4.5 Array Location-Dependent Photometric Corrections for Compact Sources with Stellar Spectral Slopes
Point source photometry requires an additional correction that arises from the way in which the data are flat-fielded. Flat-fielding is a way of removing pixel-to-pixel gain variations. The IRAC flat-field was derived by imaging the high surface brightness zodiacal background. The way the IRAC flat-field was derived has a few consequences on making photometric measurements using IRAC data.
First, the zodiacal background is extended and essentially uniform over the 5.2 arcminutes x 5.2 arcminutes IRAC field of view. The vast majority of objects seen by IRAC were not like this. Many were compact, being either stars or background galaxies. IRAC had significant scattering, as well as distortion. As a result, the extended source effective gain was 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 the objects seen by IRAC were 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 were 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 the angle of incidence, which in turn depends on the exact position of an object on the array (Quijada et al. 2004). 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 showed that this effect tended 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 the IRAC web pages at IRSA. Note that the correction images for the cryogenic mission and the warm mission are different, and users should use the appropriate mission corrections. Note also 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 were flipped around their vertical axis during the reduction by the BCD pipeline, hence the correction 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 had 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.
· The existing flat-field flattens the zodiacal background. After applying the 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 photometric 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 photometric 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 possible using the MOPEX tool. MOPEX can be found at the IRSA website for Spitzer documentation.
Figure 4.5: Array location-dependent photometric correction images. The cryogenic mission corrections are at the top, and the warm mission corrections at the bottom. For the cryogenic mission correction images, channel 1 is in the upper left, channel 2 in the upper right, channel 3 in the lower left, and channel 4 in the lower right. White is the largest value (about 1.046) and black is the smallest value (about 0.915). For the warm mission correction images, channel 1 is on the left, and channel 2 on the right. Again, white is the largest value (about 1.01757, 1.03167 in channels 1 and channel 2), and black is the smallest value (0.9476, 0.9223).
A note the on correction image filenames: The filenames for the cryogenic mission are in the pattern ch[1-4]_photcorr_rj.fits where “rj” means “Rayleigh-Jeans,” and for the warm mission in the pattern ch[1-2]_photcorr_ap_5.fits