Spitzer Documentation & Tools
Array-Location-Dependent Photometric Corrections for Compact Sources with Stellar Spectral Slopes

1. Cryogenic Mission 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 zodiacal background (or any extended object) with a similar spectral slope, it is incorrect for many other objects.

This effect has been directly measured. Througout the cryogenic mission, flux calibration stars were sampled at many different locations on the array, and their flux measured from the BCD images. We performed a simultaneous fit of a quadratic model for the array location dependence (Channels 1 through 4), and a "double gaussian" model for the intra-pixel (pixel phase) gain variations (Channels 1 and 2 only). An IDL function for correcting aperture photometry for both these functions can be downloaded here. The amplitude of the array location dependent correction may reach 15% 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 (but can be applied to (C)BCD frames).
  • 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.
  • We have normalized to the mean across the array, neglecting bad pixels. Note that Reach et al. (2005) normalized to the central pixel of the array, whereas Hora et al. (2008) normalized to the median value across the whole array.

To apply the correction from these images to photometry on a single BCD image, a) perform photometry on your BCD image, b) measure the value from the correction images below 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 flux density value. To apply the correction from these images to photometrical measurements made on a mosaic image, you will need to first mosaic the correction images in the same way as the science images. Making the correction mosaic is now possible using the MOPEX tool.

Please see Section 4.5 of the IRAC Instrument Handbook for more information about the location-dependent photometric correction.

IRAC cryogenic array location-dependent photometric correction, showing photometric variation for a source with a stellar spectrum as a function of pixel.

Click here (Tar File, 1.8 MB) to download the files.

2. Warm Mission Corrections

Based on our analysis of observations of IRAC calibration stars, the location-dependent photometric correction has changed from the cold IRAC correction. After the final array setpoint determination, a standard star (BD+67 1044) was observed over a densely spaced grid on each array and the position dependency of the photometry mapped. As with the cryogenic location-dependent correction, the variation is reasonably fit by a 2nd order polynomial in array coordinates (x, y).

From the Warm SOM Figure 6.13: Photometric correction images at 3.6 microns (left) and 4.5 microns (right). The contours start at 0.95 for 3.6 microns and 0.94 for 4.5 microns and increment by 0.01. The correction images are normalized to the average response of the array.

Download the channel 1 and channel 2 and correction images. Either S19.1 channel 1 and channel 2 or S19.2 channel 1 and channel 2 images are available. S19.2 images should be used for all new data downloads from the SHA, corresponding to reprocessed data. If you downloaded images earlier, please check the (C)BCD image header keyword CREATOR. If it says S19.1... then use the S19.1 correction images, otherwise S19.2 correction images. Photometry of blue sources (sources with SEDs falling with increasing wavelength) should be multiplied by the value of correction image at the centroid location of the source.