The IRAC system throughput and optical performance was governed by a combination of the system components, including the lenses, beamsplitters, filters, mirrors, and detectors. The system parameters are summarized in Table 2.2. The system response was based on measurements of the final in-flight system, including the beamsplitter, filter, ZnS & ZnSe coating transmissions, mirror reflectance, BaF2 and MgF2 coating transmissions, and detector quantum efficiency.
Table 2.2: IRAC channel characteristics.
Channel
Effective
(μm)
Bandwidth (µm)
Average transmission (ηI)
Minimum in-band trans-mission
Peak trans-mission
1
3.551
0.750 (21%)
0.426
0.339
0.465
2
4.493
1.010 (23%)
0.462
0.330
0.535
3
5.730
1.420 (25%)
0.150
0.119
0.170
4
7.873
2.930 (37%)
0.280
0.199
0.318
At each wavelength, the spectral response curve gives the number of electrons produced in the detector per incoming photon. While the curves provided are best estimates of the actual spectral response, it is recommended that the curves are used in a relative sense for color corrections, and the supplied photometric scaling (implicit in Level 1 products [“BCDs”] and described in Reach et al. 2005and updated inCarey et al. 2012) is used for absolute photometric calibration. Tests during the IOC/SV showed that the out-of-band leaks were less than the astronomical background at all locations, for sources of any temperature detectable in the IRAC channels.
The spectral response curves presented below in Figure 2.4 reflect our best knowledge of the telescope throughput and detector quantum efficiency. The response curves used measurements of the filter and beamsplitter transmissions over the range of angles of incidence corresponding to the distribution of incident angles across the fields of view of the IRAC detectors (Quijada et al. 2004).
We provide three sets of curves for each IRAC channel: an average response curve for the entire array, an average curve for the subarray (see Section 3.1) field of view, and a data cube of the response curves on a per pixel basis. The average curves are useful for making color corrections to photometry of well-dithered (four or more dithers) observations. The response cubes can be used for more rigorous color corrections on per instance basis. For most purposes, the average curves are sufficient. A more detailed discussion of the spectral response curves is given by Hora et al. (2008). IRSA’s Spitzer/IRAC section web page
contains links to the tabulated spectral response curves.
Figure 2.4: Spectral response curves for all four IRAC channels. The full array average curve is displayed in black. The subarray average curve is in green. The extrema of the full array per-pixel transmission curves are also shown for reference. The red curves are for the pixel in the array with the highest nominal wavelength bandpass and the blue curves for the pixel in the array with the lowest nominal wavelength bandpass. For definition of the “nominal wavelength” see Hora et al. (2008). The spectral response curve data are available at https://irsa.ipac.caltech.edu/data/SPITZER/docs/irac/calibrationfiles/spectralresponse/ . Click on the figure for a higher resolution image.
Figure 2.5: Optical image distortion in IRAC channels in the cryogenic mission (above) and warm mission (below). The panels show the image distortions as calculated from a quadratic polynomial model (above) and a fifth-order polynomial (below) that was fit to in-flight data. The magnitude of the distortion and the direction to which objects have moved from their ideal tangential plane projected positions is shown with arrows. The length of the arrows has been increased by a factor of ten for clarity. The maximum positional deviations across the arrays for this quadratic distortion model are less than 1.3, 1.6, 1.4, and 2.2 pixels for channels 1 - 4, respectively. The derivation of the pixel scales that are listed in Table 2.1 fully accounted for the quadratic (cryogenic data) and fifth order (warm data) distortion effects shown here.
2.2.4 Distortion
Due to the off-axis placement of IRAC in the Spitzer focal plane, there was a small amount of distortion over the IRAC FOV (there would have been some distortion even if the IRAC FOV had been centered in the Spitzer focal plane). The maximum distortion in each IRAC channel was < 2.2 pixels (compared to a perfectly regular grid) over the full FOV. Figure 2.5 shows the distortion across all four IRAC channels in the cryogenic mission, as determined from data taken during the IOC/SV. It also shows the fifth order distortion in the warm mission.
The warm mission distortion correction improved the accuracy by using a fifth order polynomial. Using cluster observations and relative astrometry, the new correction has an uncertainty less than 30 milliarcseconds. The new distortion correction was verified by, and agrees with, two other independent derivations using independent data sets. For more information, see Lowrance et al. (2016).
Table 2.3: IRAC read noise. The second set of numbers in channels 1 and 2 are for the warm mission.
Ch.
Read noise for frame with specified frame time (electrons)
Subarray (frame time in seconds)
Full array (frame time in seconds)
0.02*
0.1*
0.4*
2*
0.4
0.6
1.2
2
6
12
30
100
1
25.4
16.9/13.4
10.8
10.8
22.4
22.4
22.4
11.8/16
16
9.4/ 15
7.8/ 14.6
8.4/21
2
23.7
16.8/12.1
9.4
9.4
23.7
23.7
23.7
12.1/12.1
10
9.4/ 10.4
7.5/ 10.4
7.9/16
3
-
9.0
-
-
-
-
-
9.1
-
8.8
10.7
13.1
4
-
8.4
-
-
-
-
-
7.1
-
6.7
6.9
6.8
**
* Per single subframe (one of the 64 planes in the BCD cube).
