Infrared range finder


Posted on Oct 26, 2012

A rangefinder is described that uses the propagation time of a beam of infrared radiation to measure distances of up to 120 meters with errors of less than 50 µm. The instrument is capable of measuring up to five different distances per second by directing an amplitude modulated infrared beam to a retroreflector located at the far end of each path. The instrument is being tested with the expectation that such a system could be used to make precise and rapid measurements on a large radio telescope. Such measurements would be used to adjust the shape of the telescope reflector surface to correct for thermal and gravitational deformations taking place during astronomical observations. It is possible that the measuring system could be extended to relate the positions of certain moving parts of the structure to a reference system fixed in the ground, thus opening the possibility of making corrections to the telescope pointing.



The National Radio Astronomy Observatory (NRAO) is constructing a fully-steerable radio telescope at Green Bank, West Virginia, to be known as the Green Bank Telescope (GBT). The telescope design provides a reflector surface which is a part of a paraboloid, so positioned that radiation can reach the reflector and then pass to one of two focal points without meeting any obstructions. This clear aperture collects radiation from a circular area of 100 meters diameter. The goal of the design is to have an instrument which performs well at radio wavelengths down to 6 mm. It may be possible in the future to lower this short-wavelength limit to 3 mm; this depends on several factors, one of which is the outcome of the present work. To achieve satisfactory performance at short wavelengths, a radio telescope must meet two main requirements: The reflector surface must maintain its required shape. The position of the telescope beam on the sky must always be controlled with precision. These can be thought of as the "surface" and "pointing" requirements; the precision with which they must be met is related to the shortest wavelength at which the telescope is to be used, and both are very dependent on the environmental conditions at the telescope site. As an example, if the GBT were being used at a wavelength of 3 mm, it would be expected to have a surface whose departures in shape from perfection had an RMS value of at most 0.2 mm,...




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