Shimmer on Sts-112: Development and Proof-of- Concept Flight

The Spatial Heterodyne Imager for Mesospheric Radicals (SHIMMER), which is based on a new interferometric technique called Spatial Heterodyne Spectroscopy (SHS), flew on the Space Shuttle Atlantis mission STS-112 in October 2002. SHS has the advantages of high throughput, high spectral resolution, small size, low mass, all in a rugged instrument with no moving optical components. The SHS proof-ofprincipal flight successfully demonstrated the suitability of SHS for spaceflight applications where high spectral resolution measurements over a relatively narrow spectral band are required. In addition, the highest spectral resolution measurement of middle atmospheric hydroxyl (OH) solar resonance fluorescence ever achieved was made by SHIMMER during this mission. INTRODUCTION The Spatial Heterodyne Imager for MEsospheric Radicals (SHIMMER), designed specifically for flight on the Space Shuttle middeck, is based on a newly developed interferometric technique called Spatial Heterodyne Spectroscopy (SHS) developed by Roesler and Harlander at the University of Wisconsin and St. Cloud St. University, respectively. The SHS technique is similar to a Michelson interferometer or Fourier transform spectrometer (FTS), with the mirrors in the interferometer arms replaced by fixed, tilted gratings equidistant from the beamsplitter. Diffraction at the gratings results in a wavenumber-dependent tilt of the wavefronts recombining at the beamsplitter, and interference of the tilted wavefronts creates Fizeau fringes at the instrument’s charge-coupled device (CCD) detector. The Fourier transform of the interferogram produced at the CCD yields the incident spectrum. When viewing the earth’s limb from space, the scene can be imaged on the gratings and the gratings imaged on the CCD, with the dispersion plane parallel to the horizon. Using this technique, limb scanning can be avoided since the detector simultaneously records interferograms in each horizontal row of the CCD, corresponding to discrete altitudes in the range subtended by the field-of-view of the instrument. Under the sponsorship of the DoD Space Test Program, SHIMMER flew on the Space Shuttle Atlantis mission STS-112 in October 2002. The 500 mm focal-length telescope (see Figure 1) was designed to image the earth’s limb, over the altitude range of approximately 30 – 100 km, on the gratings. Relay optics focus the fringe localization plane at the gratings onto a UVsensitive CCD. The CCD rows are binned on-chip resulting in thirty two 1024-element interferograms, each with approximately 2 km altitude resolution and 60 milli-Angstrom spectral resolution over the 3 nm ultraviolet passband 307 – 310 nm. This passband was chosen because it includes solar resonance flourescence from the (0,0) band of hydroxyl (OH). The primary goal of the instrument development and flight was to assess the suitability of SHS instruments for spaceflight and to determine the performance of SHIMMER by measuring high spectral resolution solar spectra and the much dimmer superimposed ultraviolet spectrum emitted by OH molecules in the 30 – 100 km altitude range of the atmosphere. OH is one of the key radicals in middle atmospheric chemistry. It is highly reactive, historically one of the least measured atmospheric constituents, and is critical to ozone chemistry throughout the atmosphere, particularly above 50 km where it participates in the only known ozone-destroying chemical process. Its observation above 65 km can also provide an indirect measure of water vapor, which photodissociates to produce OH. Its measurement is a rigorous test of any hyperspectral imaging technology because retrieval of the OH radiance profile, entailing the removal of the bright and spectrally complex ozone-attenuated Rayleigh-scattered solar background, demands highly accurate knowledge of the instrumental line shape function, wavelength calibration, radiometric calibration, and instrumental contributions to the signal and noise. The first global OH observations were acquired by NRL during flights of its Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) spectrometer on STS-66 and STS-85. The size and weight of conventional grating spectrometers like MAHRSI, however, are not suitable for future space missions. In contrast, the practical advantages of SHS for future space shuttle, satellite, and interplanetary Space 2003 23 25 September 2003, Long Beach, California AIAA 2003-6224 This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. Report Documentation Page Form Approved