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Optics-based instruments will help crack the mysteries of the cosmos

Gary Boas, Contributing Editor,

What would we know about the universe without the aid of optics? A whole lot less than we do now, to be sure. Optics has always played a significant role in both manned and unmanned space missions, while astronomical study would be largely inconceivable without the telescope and the myriad imaging modalities introduced in the four centuries since Galileo.

As much as optics-based instruments have told us already, though, continued development of the technology is enabling us to look farther and probe deeper than ever before.

The Mars missions offer a good example of how optics and photonics can help to reveal yet more about the universe. “Curiosity,” the next Mars rover, will seek to determine whether the environment on the red planet once was – or might still be – able to support microbial life. Due to launch in November 2011, the rover will carry 10 instrument packages, including ChemCam, which will employ laser-induced breakdown spectroscopy (LIBS) to measure the chemical content of rocks at distances of up to 22 feet.

The next Mars rover will carry a number of instruments to aid in the search for habitability on the planet. Among these will be ChemCam, which will use laser-induced breakdown spectroscopy to study the preservation of carbon-bearing materials. Courtesy of Jean-Luc Lacour, CEA, and Los Alamos National Laboratory.

This latter capability is essential. One of the reasons ChemCam was selected for the mission, said Roger Wiens of Los Alamos National Laboratory in New Mexico, the instrument’s principal investigator, was that planetary exploration with purely passive methods has proved something of a challenge. One example: In 2004, “Opportunity,” the second of two rovers in the Mars Exploration Rover (MER) mission, landed directly in a small crater, much to the delight of mission specialists. Taking full advantage of this bit of good fortune proved difficult, though.

“When the rover’s cameras were deployed,” Wiens said, “lo and behold, they saw the very first extraterrestrial rock outcrop right before them. It turns out it was sedimentary rock, and yet the remote sensing really couldn’t tell it was sedimentary. That kind of stoked the desire to have an active technique that would be able to brush away dust, so to speak, without actually touching the rock.”

ChemCam will allow NASA to do just that. By firing laser pulses, it can penetrate dust layers on the rock and even obtain depth profiles to determine whether there are chemical weathering layers – which can reveal quite a bit about weather over very long periods on Mars, Wiens said. And of course, clearing away the dust also enables study of the pristine rock underneath.

ChemCam can determine the elements present in a Mars rock by observing the light-emitting plasma from material ablated from the rock and by matching the spectral emissions against a library of known chemical compounds. Courtesy of Los Alamos National Laboratory.

Here’s how it works: ChemCam’s laser, imager and telescope reside in the instrument’s mast unit, mounted on the rover’s remote-sensing mast 2.01 m above the ground. Brief, 5-ns pulses from the 30-mJ Nd:KGW laser operating at 1067 nm produce light-emitting plasmas from material ablated from the rock. The telescope observes the light and delivers it to three spectrometers in ChemCam’s body unit, nestled inside the body of the rover. The spectrometers, with a combined spectral range of 240 to 860 nm, have resolutions between 0.15 and 0.65 nm full width half-maximum. The spectral emissions can be matched against a library of known chemical compounds; thus, scientists can determine the elements present in the sample.

LIBS analysis generally consists of 50 laser pulses at 3 Hz, the first several of which will be for cleaning. The spectra from the remaining pulses typically are averaged together to achieve better statistics. In some cases, however, as in weathering rind analyses, they can be analyzed separately.

Although the LIBS technique and the technology used in ChemCam have been established, the researchers found that, off the shelf, they weren’t always suitable for this application. As a result, Wiens said, they had to revise the design several times while developing the instrument. One example: The original design included three optical fibers connecting the telescope to each of the spectrometers. Images in the LIBS literature showed larger plasmas measuring more than a centimeter wide at Mars pressures, which should have allowed the light to be captured across several fibers at the focal point of the telescope.

However, when the team members did their homework and checked the spatial distribution of the plasma light, they found that the portion of the bright plasma yielding strong emission line signals was much smaller – less than a millimeter in diameter. So they redesigned the instrument to use a single optical fiber connecting the telescope on the mast to the spectrometers in the body.

The team also found that it had to replace the original spectrometer CCDs – very simple and inexpensive commercial CCDs – with more complex devices, as the simpler ones failed a radiation test.

Using these various optical technologies, ChemCam will assist in the search for habitability on Mars. The MER rovers and orbital spacecraft of the past decade found abundant evidence for water in Mars’ past, fulfilling NASA’s mantra to “follow the water,” Wiens said. The Curiosity rover takes Mars exploration to the next level by studying the preservation of carbon-bearing materials.

The landing site selection is focused on concentrations of clay minerals and features such as river deltas. The rover is equipped with sensitive mass spectrometers and a gas chromatograph for detecting and identifying carbon-bearing compounds, a critical elemental capability lacking in previous rovers. ChemCam will also aid in the habitability search by remotely detecting carbon, hydrogen and other elements important to the question of habitability.

The ChemCam instrument suite was a joint development between France and the US, with the mast unit contributed by CNES, the French Space Agency, and the body unit developed and built at Los Alamos National Laboratory, which led the project. The laser was developed and built by Thales. The spectrometer optical design was provided by Ocean Optics. Jet Propulsion Laboratory, which is building the Curiosity rover, and Goddard Space Flight Center also contributed optical expertise for the ChemCam instrument.

Peering across space and time

The next several years will see the launch of other important optics-based instruments, such as those developed for the James Webb Space Telescope, or JWST – the successor to the Hubble. The JWST will consist of three major elements: the Spacecraft Element; the Optical Telescope Element, which will gather the light from afar and deliver it to the instruments; and the Integrated Science Instrument Module, with the instruments themselves.

These will include the Mid-Infrared Instrument, or MIRI, which was designed to aid scientists in a number of areas: studies of the first light in the early universe and of the formation of planets around distant stars, among others. Such events are often hidden from ground-based telescopes and even the Hubble by clouds of dust surrounding newly formed stars, for example. With MIRI, though, scientists will be able to peer through the dust; the instrument will be sensitive to light in the range of 5 to 28.3 µm, which can penetrate the clouds.

“It will be the most sensitive space- or ground-based telescope – the most sensitive astronomical instrument – working in that waveband by several orders of magnitude,” said John Pye, Space Research Centre manager at the University of Leicester and the university’s lead staff member for MIRI.

The instrument will consist of two main modules: an imager and a medium-resolution spectrograph. The imager will offer broad- and narrowband imaging, phase-mask coronagraphy, Lyot coronagraphy and prism low-resolution split spectroscopy in the 5- to 10-μm range, using a 1024 x 1024-pixel arsenic-doped silicon (Si:As) sensor chip assembly. The medium-resolution spectrograph will record spectral and spatial data, using four integral field units – here providing four simultaneous fields of view. This instrument will employ two 1024 x 1024-pixel Si:As sensor chip assemblies.

The James Webb Space Telescope – an artist’s impression of which is shown here – will carry, among other devices, a Mid-Infrared Instrument (MIRI) that will advance understanding of, for example, the first light in the early universe and the formation of planets around distant stars. Courtesy of the European Space Agency.

The greatest challenge in developing MIRI, Pye said, was ensuring operation at temperatures within 7° of absolute zero. The James Webb Telescope ultimately will make its home at L2, a gravitational pivot point some 150,000 km from Earth – moving away from the sun. Here, shaded from the light of both the sun and the Earth, it is cool enough for the instrument to record in the mid-infrared, and thus to obtain the measurements that will help scientists understand a range of phenomena.

“Although the instrument was assembled at room temperature,” Pye said, “we had to make sure it maintains alignment as it cools down.” They achieved this by using aluminum as much as possible in the design of the instrument, including most of the optical components. The filters are not aluminum. The material is light, strong and relatively low-cost, Pye said, and at the appropriate grade has extensive “space heritage.”

MIRI is currently undergoing testing at Rutherford Appleton Laboratory in the UK. Once completed, the instrument will be shipped to NASA’s Goddard Space Flight Center in Maryland, where it will be integrated with the James Webb Telescope’s Integrated Science Instrument Module. Courtesy of MIRI European Consortium, Rutherford Appleton Laboratory.

MIRI, which is being developed by NASA and a consortium of European partners allied with the European Space Agency, is being prepared for eventual deployment. The MIRI team is led jointly by professor Gillian Wright of the Science & Technology Facilities Council (STFC) Astronomy Technology Centre in Edinburgh, UK, and professor George Rieke of the University of Arizona, Tucson. The subassemblies – including the imager, the spectrometer optics, and the input-optics and calibration module – have been integrated into a flight model of MIRI, which is currently undergoing testing at STFC’s Rutherford Appleton Laboratory in the UK, using a chamber specially designed to reproduce the environment at L2.

Once the tests are completed, MIRI will travel to NASA’s Goddard Space Flight Center in Greenbelt, Md., where it will be integrated with the James Webb Telescope’s Integrated Science Instrument Module. Launch is planned for no sooner than 2014.

Photonics Spectra
Dec 2010
The scientific observation of celestial radiation that has reached the vicinity of Earth, and the interpretation of these observations to determine the characteristics of the extraterrestrial bodies and phenomena that have emitted the radiation.
astronomyBasic ScienceBoasbroad-band imagingcamerasCCDChemCamCNESCuriosityEuropean Space AgencyFeaturesGaryGary BoasGeorgeGeorge RiekeGillianGillian WrightGoddard Space Flight Centerimagingimaging and sensingJames Webb Space TelescopeJet Propulsion CenterJohnJohn PyeJWSTlaser-induced breakdown spectroscopylasers and laser systemsLIBSLos Alamos National LaboratoryLyot coronographyMarsMars Exploration RoverMars missionsMars roverMERSMid-Infrared InstrumentMIRInarrow-band imagingNASAOcean Opticsphase-mask coronographyprism low-resolution slit spectroscopyPyeRiekeRogerRoger WiensScience & Technology Facilities Council Astronomy Technology CentreSensors & DetectorsspectroscopySTFCThalesTucsonUniversity of ArizonaUniversity of LeicesterWiensWrightlasers

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