Effect of telescope alignment on a stellar interferometer.

For a ground-based stellar interferometer, we investigate the effect of wave-front distortions that are due to telescope alignment errors and other factors. We apply the results to the IR/Optical Telescope Array (IOTA) interferometer. We present the computational method used in our simulation program to calculate explicitly the wave-front shape from an arbitrarily misaligned telescope. We calculate the wave-front shape and variance for a suite of misalignment conditions and interpret these results to find allowable tolerances on the positions and tilts of the telescope mirrors. We calculate the expected Strehl ratios from a total of ten types of factor, including telescope alignment, that are expected to be important in a real interferometer. Ranking the expected wave-front perturbations, we find that three of them, the wave-front curvature from atmospheric turbulence, the servo system time constant, and the flatness of the relay optics surfaces, are more significant than the telescope alignment factor. We compare observational experience at IOTA with our model estimate of the overall Strehl ratio in the visual and the infrared, finding moderately good agreement and, more important, a guide for future instrumental improvements.

Fourier-transform spectroscopy: new methods and applications: introduction by the feature editors.

We are pleased to introduce this special issue of papers on Fourier-transform spectroscopy, which grew out of a recent topical meeting sponsored by the Optical Society of America. The topical meeting welcomed all researchers who practice the art of Fourier-transform spectroscopy in the laboratory, in the atmosphere, and in space. The power and the wide applicability of Fourier-transform spectroscopy unite these fields with a common mathematical and instrumental bond. The meeting probed each of these areas in depth, bringing out new ideas for instrumentation, analysis, and applications. There was a strong sentiment at the meeting that the quality of papers and posters was exceptionally high and that it would be important for future progress in the field to have the results of this meeting captured in print. This special issue is the fruit of that effort.

New designs for far-infrared beam splitters.

We explore theoretically the design of multilayer beam splitters for the far-infrared that are based on coated pellicles as well as on solid substrates. The specific design criterion considered is as high a value as possible of the efficiency E = 4RT throughout the 10-140-µm spectral region. Here R and T are the reflectance and the transmittance of the beam splitter, respectively. In the numerical study the refractive indices of the substrates and coating materials varied between 1.50 and 4.00. To survey the range of designs, we make a number of simplifying assumptions, the significance of which is later investigated. Various potential manufacturing problems are considered. It is shown that the performance of the beam splitters is not sensitive to the accuracy with which the layer thicknesses can be controlled. However, it does depend strongly on the lowest available refractive index of the coating materials. The performance is particularly sensitive to the extinction coefficients of solid substrate materials. Multilayer designs presented should be useful for use in Fourier-transform spectrometers, as well as in other applications that do not require as high a spectral resolution.

Phase determination from mostly one-sided interferograms.

We show how to detect and correct for nonlinear phase shifts in a mainly one-sided interferogram of an emission-line source. We simultaneously detect and correct for an out-of-phase emission background from the spectrometer. The method requires two auxiliary spectra, one of a strong continuum source, and one of an emission-line source with little or no continuum.

Stratospheric hydroperoxyl measurements.

The hydroperoxyl radical (HO(2)) plays a key role in stratospheric chemistry through the HOx catalytic cycle of ozone destruction. Earlier measurements of stratospheric HO(2) have given mixed results; some measured mixing ratios greatly exceed theoretical predictions. Measurements of HO(2) have now been made with a balloon-borne farinfrared spectrometer. The measured daytime profile is in excellent agreement with theory up to 40 kilometers; above this level the measurements exceed theory by 30 percent, perhaps because of underprediction of ozone at these altitudes. The nighttime HO(2) profile is strongly depressed with respect to the daytime profile, in general agreement with theory.

Theoretical atmospheric transmission in the mid-and far-infrared at four altitudes.

The ir transmission of the terrestrial atmosphere is calculated at four altitudes of interest: Mauna Kea at 4.2 km (2-1000 microm), aircraft at 14 km (5-1000 microm), and balloon at 28 km and 41 km (10-1000 microm). We show both high resolution spectra (0.05 cm(-1)) and broadband averages. The results are intended to serve both as a detailed guide to the interference that is expected from the atmosphere for astronomical spectroscopy and also as an indicator of the relative change in absorption and emission that can be expected at various observing altitudes. One salient result for the spectral region around 100 microm is that the absorption (and emissivity) of the atmosphere drops by a factor of 10 for each increase in altitude of 15 km throughout the aircraft and balloon range; thus balloon-borne astronomical photometry and spectroscopy should both enjoy a considerable advantage over aircraft observations in the 30-300-microm region.

Detection of molecular oxygen on Mars.

Molecular oxygen was detected in martian spectra near 7635 angstroms and its abundance measured both during and after the 1971 dust storm. Its column abundance in the clear martian atmosphere is about 10.4 +/- 1.0 centimeters amagat, giving a mixing ratio of molecular oxygen to carbon dioxide of 1.3 x 10-(3). The mixing ratio of molecular oxygen to carbon monoxide (1.4 +/- 0.3) is quite different from the value of 0.5 that would result from the photolysis of a pure carbon dioxide atmosphere, which indicates that there is or was a net source of oxygen relative to carbon (probably water) in the martian atmosphere.