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.
The Infrared Array Camera (IRAC) is one of three focal plane instruments in the Space Infrared Telescope Facility (SIRTF). IRAC is a four-channel camera that obtains simultaneous images at 3.6, 4.5, 5.8, and 8 microns. Two adjacent 5.12 X 5.12 arcmin fields of view in the SIRTF focal plane are viewed by the four channels in pairs (3.6 and 5.8 microns; 4.5 and 8 microns). All four detectors arrays in the camera are 256 X 256 pixels in size, with the two shorter wavelength channels using InSb and the two longer wavelength channels using Si:As IBC detectors. We describe here the results of the instrument functionality and calibration tests completed at Goddard Space Flight Center, and provide estimates of the in-flight sensitivity and performance of IRAC in SIRTF.
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.
The Infrared Array Camera (IRAC) is one of three science instruments that will fly aboard the Space Infrared Telescope Facility mission scheduled for launch in December, 2001. This paper summarizes the `as built' design of IRAC along with important integration and testing results.
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.
The Composite Infrared Spectrometer of the Cassini mission to Saturn has two interferometers covering the far infrared and mid infrared wavelength region. The instrument was aligned at ambient temperature, but operates at 170 Kelvin and has challenging interferometric alignment tolerances. Cryogenic alignment tests of the instrument indicated that it should suffer minimal degradation due to the cooldown from ambient to operational temperature. System level tests performed by the calibration team indicated a lower than expected signal level on the mid infrared channel, while providing ambiguous optical throughput data. Therefore it became imperative to develop a metric that could be used to determine the instrument performance at both the instrument and system levels, at ambient and cryogenic temperature. Modulation efficiency and throughput measurements were performed and new analytical models developed to evaluate the status of the instrument. Methodologies are detailed, empirical and analytical data are reconciled and deviations from design values explained.
Measured infrared reflectance and transmittance data, as a function of wavelength and incidence angle, are presented for polarization inducing components on a typical Geostationary Operational Environmental Satellite (GOES) atmospheric sounding instrument. The data are then used to construct Mueller matrices from which instrument polarization sensitivity, not only to incoming radiance from an observed scene, but also to the instrument's own radiant signature. A comparison is made between the polarizing characteristics of a typical filter-type infrared sounding radiometer and a proposed Micheson-type FOurier transform alternative.
The CIRS instrument to be flown on the Cassini mission to Saturn is a cryogenic spectrometer with far-IR (FIR) and mid-IR (MIR) channels. The CIRS FIR channel is a polarizing interferometer that contains three polarizing grid components. These components are an input polarizer, a polarizing beamsplitter, and an output polarizer/analyzer. THey consist of a 1.5 micron thick substrate with 2 micrometers wide copper wires, with 2 micrometers spacing, photolithographically deposited on the substrate. Mylar and polypropylene were chosen as the flight candidate substrates. After the testing was performed, mylar was chosen over polypropylene for the CIRS instrument due to a better cryogenic reflectance performance. These elements were fabricated at Queen Mary and Westfield College in London. This paper details the flight qualification of the mylar substrate and the characterization of the polypropylene substrate. Performance tests included cryogenic optical flatness, cryogenic polarization sensitive reflectance and transmittance measurements. Environmental tests included vibration, acoustic, humidity, and radiation survivability.
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.
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