The scientific motivation, design criteria, and specifications for a new ground-based instrument to observe the Sun in the He i 1083-nm spectral line is described. The instrument employs a liquid-crystal tunable Lyot-type spectral filter and an array detector that allows the full solar disk to be observed with a time cadence of minutes. We describe the telescope's optical and mechanical features and discuss computer interface and data-reduction procedures employed. Instrument performance during the initial year of operation of the telescope at its high-altitude site is summarized.

A Lyot-Ohman filter for imaging near the solar He i 1083-nm line is described. Fast and continuous spectral tunability is provided by nematic liquid crystals. This solid-state filter has a free spectral range of 2.35 nm and a spectral resolution of 0.135 nm at the operating wavelength of 1083 nm. A wide-fielded design was used for both static and electro-optic retarder elements, facilitating use in fast imaging systems. A first-light He i image of the Sun is presented.

A method for predicting scattering coefficients in the atmospheric windows from 1.0 micro to 15.0 micro is presented. The method avoids the assumptions that the index of refraction and the aerosol distribution are constants over the range of relative humidity values. Relative humidity is taken as the independent variable with the aerosol distribution and index of refraction dependent upon the value of the relative humidity. A two-component continental and maritime aerosol distribution is used. Although strictly applicable to the 1.0-15.0 micro wavelength region, the calculations are extended with a lesser degree of accuracy into the visible region so that comparisons with measured data may be made.

Atmospheric transmittance spectra from 0.56 micro to 10.7 micro obtained over a 25-km horizontal path at sea level, are presented. The sources were six 150-cm diam carbon are searchlights. A radiometrically calibrated spectrometer equipped with a NaCl prism and a thermocouple detector was used to record the spectral distribution of the radiation transmitted through the atmosphere. The transmittance spectra were obtained for values of precipitable water vapor ranging from 21.5 cm to 43.3 cm. From these spectra the selective transmittance of four atmospheric windows was measured. A plot of selective transmittance as a function of the square root of the precipitable water vapor for the four windows shows that the transmittance in these windows is well described by the statistical model of band absorption.

An interferometric method has been developed to determine the linear coefficient of thermal expansion of optical materials that are available only in short sample lengths (1.5 cm or less). The method utilizes the simple fringe pattern resulting from a thin air wedge formed between two fused-quartz (fused-silica) optical flats; the top flat is supported by the sample by means of a fused-quartz screw. The expansion of the material is determined by relating its change of length to the measured change of fringe spacing, as temperature is changed. Three different techniques for controlling the temperature of the sample are discussed. Measurements have been made in the temperature range 0°–90°C for various optical materials: single crystals, polycrystals, and glasses.