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comparator(kom -pă-ray-ter, kom-pa -ră-) An instrument by means of which two photographs of the same area of the sky may be compared and any difference in the brightness or position of an object quickly detected without resorting to exact measurements. The movement of a comet, the proper motion of a star, or the presence of a variable star may thus be revealed. In the blink comparator the two photographs are viewed in rapid succession through an eyepiece: changes in position produce an apparent movement of an image while variable stars are detected as a pulsation in brightness. In the stereocomparator the two photographs are viewed simultaneously by binocular vision: objects that have changed in position or brightness (i.e. size) appear to stand out of the plane of the picture.
in astronomy, a measuring device whose operation is based on the principle of comparing two astrophoto-graphs, spectrograms, and the like, one of which is taken as the standard. There are several types of comparators, which are distinguished by their purpose and method of comparison. Some comparators compare the second pulses of clocks with the time of contact of micrometers (tape recorded) of transit instruments. In measuring the tape in such comparators, the distance between the second impulses of clocks is the standard. In spectrocomparators, spectrograms of stars are compared and the displacement of one or another group of spectral lines belonging to one star from the same group of lines belonging to another star (or laboratory light source) is measured. In this way, the radial velocity of a specific star can be established if the radial velocity of the reference star (comparison star) is known.
Comparators also include certain instruments that help detect changes that have occurred in celestial bodies; this is done by comparing two astrophotographs or spectrograms obtained at different times. Such comparators are blink comparators, in which the comparison is accomplished by alternately looking at two photographs (blinking). In stereocomparators the change in position of a celestial object relative to other objects is revealed by simultaneously looking at two astrophotographs by means of a stereoscopic image of the shift.
E. A. IUROV
a measuring instrument used to compare linear quantities with a measure or scale. A comparator measures the difference between two quantities whose nominal magnitudes are close; this provides high precision of measurement. A comparator may be used to compare a dimension of the object being measured with the distance between lines on a standard scale (line comparator) or with the end block (end comparator). Instruments of a different type, which also use a comparative method, are sometimes also called comparators (for example, an interference comparator).
Measuring devices used in comparators include microscopes with vernier, scale, or optical eyepiece micrometers; photoelectric microscopes with digital readout; and interferometers.
Lengths measured with comparators range from fractions of a millimeter to dozens of meters. Comparators are used in metrology for comparing length standards 0.1 to 4 m long, in machine building for checking part dimensions up to 1 m, and in geodesy.