Laser spectral analysis systems are increasingly being considered for in situ analysis of the atomic and molecular composition of selected rock and soil samples on other planets . Both Laser Induced Breakdown Spectroscopy (LIBS) and Raman spectroscopy are used to identify the constituents of soil and rock samples in situ. LIBS instruments use a high peak-power laser to ablate a minute area of the surface of a sample. The resulting plasma is observed with an optical head, which collects the emitted light for analysis by one or more spectrometers. By identifying the ion emission lines observed in the plasma, the constituent elements and their abundance can be deduced. In Raman spectroscopy, laser photons incident on the sample surface are scattered and experience a Raman shift, exchanging small amounts of energy with the molecules scattering the light. By observing the spectrum of the scattered light, it is possible to determine the molecular composition of the sample.
For both types of instruments, there are advantages to physically separating the light collecting optics from the spectroscopy optics. The light collection system will often have articulating or rotating elements to facilitate the interrogation of multiple samples with minimum expenditure of energy and motion. As such, the optical head is often placed on a boom or an appendage allowing it to be pointed in different directions or easily positioned in different locations. By contrast, the spectrometry portion of the instrument is often well-served by placing it in a more static location. The detectors often operate more consistently in a thermally-controlled environment. Placing them deep within the spacecraft structure also provides some shielding from ionizing radiation, extending the instrument’s useful life. Finally, the spectrometry portion of the instrument often contains significant mass, such that keeping it off of the moving portion of the platform, allowing that portion to be significantly smaller, less massive and less robust.
Large core multi-mode optical fibers are often used to accommodate the optical connection of the two separated portions of such instrumentation. In some cases, significant throughput efficiency improvement can be realized by judiciously orienting the strands of multi-fiber cable, close-bunching them to accommodate a tight focus of the optical system on the optical side of the connection, and splaying them out linearly along a spectrometer slit on the other end.
For such instrumentation to work effectively in identifying elements and molecules, and especially to produce accurate quantitative results, the spectral throughput of the optical fiber connection must be consistent over varying temperatures, over the range of motion of the optical head (and it’s implied optical cable stresses), and over angle-aperture invariant of the total system. While the first two of these conditions have been demonstrated, spectral observations of the latter present a cause for concern, and may have an impact on future design of fiber-connected LIBS and Raman spectroscopy instruments. In short, we have observed that the shape of the spectral efficiency curve of a large multi-mode core optical fiber changes as a function of input angle.
The MIRI is the mid-IR (5-28μm) instrument for NGST and provides for imaging, cororographic, high- and low-resolution spectroscopic capabilities. Unlike to the other instruments on NGST, the MIRI must be cooled - to reduce the thermal background from the optics and because the detectors require an operating temperature of about 7k.. In this paper we summarise the science goals, the proposed overall opto-mechanical concept, the thermal design aspects, the detectors and the expected sensitivity of the instrument.
The visual and infrared mapping spectrometer (VIMS) is a remote sensing instrument developed for the Cassini mission to Saturn by an international team representing the national space agencies of the United States, Italy, and France. A dual imaging spectrometer, VIMS' unique design consists of two optical systems boresighted and operating in tandem, coordinated by a common electronics unit. The combined optical system generates 352 2D images simultaneously, each in a separate, contiguous waveband. These are combined by the electronics to produce 'image cubes' in which each image pixel represents a spectrum spanning 0.3 to 5.1 microns in 352 steps. VIMS images will be used to produce detailed spatial maps of the distribution of mineral and chemical species of Saturn's atmosphere, rings, and moons, and the atmosphere of Titan. At some wavelengths VIMS will penetrate Titan's atmosphere to map its surface, and image the night side of many Saturnian objects.
If the solar spectral irradiance and the orientation and directional reflectance of a solar diffuser are known, then the spectral radiance of the diffuser is readily calculated and it can be used for the accurate absolute calibration of a satellite sensor. However, the solar diffuser is exposed during in-flight satellite sensor calibration to high-energy ultraviolet irradiance, particle impacts and atomic oxygen effects. This paper describes desirable solar diffuser characteristics and the results of proton and UV irradiation on the directional-hemispheric spectral reflectance, the bidirectional spectral reflectance factor and the polarization properties of candidate diffuser materials.