may reach 10% peak-to-peak, depending on the detector array. This is larger than
any other source of uncertainty in IRAC calibration. For well-dithered data, experiments have shown that
this effect tends to average out so that the amplitude of the effect is very small (less than 1%). However,
depending on the exact details of mapping and dithering, it is not uncommon to have small areas of data
where the mean correction approaches the full 10%.
Correction images may be downloaded from IRAC instrument web pages. Users should note the
following:
• The correction images are oriented so that they apply multiplicatively to the (C)BCD images.
Among other things, the channel 1 and 2 arrays are flipped around their vertical axis during the
reduction by the BCD pipeline, hence these images cannot be directly applied to the raw data.
• The correction images are for compact, or point-like sources.
• The correction images are for a Rayleigh-Jeans (stellar, Vega-like) spectrum. Spectral indices
differing from this will have different corrections. Generally, most IRAC objects have spectral
slopes that are bracketed by the two extremes of the red zodiacal spectrum and the blue stellar
spectrum, so the corrections will lie between zero and that in the correction image.
• Note that the existing flatfield flattens the zodiacal background. After correction, although the
point sources may be correctly measured, the background will no longer be flat.
To apply the correction from these images to photometry on a single (C)BCD image, a) perform
photometry on your (C)BCD image, b) measure the value from the correction images at the central pixel
of your target for which you are performing photometry, c) multiply your photometrical flux
measurement by the measured correction value for the central pixel of your target to obtain a corrected
Calibration 43 Array Location-Dependent
Photometric Corrections for
Compact Sources with Stellar
Spectral Slopes