IRAC: Array-Location-Dependent Photometric Corrections

IRAC Basic Calibrated Data (BCD) are corrected for pixel-to-pixel gain variations, a process commonly known as "flat-fielding." The IRAC flat-field is derived by imaging the high surface-brightness zodiacal background. There are two relevant points about this approach. First, the zodiacal background is extended and essentially uniform on IRAC size-scales, and thus uniformly fills the field of view. Second, the zodical background is very red with a color temperature of just a few hundred degrees, and peaks redward of the IRAC filters. Unfortunately, the vast majority of objects seen by IRAC are not like this. Many are compact, being either stars or background galaxies. 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.

Unfortunately, there are several effects in IRAC that interact with these two points. IRAC has significant scattering, as well as spatial distortion. As a result, the extended and point source effective gains are slightly different. In addition, there is a variation in the effective filter bandpass 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, SPIE 5487, 244). As a result of this, while the flat-field perfectly corrects the extended zodical background (or any extended object) with a similar spectral slope, it is incorrect for many other objects.

This effect has been directly measured. Objects were sampled at many different locations on the array, and their flux measured from the BCD images. 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 the IRAC calibration.

Below we include a set of correction images. One should note the following:

• The correction images are oriented so that they apply multiplicatively to the 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 flat-field flattens the zodiacal background. After correction, although the point sources may be correct, the background will no longer be flat.

3.6 microns	4.5 microns
5.8 microns	8.0 microns
Click on the images above to download the files, or click here. 12/15/05: Subarray images are included in the tarball.

Go back to IRAC page