Modeling the partially coherent behavior of few-mode far-infrared grating spectrometers.

Modeling ultra-low-noise far-infrared grating spectrometers has become crucial for the next generation of far-infrared space observatories. Conventional techniques are awkward to apply because of the partially coherent form of the incident spectral field, and the few-mode response of the optics and detectors. We present a modal technique for modeling the behavior of spectrometers that allows for the propagation and detection of partially coherent fields, and the inclusion of straylight radiated by warm internal surfaces. We illustrate the technique by modeling the behavior of the long wavelength band of the proposed SAFARI instrument on the well-studied SPICA mission.

Design and performance of cryogenic, scanning Fabry-Perot interferometers for the Long-Wavelength Spectrometer on the Infrared Space Observatory.

The design of cryogenic, scanning Fabry-Perot interferometers for the Long-Wavelength Spectrometer on the ESA Infrared Space Observatory is presented. The interferometers were designed to provide a spectral resolving power of 10(4) over the wavelength range 45-180 µm, with the highest possible transmission efficiency consistent with this requirement. Metal meshes, custom designed with the aid of a theoretical model of metallic reflection, were used as the reflecting elements. The scanning mechanism featured a spring-suspended plate, which was servocontrolled by moving coil actuators and monitored by capacitance micrometers. The spectroscopic performance of the interferometers was measured in the laboratory and is compared with the model developed for the interferometer design. Although the measured resolving powers were somewhat lower than expected because of the laboratory measurement conditions, the transmission efficiencies were in approximate agreement with the design specification.

A versatile optical alignment tool.

An optical alignment tool is described, which acts like an optical tee to produce two collinear beams whose direction can be adjusted. The device can be used to align optica subsystems.

Eliminating channel spectra in Fourier transform spectroscopy.

A numerical method for eliminating channel spectra from data obtained with a bandlimited infrared Fourier transform spectrometer is developed. The method is compared with others in common use on a synthetic model spectrum.

Balloon-borne stratospheric far-IR spectral absorption measurements: the design of a solar telescope and high-resolution Michelson interferometer combination.

The design, construction, and performance of a combined solar telescope and Michelson interferometer for balloon-borne stratospheric far-IR absorption studies is discussed in terms of the relevant parameters and precautions necessary to obtain high resolution radiometrically calibrated spectra of the stratosphere between 20 and 100 cm(-1). Preliminary data are presented to indicate that the system performed as designed and provided constituent concentration values for H(2)O and O(3) from measurements of strong absorption lines in this spectrum.

Mylar beam-splitter efficiency in far infrared interferometers: angle of incidence and absorption effects.

Absorption and complete multipath interference have been included in the calculation of the modulation efficiency of a far i.r. Michelson interferometer with 25-mum and 100-mum Mylar beam splitters over the range of wavenumbers between 20 cm(-1) and 125 cm(-1) for angles of incidence between 45 degrees and 80 degrees . These results show that optimum performance of an interferometer in terms of highest and most uniform modulation efficiency for unpolarized radiation will be obtained by selecting the beam-splitter thickness to cover the wavenumber range of interest within the first interference lobe and by utilizing higher angles of incidence than the mechanically convenient 45 degrees . Practical constraints will usually limit this angle to a value somewhat less than the optimum of about 75 degrees for Mylar beam splitters.