Kenneth P. Stewart

Fourier-transform optical microsystems.

The design, fabrication, and initial characterization of a miniature single-pass Fourier-transform spectrometer (FTS) that has an optical bench that measures 1 cm x 5 cm x 10 cm is presented. The FTS is predicated on the classic Michelson interferometer design with a moving mirror. Precision translation of the mirror is accomplished by microfabrication of dovetailed bearing surfaces along single-crystal planes in silicon. Although it is miniaturized, the FTS maintains a relatively high spectral resolution, 0.1 cm-1, with adequate optical throughput.

Transmittance and reflectance of crystalline quartz and high- and low-water content fused silica from 2 μm to 1 mm

The transmittances and reflectances of cultured crystalline quartz, Suprasil, Suprasil W, and Infrasil were compared over the wavelength region from 2 to 1000 μm. The high-water content of Suprasil and the low-water content of cultured crystalline quartz, Suprasil W, and Infrasil were determined by their transmittances measured at 2.73 μm where water content causes high absorption in optical materials. The fact that the fused silicas, both with high- and low-water content, had identical far-IR transmittances and that their transmittances were greatly inferior to that of crystalline quartz led to the conclusion that their inferior transmittance is due to their amorphous structure and not to their water content.

The Cyro-Testing Of Infrared Filters And Beamsplitters For The Cosmic Background Explorer's Instruments

The cryo-optical methods used to measure the spectral transmittances of filters and beamsplitters for the Cosmic Background Explorers instruments are described. Measured results demonstrate the temperature sensitivity, or insensitivity, of various infrared filter designs within the wavelength range from 1μm to 1000μm.

Near infrared cryogenic tunable solid Fabry-Perot spectrometer

We have developed a high resolution near-infrared temperature tunable cryogenic spectrometer with solid Fabry-Perot etalons. It is designed and built for diffuse emission of ionized hydrogen Brγ studies, although with the appropriate pre-filter it can be configured for any near infrared lines. The etalons made from silicon and germanium operate near 77K. The high refractive index of these etalons allows for the construction of a very compact spectrometer. Germanium etalon with 20mm clear aperture is equivalent to a gas spaced Fabry-Perot interferometer of about 80 mm in diameter. A strong temperature dependence of the refractive index for these two materials makes it easy to tune etalons. Combination of these factors allowed to build a compact, high resolution (R=12000) high throughput instrument.

Cryotransmittance and -reflectance of filters and beamsplitters for the SIRTF infrared array camera

The Space Infrared Telescope Facility Infrared Array Camera (IRAC) uses two dichroic beamsplitters, four bandpass filters, and four detector arrays to acquire images in four channels at wavelengths between 3 and 10 micrometers . Accurate knowledge of the pass bands is necessary because, in order to meet the science objectives, IRAC is required to do 2% relative photometry in each band relative to the other bands. We report the in-band and out-of-band polarized transmittance and reflectance of these optical elements measured near the instrument operating temperature of 1.4 K. Details of the experimental apparatus, which include a continuous flow liquid helium optical cryostat and a Fourier transform infrared spectrometer are discussed.

Low frequency antenna options for the lunar surface

There are a number of scientific motivations for low frequency radio observations from space. These include the possibility of detecting, and eventually imaging, the spectral line of neutral Hydrogen from the cosmic Dark Ages, before the first stars formed. This is the period between the creation of the cosmic microwave background and the formation of the first stars, massive black holes, and galaxies, a period of large-scale structure formation in the universe. The Hydrogen line from the Dark Ages is redshifted down to frequencies of tens of MHz. In addition, imaging and tracking of emission from solar and interplanetary radio bursts, and detecting electron cyclotron emission from extrasolar gas giant planets, all require observations at and below Earths ionospheric cutoff. Observations far from Earth can completely eliminate ionospheric absorption and distortion, and greatly reduce terrestrial interference (both natural and human-generated). But efficient antennas at low frequency are physically large. Consequently there has been a lot of work exploring concepts for low mass, wide bandwidth low frequency antennas that can be deployed in space. This paper summarizes recent work on deployable low frequency antennas, including thin film antennas that could be unrolled on the lunar surface and inflatable antennas for both free space and lunar locations.

Curved infrared screens.

We address curved IR screens for multiwavelength systems. To first-order of the approximation, a curved screen may be viewed as composed of many local flat screens. On the other hand, the validity of such an approximation is not clear a priori. We provide experiments and simulations to show that such an approximation works well for cylindrically curved IR screens while monitoring their peak transmission as a function of the screen curvature.

Infrared filters for cryogenic radiometers

This paper will present transmittance data, in the 1- to 1000-micron wavelength region, at temperatures from 300K down to near 4K, for a selection of filters composed of multilayer thin films on transmitting substrates, reststrahlen crystals, mesh-grid elements, and hybrids of these types. Polyethylene laminates and vapor deposited parylene will be compared as antireflection layers for high refractive index infrared crystals at long wavelengths.

Noise tube sources for the far IR and millimeter region.

The radiant output of a noise tube (C. P. Clare Model TN-167), designed for the 90-140-GHz (3.3-2.1-mm) frequency range, has been compared with that from mercury lamps over the wavelength region from 0.4 to ~6 mm. Lamellar grating and Michelson Fourier transform spectrometers were used in conjunction with He cooled bolometers of NEP from 10(-12) to 10(-14) W/(H(2))(1/2) to measure relative spectral irradiance. With this instrumental arrangement, the radiant power emitted by the noise tube was observed to be less than that from a mercury lamp, at least to a 3-mm wavelength, but it produced less source noise than an ac operated mercury lamp. When the noise tube operating current was reduced, the spectral irradiance peak shifted to longer wavelengths.

Beamsplitter material comparison: KBr and ZnSe

This paper will present measured reflectance, transmittance, surface figure and roughness data for KBr and ZnSe beamsplitters and compensators that were made for use on spaceflight Michelson-type Fourier transform spectrometers. Measured data for visible and infrared wavelengths, at room temperature and cryogenic temperatures, will be shown. Calculated performance data for KCl substrates will be included for comparison.