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An astronomical detector receives photons from a source and produces acorrespondingsignal. The signal characterizes the incoming photons: it maymeasure their rate of arrival, their energy distribution, or perhaps their wavephase or polarization. Although detecting the signal may be an exact science, itscharacterization of the source is rarely exact. Photons never pass directly fromsource to detector without some mediation. They traverse both space and theEarth’s atmosphere, and in both places emissions and absorptions may modifythe photon stream. A telescope and other elements of the observing system, likecorrecting lenses, mirrors, filters, optical fibers, and spectrograph gratings col-lect and direct the photons, but also alter them. Only in the end does the detectordo its work. Figure 8.1 illustrates this two-stage process of signal generation:background, atmosphere, telescope, and instruments first modify light from thesource; then a detector detects.An astronomer must understand both mediation and detection if she is toextract meaning from measurement. This chapter describes only the second stepin the measurement process, detection. We first outline the qualities an astron-omer will generally find important in any detector. Then we examine a fewimportant detectors in detail: the CCD, a few photo-emissive devices, the infraredarray, and the bolometer.235
8.1 Detector characterizationWhy does an astronomer choose one detector instead of another? Whydid optical astronomers in the 1980s largely abandon photography, the then-dominant detector for imaging, in favor of solid-state arrays? Why are thesesame arrays useless for other purposes, such as measuring very rapid changes inbrightness? Is there aperfectdetector? We begin an answer with a list of severalcritical characteristics of any detector.8.1.1 Detection modesWe can distinguish three distinct modes for detecting light.Photon detectorsproduce a signal that depends on an individual photonaltering the quantum mechanical state of one or more detector electrons.For example, in the last chapter, we saw how a change in electron energy ina photoconductor or photodiode can produce a change in the macroscopicelectrical properties like conductivity, voltage, or current. Other changes inquantum state might produce chemical reactions (as in photography) or a pulseof free electrons, as in vacuum photomultipliers. Photon detectors are partic-ularly suited to shorter wavelengths (infrared and shorter), where the energiesof individual photons are large compared to the thermal energies of the elec-trons in the detector.Thermal detectorsabsorb the energy of the incoming photon stream andconvert it into heat. In these devices the signal is the temperature change inthe body of the detector. Although thermal detectors are in principle useful at allwavelengths, in practice, thermal detectors, especially a class calledbolometers,have been fundamentally important in the infrared and microwave regions, aswell as very useful in the gamma and X-ray regions.