IRAC: Data Calibration
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Table of Contents:
Introduction
Astronomical flux standards
IRAC zero magnitude flux densities in Jy
Sky Flats
Skydarks
Distortion and PSF Map
Linearization
Routine (Frequent) Calibrations for Normal Operations
Introduction
The SSC performs routine calibration of IRAC using observations of standard stars and other astronomical objects. Additional diagnostic data may also be taken at various times to perform certain calibration operations. The data obtained in these observations are used to construct the necessary calibration inputs to the pipeline for the IRAC data processing of science observations. The calibration data files, as well as the pipeline inputs, are available to the general user in the Spitzer archive maintained by the SSC. For more information, consult the IRAC Data Handbook, the IRAC Pipeline Decription Document, and the paper by Reach et al.
Astronomical flux standards
A number of astronomical standard stars are observed in each instrument campaign to obtain a valid absolute flux calibration. Stars with a range of fluxes are observed at a number of positions across the array and many times throughout the mission to monitor any changes that may occur. Calibration stars with known spectral types and accurate absolutely calibrated fluxes in the IRAC bands have been determined. These absolute calibration stars are in the continuous viewing zone (CVZ) so that they can be observed at any time necessary and can be monitored throughout the mission.Four stars are observed in the CVZ at the beginning and end of each instrument campaign. These standards will remain the same throughout the mission, and provide the absolute flux reference for IRAC. Additionally, a (secondary) calibrator near the ecliptic plane (which will be different for each campaign) will be observed every twelve hours. Its placement in the ecliptic plane is meant to minimize telescope slews. This calibrator is used to monitor any short-term variation in the photometric stability.
Analysis of the flux calibrator data to date indicates that absolute flux calibration is accurate to 3%. Repeatability of measurements of individual stars is good to better than 1.5% and can be as good as 0.01% with very careful observation design (Charbonneau et al. 2005, ApJ 626, 523). The absolute calibration is derived taking several systematic effects into account. The steps are described in detail by the Reach et al. paper. If this methodology is not applied then point source photometry from the BCDs can be in error by up to 10%..
IRAC zero magnitude flux densities in Jy
| Channel | flux density |
|---|---|
| Channel 1 | 280.9 +/- 4.1 Jy |
| Channel 2 | 179.7 +/- 2.6 Jy |
| Channel 3 | 115.0 +/- 1.7 Jy |
| Channel 4 | 64.13 +/- 0.94 Jy |
Sky Flats
To get the most accurate measure of the full system gain, including the effects of the telescope and the IRAC pickoff mirrors, one must use observations of the sky.This is done using many dithered observations of a network of 22 high zodiacal background regions of the sky in the ecliptic plane, which ensures a relatively uniform illumination with a reasonable amount of flux. One such region is observed in each campaign. The data are combined with object identification and outlier rejection, producing a product analogous to a median sky flat, as is commonly constructed during ground-based observing, and which will be an image of the presumably smooth celestial background, further smoothed by the dither pattern. The resulting flat field will be divided into the science data. Pixel-to-pixel accuracy of the flat-fielding derived from a single observing campaign is 2.4%, 1.2%, 1.0%, and 0.3%, 1-sigma, for bands 1 through 4, respectively. Using combined flats ( super sky flat ) from the first two years, the estimated pixel-to-pixel accuracy is 0.5, 0.2, 0.2, and 0.05% in channels 1-4, respectively. Users should note that the flat field data are 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 the IRAC Data Handbook 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 could be determined by the actual survey frames themselves, rather than using the standard set of dedicated observations of some other part of the sky. The dither and mapping pattern need to be specified to optimally relate each pixel in the array to all the others, as well as mapping out the region at the required sensitivity. Tests of self-calibration with suitable datasets have shown that the SSC pipeline results are comparable. The SSC will not, as a matter of course, undertake such special processing (self-calibration) as part of the automated pipeline. The SSC-generated data products will always use the dedicated calibration data.
Skydarks
Dark current and bias offsets are calibrated via the standard ground-based technique of dark subtraction. As part of routine operations, the SSC observes a dark region of the sky ( skydark ) near the north ecliptic pole at least twice per campaign (at the beginning and end). These data are reduced and combined in such a way as to reject stars and other astronomical objects with size-scales smaller than the IRAC array. The resulting image of the minimal uniform sky background contains both the bias and dark current. When subtracted from the routine science data, this eliminates both of these instrumental signatures. Naturally, this also subtracts a component of the true celestial background. The SSC includes a COBE-based model estimate of the true celestial background, which is the same as that returned by Spot. Note that our lack of an isolated measurement of the dark current and bias offset during shutterless operations limits the ability of IRAC to measure the true celestial background.Distortion and PSF Map
The PSF over the field of view of each of the four bands has been characterized, and the optical distortion has been measured. This has been done by observing an open star cluster for which we have good ground-based astrometry. The distortion is included in the WCS header keywords. The positions of both IRAC fields of view have also been determined relative to the PCRS and other science instruments.
Linearization
Non-linearities in the IRAC detectors were extensively calibrated during groundtesting using special ground support equipment and also in flight using extended astronomical objects. These calibration data allow the detectors to be linearized to 1% for 90% of their full-well capacity. The linearity solutions are spotchecked during flight, on timescales of many months, via observations of bright, extended objects such as elliptical galaxies. If we find a significant deviation from the previous linearity characterization, new solutions will be derived.
Routine (Frequent) Calibrations for Normal Operations
Some calibrations occur extremely infrequently. Examples of these are linearization, distortion mapping, etc. Other calibration measurements are repeated more frequently, and in particular at least every time IRAC is powered on. These calibration observations will be performed at specific intervals as required, rather than having specific calibration observations for each individual data set. Every 12-24 hours, observations of secondary calibrator stars will be obtained, for example. Skyflats and darks are derived at the beginning and end of each campaign.For more information, see the IRAC chapter of the SOM
