An astronomical detector receives photons from a source and produces a

An astronomical detector receives photons from a

This preview shows page 249 - 251 out of 460 pages.

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