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We have developed epoxy-based, broadband antireflection coatings for millimeter-wave astrophysics experiments with cryogenic optics. By using multiple-layer coatings where each layer steps in dielectric constant, we achieved low reflection over a wide bandwidth. We suppressed the reflection from an alumina disk to 10% over fractional bandwidths of 92% and 104% using two-layer and three-layer coatings, respectively. The dielectric constants of epoxies were tuned between 2.06 and 7.44 by mixing three types of epoxy and doping with strontium titanate powder required for the high dielectric mixtures. At 140 K, the band-integrated absorption loss in the coatings was suppressed to less than 1% for the two-layer coating, and below 10% for the three-layer coating.

We are developing superconducting transition-edge bolometers for far-infrared and millimeter wavelengths. The bolometers described here are suspended by thin legs of silicon nitride for thermal isolation. At frequencies between 200 mHz and 10-50 Hz these devices show white noise at their thermal fluctuation limit (NEP approximately 10(-17) W/ radicalHz). At higher frequencies a broad peak appears in the noise spectrum, which we attribute to a combination of thermal fluctuations in complex thermal circuits and electrothermal feedback. Detailed noise calculations fit the noise measured in three different devices that were specifically designed to test the model. We discuss how changes in bolometer materials can shift the noise peak above the frequency range of interest for most applications.

We present an experimental study of a composite voltage-biased superconducting bolometer (VSB). The tested VSB consists of a Ti-film superconducting thermometer (T(c) ~375 mK) on a Si substrate suspended by NbTi superconducting leads. A resistor attached to the substrate provides calibrated heat input into the bolometer. The current through the bolometer is measured with a superconducting quantum interference device ammeter. Strong negative electrothermal feedback fixes the bolometer temperature at T(c) and reduces the measured response time from 2.6 s to 13 ms. As predicted, the measured current responsivity of the bolometer is equal to the inverse of the bias voltage. A noise equivalent power of 5 x 10(-17) W/ radicalHz was measured for a thermal conductance G ~ 4.7 x 10(-10) W/K, which is consistent with the expected thermal noise. Excess noise was observed for bias conditions for which the electrothermal feedback strength was close to maximum.

We developed a far-infrared Fabry-Perot filter constructed from a single silicon substrate. The limiting resolving power caused by beam divergence of a silicon-gap Fabry-Perot filter is approximately 10 times higher than that of a vacuum-gap Fabry-Perot filter because of the large index of refraction of silicon. The filter thus permits compact, high-throughput optical systems. Metal mesh patterns microlithographed on each face provide enhanced, wavelength-dependent face reflectivity. We tested the performance of filters with metal mesh patterns consisting of inductive crosses and capacitive squares. A Fabry-Perot filter developed for a rocketborne astrophysics experiment with a capacitive square metal mesh pattern achieves a resolving power of λ/Δλ(FWHM) =160 at λ= 158 µm, with a peak transmittance of 37% over an active aperture of 6.9 mm for an ƒ/3.8 optical beam at 15° incidence. The absorptivity of a 240-µm thick silicon substrate patterned with capacitive metal mesh is A ≲ 1% per pass, including loss in both the silicon and the metal mesh.

An optimized leaf-spring apparatus for applying uniaxial stress to a Ge:Ga far-infrared photoconductor has been designed and tested. This design has significant advantages for space applications which require high quantum efficiency and stable operation over long periods of time. The important features include adequate spring deflection with relatively small overall size, torque-free stress, easy measurement of applied stress, and a detector configuration with high responsivity. One-dimensional arrays of stressed photoconductors can be constructed using this design. A peak responsivity of 38 A/W is achieved in a detector with a cutoff wavelength of 200 microm, which was operated at a temperature of 2.0 K and a bias voltage equal to one half of the breakdown voltage.

The fabrication and optimization of composite submillimeter wave bolometers with metal film absorbing elements and doped Ge thermometers are described. Performance characteristics are given for 4 x 4-mm bolometers designed for operation at both (4)He and (3)He temperatures. The performance expected from such bolometers when they are optimized for various values of background loading is calculated. Current dependent noise, which can arise from any of several sources, is included in the analysis. This can make it impossible to reach the background fluctuation noise limit. Feed structures for these bolometers which employ Winston light concentrators are described.

A horn antenna with low sidelobe response has been fabricated for a rocket-borne observation of the diffuse brightness of the sky at submillimeter wavelengths. The antenna consists of a Winston concentrator which defines the field of view and a flared baffle which minimizes diffraction. The sidelobe response for lambda = 337 microm was measured to be <10(-9) at an angle of 70 degrees from the optical axis. Calculations are reported which illustrate the contributions of diffraction and scattering to the sidelobe response.

We have fabricated low pass filters made of spherical glass beads embedded in polyethylene and measured the transmittance at room temperature and 1.3 K as a function of the diameter and the filling factor of the glass beads. The cutoff frequency is a function of the bead diameter. Compared with the transmittance of thin glass plates or Fluorogold filters, these filters have a sharper cutoff due to the effect of scattering by the spherical beads. This effect becomes prominent at low temperature, because of the decrease in the absorption coefficient of glass itself. The glass bead filters have very low transmittance above the cutoff frequency. One of the filters we fabricated has a transmittance of <5 x 10(-4) between 50 and 1000 cm(-1).

The properties of bandpass filters for broadband photometry are reported in the 3-12-cm(-1) frequency range. The filters are based on a combination of capacitive grids deposited on thick Mylar substrates and are designed to have very high out-of-band rejection. Low frequencies are blocked by a thick grill that consists of a hexagonal grid of circular holes in a thick metal plate.

We report the performance of a rocket-borne absolute radiometer which was designed to measure the diffuse brightness of the sky in six passbands between 100 microm and 1 mm. The radiometer consisted of a horn antenna and a photometer, both of which were cooled to 1.2 K by liquid (4)He. The performance of the instrument was satisfactory, but no astrophysical data were obtained during the flight because a lid covering the horn antenna failed to pen. Another flight of a similar apparatus is planned for 1986.

A detailed analysis is presented of the Michelson polarizing interferometer suggested by Martin and Puplett. This instrument has many favorable properties for use as a far ir Fourier spectrometer. The effect of misalignments and imperfections of the optical components on the instrumental performance is calculated. Based on these results, we give a method of aligning the interferometer which optimizes its performance. In addition, this instrument may be used to measure the optical transfer function of a component in the output beam as a function of both spatial and optical frequency. A procedure is described by which this may be done.