Following up on Cassini/CIRS, we are building the next-generation Composite InfraRed Spectrometer for deep-space
planetary exploration. CIRS-Lite combines Mid & Far-IR channels into a single instrument with 4x the spectral
sensitivity of CIRS. Here we discuss the instrument optical design, the design process, and the system performance.
Planetary Fourier transform spectroscopy (FTS) has a long history at the Goddard Space Flight Center. Dr. Rudy Hanel developed a series of such instruments for Earth, Mars and the two Voyager spacecraft. More recently as part of the Cassini mission, the CIRS (Composite Infrared Spectrometer) FTS was launched in 1997 for the 2000-2001 Jupiter flyby and the 2004-2008+ Saturn tour. At about 40 kg, CIRS is both too heavy and too light for future planetary missions. It is too heavy for future Discovery and New Frontier missions, where the emphasis is on low-mass, low-power instrumentation. On the other hand, CIRS could be heavier to take full advantage of future Prometheus missions such as JIMO. Here we discuss future development of CIRS-like FTS’s for both Discovery/New Frontier and for Prometheus flight opportunities. We also briefly discuss possible applications in the Moon/Mars exploration initiative.
The performance of a high Tc (~90 K) transition-edge superconducting (TES) bolometer on a monolithic sapphire membrane is presented and discussed. It is compared to the performance of a previous TES bolometer on non-monolithic sapphire substrate. The development and optimization of monolithic sapphire membranes is critical for the fabrication of 1 and 2-D arrays of TES bolometers. Moderately cooled and optimized TES bolometers are expected to be the replacements of choice for thermopiles and other room temperature thermal sensors on far IR instruments on future planetary missions.
The Composite InfraRed Spectrometer (CIRS) instrument aboard the Cassini spacecraft en route to Saturn is a cryogenic spectrometer with far-infrared (FIR) and mid-infrared channels. The CIRS FIR focal plane, which covers the spectral range of 10 - 600 cm-1, consists of focusing optics and an output polarizer/analyzer that splits the output radiation according to polarization. The reflected and transmitted components are focused by concentrating cones onto thermoelectric detectors. These thermoelectric detectors consist of a gold black absorber on top of a gold foil that is welded to a thermoelement consisting of two semiconductor pyramids. After the detectors were integrated into the focal plane assembly and the CIRS instrument, the detectors proved to be extremely susceptible to two environmental survivability conditions: acoustics and airflow. Several changes were investigated to improve the integrity of the detectors including detector airflow geometry, structural changes to the detectors, and more intensive screening methods. The geometry of the air paths near the sensing elements was modified. Two structural modifications were implemented to improve the stability of the sensing elements. These were changes in the geometry of the thermoelectric pyramids by ion milling, and a change in the gold foil thickness. New screening methods, centrifuge and modulated force testing, were developed to select the most rugged detectors. Although several methods gave significant improvements to the detector's stability, the modification that allowed the detectors to meet the environmental survivability requirements was the change in the geometry of the air paths near the sensing elements.
The composite infrared spectrometer (CIRS) is a remote sensing instrument to be flown on the Cassini orbiter. CIRS will retrieve vertical profiles of temperature and gas composition for the atmospheres of Titan and Saturn, from deep in their tropospheres to high in their stratospheres. CIRS will also retrieve information on the thermal properties and composition of Saturn's rings and Saturnian satellites. CIRS consists of a pair of Fourier Transform Spectrometers (FTSs) which together cover the spectral range from 10-1400 cm-1 with a spectral resolution up to 0.5 cm-1. The two interferometers share a 50 cm beryllium Cassegrain telescope. The far-infrared FTS is a polarizing interferometer covering the 10-600 cm-1 range with a pair of thermopile detectors, and a 3.9 mrad field of view. The mid-infrared FTS is a conventional Michelson interferometer covering 200-1400 cm-1 in two spectral bandpasses: 600-1100 cm- 1100-1400 cm(superscript -1 with a 1 by 10 photovoltaic HgCdTe array. Each pixel of the arrays has an approximate 0.3 mrad field of view. The HgCdTe arrays are cooled to approximately 80K with a passive radiative cooler.
A simple technique is presented for estimating the thickness of a transparent plate. Using a laser source and detector, the transmitted light is measured as a function of angle of incidence. With prior knowledge of the refractive index of the plate at the laser wavelength, the observed fringes can be processed to estimate the plate thickness. The technique works with a modest degree of wedging, as long as fringes are discernible. Formal estimates of the error budget indicate the accuracy of this method is approximately 1 μm. If fringes are measured versus both angle and wavelength, it should be possible to estimate both thickness and refractive index.
A simple polishing technique was developed for thinning the LaAlO3 substrates for high-quality Tc bolometer films, and thus reducing their heat capacity. A 150-ms bolometer was made on a LaAlO3 substrate with a 5-Hz D* value of 1.5 x 10 exp 8. It is shown that passive temperature stabilization is adequate for operation at the transition. There remained excess noise at the transition, but this noise appears to be of nonbolometric origin.
An infrared spectroscopy instrument for infrared remote sensing from the Cassini orbiter is being breadboarded in the laboratory. The Composite Infrared Spectrometer (CIRS) consists of a pair of Fourier Transform Spectrometers (FTS) which together cover the range from 10 - 1400/cm with a spectral resolution up to 0.5/cm. The far-infrared FTS is a polarizing interferometer covering the 10 - 300/cm range. The mid-infrared FTS is a conventional Michelson FTS covering 200 - 1400/cm in three spectral channels. CIRS will retrieve information on the atmospheres of Titan and Saturn with good vertical resolution, from deep in their tropospheres to high in their stratospheres, and into the upper few centimeters of the regoliths of icy objects. The science objectives and design of CIRS are discussed.
Past and present planetary exploration is briefly reviewed, and the planned 1996 Cassini mission to Saturn and Titan is examined. The CIRS experiment aboard Cassini, which will retrieve information on the atmospheres of Titan and Saturn, is discussed. Ongoing efforts to build a high-sensitivity, high-Tc bolometer that would greatly improve detection in Titan's atmosphere are addressed.