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
