Vims

Visible and IR spectral mapping to study composition and structure of surfaces, atmospheres and rings

tigate photochemical reactions; (2) to map the distribution and properties of aerosols; and (3) to study the nature of circulation in the upper atmospheres of these worlds. We saw in Chapters 4 and 5 that UV images are sensitive to the abundance of stratospheric hazes. It is hoped that UVIS will be able to take so many images that it will be able to generate movies showing how such material is moved in the upper atmospheres of Saturn and Titan, rather like expected ISS movies, but at lower wavelengths. The observed UV spectra may reveal absorption signatures of several upper-tropospheric/lower-stratospheric gases such as ammonia. In addition, UVIS measures the fluctuations of starlight and sunlight as the Sun and stars move behind the atmospheres of Titan and Saturn and it is hoped it will be able to make new estimates of the D/H ratio in these atmospheres. Such observations are currently being analyzed.

Visible and Infrared Mapping Spectrometer (VIMS)

VIMS is a development of the NIMS instrument flown on Galileo and is designed to measure reflected and emitted radiation from 0.35 ^m to 5.1 ^m to determine com position, temperature, and structure. It actually consists of two instruments: a visible channel (VIMS-V), recording the spectrum in 96 channels between 0.35 ^m and 1.07 ^m, and an IR channel (VIMS-IR), recording the spectrum in 256 channels between 0.85 ^m and 5.1 ^m. The FOV of both channels is 32 x 32mrad and both focal planes are cooled by a passive radiative cooler to 190 K for VIMS-V and as low as 56 K for VIMS-IR.

In the visible section (VIMS-V), light collected by a 4.5 cm Shafer telescope is dispersed by a holographic grating onto a 256 x 512 silicon CCD array. The data are averaged into 96 spectral channels and 64 cross-dispersion pixels giving a pixel size of 0.5mrad. Imaging is then achieved by a single axis-scanning mirror.

The IR section (VIMS-IR) consists of a 23 cm Cassegrain telescope and a linear array of 256 cooled InSb detectors. During the time VIMS-V makes an exposure, VIMS-IR must record 64 individual spectra by stepping its two-axis scan mirror in the cross-dispersion direction which, together with the fact that the reflected radiance of Jupiter decreases with wavelength, is why the entrance aperture of VIMS-IR has to be so much larger. Imaging is then achieved by stepping the scan mirror in the dispersion direction in the same way as VIMS-V.

The atmospheric science goals of VIMS are: (1) to map the temporal behavior of winds, eddies, and other features on Saturn and Titan at multiple wavelengths (and thus altitudes) and to perform long-term studies of cloud movement and morphology in Saturn's atmosphere; (2) to study the composition of the atmospheres and distribution of different cloud species; (3) to determine the temperature, internal structure, and rotation of Saturn's deep atmosphere; and (4) to search for lightning and, for the case of Titan, for active volcanism. As has been reported in Chapters 4 and 5, many of these goals have already been achieved.

Composite Infrared Spectrometer (CIRS)

CIRS is a development of the IRIS spectrometers flown on Voyagers 1 and 2 (Calcutt et al, 1992). Light is gathered by a 50 cm telescope and fed to two interferometers, one working in the mid-infrared from 600 cm-1 to 1,400 cm-1 and one operating in the far-infrared from 10 cm-1 to 600 cm-1 (Figure 7.45). Both sections share the same mirror drive assembly and the maximum path difference that can be introduced is 2 cm. Hence, the maximum spectral resolution is 0.5 cm-1.

The mid-IR section is itself split into two parts. The spectrum from 600 cm-1 to 1,100 cm-1 is recorded by the FP3 array of ten mercury-cadmium-telluride (HgCdTe) photoconductive detectors of 0.273 x 0.273 mrad FOV arranged as shown in Figure 7.46. Similarly, the spectrum from 1,100 cm-1 to 1,400 cm-1 is recorded by the FP4 array of ten HgCdTe photovoltaic detectors with the same FOV. The mirror system of the mid-IR interferometer utilizes corner cube reflectors, to negate any misalignments and the detectors are cooled to 80 K by a passive radiative cooler, which gives them high sensitivity.

The far-IR section, FP1, uses a polarizing interferometer and a pair of bolometer detectors. The polarizing beamsplitter and polarizing plates are formed from finely

Figure 7.45. Cassini CIRS instrument. The mid-IR section is to the left and is cooled by the passive radiative cooler. The far-IR polarizing interferometer is to the right. Courtesy of NASA.

etched metal grids, and the use of polarization grids allows for the cancellation of the offset term in the interferogram (Equation 7.6) by subtracting the signals recorded by the detector pair. This makes the spectrometer far less susceptible to instrumental drifts and increases measurement precision. The far-IR FOV is 3.9 mrad in diameter and roof top reflectors are used to guard once more against any possible misalignments in the beams.

0.273 mrad

Figure 7.46. CIRS focal plane pointing and FOV. Courtesy of NASA.

0.273 mrad

Figure 7.46. CIRS focal plane pointing and FOV. Courtesy of NASA.

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Solar Panel Basics

Solar Panel Basics

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