The Spitzer Science Center (SSC) performed routine calibrations of IRAC using observations of standard stars and other astronomical objects. The data obtained in these observations were used to construct the necessary calibration inputs to the pipeline for the IRAC data processing of science observations. The calibration data files are available to the general user in the Spitzer Heritage Archive maintained by IRSA.
4.1.1 Shutterless Skydarks
Dark current and bias offsets were calibrated via the standard ground-based technique of dark subtraction. As part of routine operations (during both the cryogenic and warm missions), the SSC observed a “dark” region of the sky (skydark) with no sources brighter than Vega magnitude V = 12 near the north ecliptic pole at least twice per campaign (at the beginning and end). These data were 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 (Figure 4.1) of the minimal uniform sky background contained both the bias and the dark current. When subtracted from the routine science data, this eliminated both of these instrumental signatures. Naturally, this also subtracted a component of the true celestial background. The SSC included a COBE-based model estimate of the true celestial background in the (C)BCD headers. Note that the lack of an isolated measurement of the dark current and bias offset during shutterless operations limited the ability of IRAC to measure the true celestial background. The skydarks are available to download from the Spitzer Heritage Archive (SHA) at IRSA, at
Figure 4.1: IRAC instrument skydark images. These images were taken during a normal campaign, and using 100 second frame time skydarks.
4.1.2 Closed Shutter Campaign
The shutter was allowed to be used on-board during the final science campaign to aid observations of the zodiacal background, and take a set of calibration observations. The obtained darks with the shutter closed were scientifically useful for zodiacal observations, however, two-thirds of the planned shutter-closed darks were lost due to an onboard command timing error. Figure 4.2 shows the channel 2 skydark compared to the channel 2 dark taken with the shutter closed. Figure 4.3 demonstrates the visual difference between the shutter darks and regular skydarks. The overall offset is 0.0746 MJy/sr at 3.6 µm and 0.251 MJy/sr at 4.5 µm for the 30 second frame times.
Figure 4.2: Row pixel values (in DN) of a shutter dark and a regular skydark. This plot shows the channel 2 skydark (red) pixel values along a row compared to the pixel values in a channel 2 shutter closed dark (black) in the same row. These observations used the 100 second frame times. The difference between the measurements is consistent with the astronomical background at the skydark position (1 DN = 3.7 electrons).
Figure 4.3: Nominal 30 second frame time skydarks (first row) and 30 second frame time shutter darks (second row) for channels 1 and 2 with relative scaling. The difference between the skydark and the shutter dark (last row) is also displayed.
Pixel-to-pixel relative gain variations are commonly known as the flat-field. To get the most accurate measurement of the flat-field, including the effects of the telescope and the IRAC pickoff mirrors, we had to use observations of the sky. Because the IRAC detectors were relatively large, there were few discrete astronomical objects large enough and bright enough to fill the detector field of view.
The flat-field 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 flat-field. The resulting flat-field 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 flat-field response on a campaign-wide basis showed that there were no changes within the cryogenic or within the warm missions. Based on this, all of the flat-field data were combined into superskyflats. The 1σ pixel-to-pixel accuracy of the cryogenic (warm) superskyflat flat was 0.14% (0.17%), 0.09% (0.09%), 0.07%, and 0.01% in channels 1 - 4, respectively. This was the flat-field used for all pipeline-processed data. During warm operations the flat-field was remeasured to a similar accuracy as that of the cryogenic flat-field. While the warm flat-field was similar in overall appearance, details were sufficiently different so that the warm and cold flats cannot be interchanged. The superskyflats are available from the “IRAC calibration and analysis files” section of IRSA’s Spitzer/IRAC documentation website:
Users should note that the flat-field data were generated from the zodiacal background, and are appropriate for objects with that color. There was 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 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 (see, e.g., Arendt et al. 2008).
Figure 4.4: IRAC superskyflats. Shown are the cryogenic mission (top two rows, including channels 1 – 4) and the warm mission (bottom, including channels 1 and 2) superskyflats.