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Title:
Tests of vacuum VS helium in a solar telescope
Authors:
Engvold, O.; Dunn, R. B.; Livingston, W. C.; Smartt, R. N.
Affiliation:
AA(Oslo, Universitetet, Oslo, Norway), AB(Sacramento Peak Observatory, Sunspot, NM), AC(Sacramento Peak Observatory, Sunspot, NM), AD(Kitt Peak National Observatory, Tucson, AZ)
Publication:
Applied Optics, vol. 22, Jan. 1, 1983, p. 10-12. Research supported by the Norges Almenvitenskapelige Forskningsrad.
Publication Date:
01/1983
Category:
Astronomy
Origin:
STI
NASA/STI Keywords:
Gas Cooling, Helium, Solar Instruments, Telescopes, Vacuum Tests, Air Cooling, Optical Properties, Thermal Conductivity, Wave Front Deformation
DOI:
10.1364/AO.22.000010
Bibliographic Code:
1983ApOpt..22...10E

Abstract

The consequences of filling a solar telescope with helium, a gas with an exceptionally low index of refraction and a high thermal conductivity, are reported. All the tests are conducted with the solar beam present to provide normal heating. Internal seeing of the telescope is evaluated by viewing the image of an He-Ne laser beam in autocollimation reflected from the entrance window for different pressures of air or helium. The total path of the laser beam through the tank (six times) becomes about 100 m. Quantitative measurements of the wave front distortions are derived by inserting a point-diffraction interferometer near the focused image of the laser beam. Excursions of the fringe pattern yield direct measures of the wave front errors in units of the 0.633-micron laser wavelength. It is concluded that a helium-filled telescope is a realistic alternative to a vacuum telescope for solar observations. The barely detectable tank seeing with helium near atmospheric pressure could possibly be rectified either by a high reflectivity coating of the mirrors, which reduces the heat load, or by a forced circulation of the helium in the tank.
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