next up previous
Next: CCDs Up: Detectors Previous: Detectors

Basic Principles and Properties

To illustrate basic terminology used for detectors, we will consider 3 examples of detectors: photographic plate, photomultiplier, and array detector.

Detectors work because they are made of some material which interacts with photons. An incoming photon generates a chemical reaction (photographic) or photoelectron. Photoelectrons are counted, either

In a photomultiplier, some fraction of incident photons hit a photosensitive material and eject a photoelectron. This electron is amplified numerous times to create a large ``swarm'' of electrons which is detected as a pulse. Thus, photons are ``counted'' as they come in. Generally, simple photomultipliers do not retain any information about the location on the detector where the photon hits. There are some modern devices, however, called microchannel plates, which are essentially arrays of small photomultipliers where positional information can be obtained; one of the more common of these is called a MAMA, and exists in several instruments on the Hubble Space Telescope. Traditional photomultipliers were the workhorse of photometry from the 50's to the late 70's.

Array detectors are in more common usage today. In these devices, incoming photons create photoelectrons which are trapped in local potential wells. The amount of energy needed to eject a photoelectron depends on the type of material used. In the optical, silicon provides a good choice, but the excitation energy for silicon is too high to be used in the infrared. In the IR, various different substances are used, including HgCdTe, InSb, and PtSi. After a specified amount of time, the photoelectrons are ``counted''. The method by which this is done differs between different types of arrays. In CCDs, the charge is physically clocked down columns of the device, a single row at a time (a parallel transfer) then read out of serial register; CCDs are inherently asymmetric in rows and columns. In IR devices, each pixel is read individually, in sequence.

When discussing detectors, one should be familiar with some terminology:


next up previous
Next: CCDs Up: Detectors Previous: Detectors
Rene Walterbos 2003-04-14