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Dawn's Visible and Infrared Mapping Spectrometer (VIR)

Angioletta Coradini
VIR Team Lead, Istituto Nazionale di Astrofisica (INAF), Rome

Hubble Space Telescope observations of Vesta's surface made ten years ago revealed a large southern polar crater and geological diversity, with regions that can be characterized spectroscopically as basalts.

Figure 1. Spectral map of Vesta's surface from Hubble. Click for larger view.

Figure 1 shows the surface composition map of Vesta produced from separate images in blue (439 nm), orange (673 nm), red (953 nm), and near-infrared (1024 nm) light. The map shows that all of Vesta's surface is igneous, indicating that either the entire surface was once melted, or lava flowing from its interior completely covered its surface. A firm identification of surface geology on Vesta requires medium-high resolution spectra. The Dawn Visible-IR Mapping Spectrometer (VIR) addresses this need with a capability to acquire high spectral and spatial resolution data. Spectral coverage is important for both Vesta and Ceres, as diagnostic minerals show absorption bands in the visible and near IR regions. For this reason we developed a single spectrometer able to cover both visible and IR spectral ranges.

Figure 2. Comparison of VIRTIS flight IR lamp (right) with the Ground IR lamp (left). Click for larger view.

The Visible and Infrared sensors, that are the heart of the VIR imaging spectrometer, are housed in the same optical subsystem. The instrument is derived from the VIRTIS experiment that is presently flying on the Rosetta mission. Figure 2 shows the result of in-flight calibration of VIRTIS, compared with the ground-based measurements. The difference at the long wavelengths is due to the higher temperature of the VIRTIS box in-flight.

Figure 3. The VIR Optics Module. Click for larger view.

The optics module OM (Figure 3) contains the optical system, scan mirror, the entrance and sunshield, cover, shutter, cryocooler, in-flight calibration units (lamps), radiators, focal plane arrays (FPAs), and the proximity electronics (PEM). The OM architecture has been maintained as similar as possible to the VIRTIS for Rosetta; only the spacecraft-instrument interfaces have been modified. The optical concept is inherited from the visible channel of the Cassini Visible Infrared Mapping Spectrometer (VIMS-V) developed at Galileo Avionica. This concept matches a Shafer telescope to an Offner grating spectrometer to disperse a line image across two FPAs. The Shafer telescope and Offner spectrometer are aligned separately, then mounted and co-aligned.

The Shafer telescope combines an inverted Burch telescope and an Offner relay (M4/6 and M5). The Offner relay takes the curved, anastigmatic VIR virtual image of the inverted telescope and makes it flat and real without losing the anastigmatic quality. Coma optical aberration is eliminated by putting the aperture stop on M5 near the center of curvature of the primary mirror and thus making the telescope monocentric. The result is a telescope system that relies only on spherical mirrors yet remains diffraction limited over an appreciable portion of the spectrum and all vertical field (slit direction). The Shafer telescope is matched to the Offner grating spectrometer because both are telecentric; the entrance pupil is positioned in the front focal length (FFL) of the optical system at 750 mm in front of the primary mirror (M1). Because the pupil optics conjugate is on the grating, the same spectral beam splitting is performed for each FOV angle. The grating profiles are holographically recorded into a photoresist and then etched with an ion beam. Higher groove density in the central 30% of the conjugate pupil area generates the higher spectral resolution required in the "visible" channel, extending from the ultra-violet to the near infrared. The smaller pupil area allows the visible channel to operate partially coherently and achieve a smaller point spread function.

A laminar grating is used for the visible channel's pupil zone to increase the grating efficiency spectrum and compensate for low solar energy and low CCD quantum efficiency in the ultra-violet and near infrared regions. This improves the instrument's dynamic range by increasing the S/N at the extreme wavelengths and preventing saturation in the central wavelengths. For the infrared zones, a blazed groove profile is used that results in a peak efficiency at 5 µm to compensate for the low signal levels expected at this wavelength. These features, combined with custom designed FPAs, result in an instrument able to collect spectra of very large dynamic range with high spatial resolution that satisfies Dawn's requirements.