The IRAC optical layout is shown in Figure 2.2 and Figure 2.3. Light from the telescope was reflected into the IRAC structure by the pickoff mirrors for the two fields of view (FOVs). Each pair of channels had a doublet lens which re-imaged the Spitzer focal plane onto the detectors. A beamsplitter reflected the short wavelength light to the InSb detectors (channels 1 and 2) and transmitted the longer wavelength light to the Si:As detectors (channels 3 and 4). Channels 1 and 3 viewed the same patch of sky (within a few pixels), while channels 2 and 4 viewed a nearby patch of sky simultaneously. The nearest edges of the two IRAC fields of view were separated by approximately 1.52 arcminutes, with no overlap on the sky. The IRAC pixel scale was nearly the same in all channels (≈ 1.2 arcseconds per pixel), providing a 5.2 arcminutes × 5.2 arcminutes FOV.
Figure 2.2: IRAC optical layout, top view. The layout was similar for both pairs of channels; the light entered the doublet and the long wavelength passes through the beamsplitter to the Si:As detector (channels 3 and 4), and the short wavelength light was reflected to the InSb detector (channels 1 and 2).
Figure 2.3: IRAC optics, side view. The Si:As detectors are shown at the far right of the figure. The InSb arrays are behind the beamsplitters