Astronomical Photometer

(also photometer), a device intended for measuring the brightness or radiance of celestial objects or of luminous fluxes emanating from them. Visual and electric astronomical photometers are used. Photometric problems are also solved by photographic methods through laboratory measurements—for example, with densitometers or microphotometers—of astronomical negatives that have been properly exposed and calibrated.

Visual astronomical photometers, which appeared in the 1930’s and 1940’s, are based on matching the brightness (radiance) of the object being studied with the brightness (radiance) of an artificial source by measurably varying the latter with polarizing devices, with a photometric wedge, or with restriction of the telescope’s aperture. An artificial source is optically introduced into the astronomical photometer’s field of view, and the source is seen simultaneously with the object under study. The reference object can also be some star which has been shown to have a brightness constant over time (reference star). If the object being studied is brighter than the reference star, it may be subjected to controlled intensity reduction. The most popular device became the polarizing astronomical photometer (Zöllner, 1861), which was perfected by the Russian astronomer V. K. Tseraskii and others. Wedge astronomical photometers were widely used to study variable stars. In visual astronomical photometry, evaluation of the equivalence of two light sources is not characterized by high accuracy; in measurements of point sources, errors can reach 5–10 percent (very subjective!). In the 1930’s electrophotometers, with significantly higher accuracies, began to replace visual astronomical photometers, which are used only in photometric studies of planets.

A stellar electrophotometer measures the response (photoelectric current) of the photocathode of the photoele-ment or of a photoelectronic multiplier to the luminous flux emanating from the object being studied. Comparison is accomplished in photometric systems by determining the spectral sensitivity of the photocathode—that is, its response to luminous fluxes equi-energetic in different wavelengths. Spectral sensitivity can vary; in particular, it can coincide with the spectral response of the human eye. In this case, an electrophotometer replaces the visual astronomical photometer but with incomparably higher accuracy, since comparison of photoelectric currents can be made with accuracies to 1 percent and less for objects that are not too faint. The change in the transparency of the atmosphere and its turbulence are the main sources of error in astronomical photometric measurements. In the case of faint sources, it is convenient to accumulate signals over time and to measure them with either a voltmeter or a photon counter. Measurements using this method can achieve accuracies of not less that 10 percent of the brightness of stars so faint that they are not visible in a given telescope (although they appear on photographs).