PHOTOMETERS
The photometer is an accessory device that the astronomer attaches to the telescope at the focal position of the objective to measure the amount of radiation coming from the astronomical object. Where the spectrograph is used to examine the spectral composition of radiation, the photometer can be made to scan the spectrum formed by the spectrograph. It measures the amount of radiant energy, on either a relative or an absolute scale, at one wavelength or in a band of wavelengths. The photometer is much like an exposure meter on a camera: Incident light is converted into an electrical current. One can use a variety of techniques to define the wavelength region for the photometer, such as a spectrograph or color filters. And the radiation detector is generally today a photoelectric device.
The photoelectric photometer is usually limited to measuring only one light source, such as a star, at a time. But the limitation is compensated for by the photoelectric photometer's very great accuracy. Because of its quick response to changes in amounts of light, the photoelectric photometer is particularly useful in continually monitoring the change in brightness of an object whose emission of radiant energy varies with time (for example, a number of stars are known to be variable light sources).
