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.