IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
All of the calibration data taken with these stars are public and are available in the Spitzer Heritage
Archive. The secondary calibrator stars were used to monitor short-term variations in the absolute
calibration. To avoid slew overheads, they were observed close to downlinks and had to therefore be
located near the ecliptic plane, in a tightly constrained window of about 20 degrees. Because of the
motion of the Earth about the Sun this window constantly moved and so any one secondary calibrator was
visible for only a campaign or two per year. In practice, the calibration values for IRAC appear to be quite
temporally stable.
The data are calibrated by means of aperture photometry, using a 10 native pixel radius (12 arcseconds)
aperture. The background was measured using a robust average in a 12−20 pixel annulus around the
centroid of the star. Unfortunately, ground-based infrared calibrators were too bright to use as calibrators
for IRAC. Therefore, one must use models to predict the actual flux for each channel as a function of
spectral type (Cohen et al. 2003, [7]). Table 4.1 lists the calibration factors that are used in the final
processing of all IRAC data. The absolute calibration is described in detail in Reach et al. (2005, [22]),
with further refinements at the 1%–3% level, based on better models for the calibration stars and a better
estimate of the corrections to photometry (pixel phase, array-location dependent photometric correction,
etc.).
Table 4.1: The photometric calibrati on and zero magnitude flux for IRAC.
λ (μm) FLUXCONV (MJy/sr)/(DN/sec) F ν0 (Jy)
3.6 0.1088 280.9±4.1
4.5 0.1388 179.7±2.6
5.8 0.5952 115.0±1.7
8.0 0.2021 64.9±0.9
The absolute gain calibration is accurate to better than 3%. The stellar photometry is repeatable at the <
1% level. The absolute fluxes of the calibration stars are known to 2% – 3% (Cohen et al. 2003, [7]). To
obtain photometry at this accuracy, photometric corrections for the location of the source within its peak
pixel, and the location of the source within the array, must be made.
Note that IRAC is not an absolute background photometer, so the total brightness in IRAC images should
be used with great caution. There was a cold shutter in the calibration assembly, but it was not operated in
flight, in order to minimize mission risk. Therefore, the offset level in IRAC images is referred to
laboratory measurements before launch, where the offset level was observed to change very significantly
from one laboratory experiment to another.
In laboratory tests, the absolute offset of IRAC images was found to vary at levels that are comparable to
the minimum celestial background in channels 1 and 2. Furthermore, the offset level changes depending
on whether the detector was recently annealed. Thus, for diffuse surface brightness measurements, we
Calibration 36 Photometric Calibration