IRAC Instrument Handbook - IRSA - California Institute of Technology

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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
IRAC Instrument Handbook 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. You can also use this IDL script. Figure 4.3. Array location-dependent photometric correction images. Ch 1 is in the upper left, ch 2 in the upper right, ch 3 in the lower left and channel 4 in the lower right. A note on correction image filenames: The filenames are in the pattern ch[1-4]_photcorr_rj.fits where "rj" means "Rayle igh-Jeans.” Calibration 44 Array Location-Dependent Photometric Corrections for Compact Sources with Stellar Spectral Slopes
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8 Introduction to Data Analysis 8.1
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Appendix A. Pipeline History Log S1
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S18.0 Pipeline History Log 140 IRAC
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Pipeline History Log 142 IRAC Instr
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Performing Photometry on IRAC Image
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Performing Photometry on IRAC Image
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C.1 Use of the Five Times Oversampl
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Point Source Fitting IRAC Images wi
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DN Data Number. FET Field Effect Tr
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WCS World Coordinate System. Acrony
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Collaborators Construction and grou
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Margaret Drennan, Graduate Student
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Darron Harris, Mechanical Technicia
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Dick Bolt, Flight Assurance Jerry B
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Charles Stone, Harness Technician I
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List of Figures 190 IRAC Instrument
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Appendix I. Version Log Version 2.0
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Bibliography 198 IRAC Instrument Ha
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IRAC Instrument Handbook - IRSA - California Institute of Technology

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