Astronomical Measuring Apparatus

laboratory apparatus for measuring the positions of images of celestial bodies on photographs of the stellar sky and positions of spectral lines on astronomical spectrograms. Astronomical measuring devices (coordinate-measuring machines) are designed for measuring one or two of the rectangular coordinates of images on the photograph or of lines on the spectrogram. Astronomical measuring apparatus have a mounting stage for holding the photograph and a measuring microscope for focusing the image or spectral line. The mounting stage and in some designs also the measuring microscope can be progressively moved into two mutually perpendicular directions and their positions determined from measurement on a scale or with the help of a micrometer screw. The precision in modern astronomical measuring apparatus is close to ± 1 micron.

The measuring process introduces errors into the coordinates measured: instrument, human (depending on the measurer), and random. Instrument errors are caused by imperfections in the astronomical measuring apparatus, and these must be carefully studied in advance. Scale errors, errors in micrometer screws, improper alignment of the mounting stage or measuring microscope, and errors in reference micrometers are investigated. To minimize human errors, measurements are taken twice, the second measurement with a reversing prism or an astronegative rotated 180°; the arithmetic average of the two measurements is then calculated. Random errors are reduced by repeated alignment of the image being measured and by computing the averages from numerous measurements.

With the development of photographic determinations of coordinates and the proper motions of numerous stars, automatic and semiautomatic astronomical measuring devices are being introduced into practice. Automatic astronomical measuring devices measure several times faster than normal devices, the mean square error being ± 0.5 micron. In semiautomatic astronomical devices, the observer aligns the object, while the coordinates are registered automatically, with data output given in a form convenient for computer processing. An example of such a device is the coordinate-measuring machine Askorekord, developed by the C. Zeiss enterprise of the German Democratic Republic. A number of astronomical measuring devices use blink comparators and stereocomparators, primarily for measuring the difference between the coordinates of two astronegatives.