V.B.2.b.ii. Photoconductor Array Camera and Spectrometer
The PACS instrument [1] is based on a photoconductive detector that uses two 25x16 Ge:Ga unstressed/stressed arrays [2] to cover two bands, 60-130µ and 130-210µ. A key element of this instrument is the very high sensitivity (NEP ~ 5 x 10-18 W/Hz- 1/2) obtainable with Ge:Ga photoconductive detectors at ~ 2 K.[3, 4]However, these detectors have photoconductive thresholds at 130µ (unstressed) and 210µ (stressed), respectively, and for observations at longer wavelength, Herschel uses either heterodyne (HIFI) or bolometer (PACS) detectors. Figure V.B.2.b.ii-1 shows the relative response of these detectors as a function of wavelength. In its photometry mode PACS will perform photometry (λ/Δλ~2) simultaneously in the two bands, with a mesh filter selected choice (60 - 90 µ/ 90 - 130µ) available on the unstressed shorter wavelength array. This two-color system is designed for the study of broad emission features of external galaxies. In its spectrometer mode PACS has a resolution of ~1500. This is accomplished with a diffraction grating in a Littrow configuration and a dichroic beam splitter to separate diffraction orders. In this configuration, multiplex spatial imaging is retained by the use of a 5 x 5 pixel detector array.
Figure V.B.2.b.ii-1. Relative response of stressed (solid line) and unstressed (dashed line) Ge:Ga detectors.
PACS complements HIFI in two important aspects and is a good illustration of the technology trade-offs that are still necessary in the THz. First, because it does not require local oscillator power for mixers, the difficulties of producing power at high frequency are eliminated. Secondly, it is much easier to build arrays of photo-detectors than it is to build focal plane arrays of heterodyne mixers. Thus, for astrophysical projects which require large scale photometric surveys, the arrays of PACS provide spatial multiplexing and a large gain in the overall photometric efficiency for the system.
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