A Detector
- A device capable of registering a specific substance or physical phenomenon, and that optionally sounds an alarm or triggers a warning.
Astrophysics:
A major advantage of film or a photoelectric detector over the eye is its ability to collect light for a long time. In the eye, ‘exposures’ are fixed at about 1/20 s. With modern detectors, exposures of several hours are possible. Therefore, the limiting magnitude for direct visual observing is not as faint as for photography or photoelectric detectors.
Quantum Efficiency:
the fraction of photons that are detected is called the quantum efficiency of the detector.
A much higher efficiency can be obtained with photoelectric devices. A photon strikes a surface, causing an electron to be ejected. This is the photoelectric effect.
The electron is accelerated towards another surface, where more electrons are ejected. The process is repeated many times, and eventually a sufficient number of electrons are moving for a current to be detected.
Dark Current:
One problem with any detector is that it produces some background level which is called the dark current, because it is present even when no light is shining on the detector. This generally results from thermal emission from the detector.
For most astronomical observations, this background is much stronger than the signal being detected.
The background has two deleterious effects:
- it must be subtracted from any measurement, to just give the value of the astronomical signal.
- it produces a random fluctuation in the measurement. The stronger the background, the higher the fluctuation. This fluctuation produces an uncertainty (statistical error) in any result.