IRAC Instrument Handbook - IRSA - California Institute of Technology

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Calibration 33 Flat Fields IRAC Instrument Handbook The flatfie ld was derived from many dithered observations of a network of 22 high zodiacal background regions of the sky in the ecliptic plane, which ensured a relatively uniform illumination with sufficient flux on all pixels such that the observations were relatively quick to perform. One such region was observed in every instrument campaign. The data were combined with object identification and outlier rejection, creating an object-free image of the uniform celestial background, further smoothed by the dither pattern. An identical observation made at the north ecliptic pole (the "skydark") was subtracted, and the result normalized to create the flatfield. The resulting flatfield was divided into the science data. Pixel-to-pixel accuracy of the flat-fielding derived from a single observing campaign was typically 2.4%, 1.2%, 1.0%, and 0.3%, 1σ, for channels 1 through 4, respectively. Analysis of the flatfield response on a campaign-wise basis showed that there were no changes throughout the cryogenic mission. Based on this, all of the flatfield data were combined into a "super skyflat". The 1-sigma pixe l-to-pixel accuracy of this flat is 0.14%, 0.09%, 0.07%, and 0.01% in channels 1−4, respective ly. This is the flatfield used for all pipeline processed data. The super skyflats are available from the “IRAC calibration and analysis files” section of the archival documentation website. Users should note that the flatfield data were generated from the zodiacal background, and are appropriate for objects with that color. There is a significant color term, of order 5%−10%, for objects with a Rayleigh-Jeans spectrum in the mid-infrared (such as stars); see Section 4.5 for more information. Note that for deep survey observations and other data sets with a large number of frames and a good dithering strategy, the system gain can be determined by the actual survey frames themselves, rather than using the standard set of dedicated observations of some other part of the sky. During warm operations the flatfield was remeasured. While it is similar in overall appearance, details are sufficiently different that the warm and cold flats cannot be interchanged.
IRAC Instrument Handbook Figure 4.2: IRAC instrument super skyflats showing the flatfield response as measured onboard, for channels 1–4. . 4.3 Photometric Calibration A number of astronomical standard stars were observed in each instrument campaign to obtain a valid absolute flux calibration. Stars with a range of fluxes were observed at a number of positions across the Calibration 34 Photometric Calibration
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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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Caltech (California Institute of Te
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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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channel, 4, 6, 9, 24, 25, 26, 36, 4
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