SHIMMER on Shuttle Flight STS-112
SHIMMER (Spatial Heterodyne Imager for Mesospheric Radicals) is a high-resolution UV interferometer, a cooperative development of NRL (Naval Research Laboratory) in Washington DC, UWM (University of Wisconsin at Madison), and St. Cloud State University in St. Cloud, Minnesota. The primary science objective is to assess the performance and to acquire global profiles of hydroxyl (OH) in the upper stratosphere and mesosphere by measuring its resonance fluorescence in the UV range. 1) 2) 3) 4) 5) 6) 7)
Launch: SHIMMER was flown on Shuttle flight STS-112, Oct. 7-18, 2002. The flight of SHIMMER as a Shuttle middeck payload, served as a proof-of-concept demonstration of this new and powerful technique, facilitating future high-resolution imaging instruments that are small, rugged, light-weight and have no moving parts.
SHIMMER is the first spaceborne instrument implementation realizing the SHS (Spatial Heterodyne Spectroscopy) technique conceived by Fred Roesler and John Harlander of the University of Wisconsin and of St. Cloud State University, respectively (see “Background on SHS” below). The SHIMMER demonstration instrument was developed by NRL under the sponsorship of the DoD Space Test Program and support of NASA and NSF (National Science Foundation).
SHIMMER is of MAHRSI (Middle Atmosphere High Resolution Spectrograph Investigation) heritage, an NRL instrument flown on Shuttle flights STS-66 (Nov. 3-14, 1994) and STS-85 (Aug. 7-19, 1997). MAHRSI measured vertical profiles of OH concentrations using conventional techniques. The SHIMMER instrument design turned out to be approximately one-seventh the mass and volume of the MAHRSI spectrometer, with the added advantage of higher spectral resolution and much greater sensitivity - and with no moving optical components.
Observation concept: In the SHIMMER FTS (Fourier Transform Spectrometer) design, 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 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 UV-sensitive 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 (FOV) of the instrument.
The major instrument elements of SHIMMER are the telescope, the relay optics, the interferometer, and the CCD camera. The telescope images the sunlit limb scene onto the gratings. The relay optics create an image of the fringe localization plane (gratings) at the CCD.
The optical elements of the interferometer (i.e. the beamsplitter, prisms, and gratings) are held by a Vascomax steel structure which allows precise positioning of each element. The CCD rows are binned on-chip resulting in 32 interferograms of 1024 elements, each with approximately 2 km altitude resolution and 60 mÅ (milli-Angstrom) spectral resolution over the 3 nm ultraviolet passband 307-310 nm. This passband was chosen because it includes the solar resonance fluorescence from the (0,0) band of hydroxyl (OH). Every pixel row of the CCD, parallel to the horizon, contains the spectral information (interferogram) for the corresponding tangent altitude of the FOV. An altitude range from 30 km to 100 km was covered (SHIMMER imaged the altitude range simultaneously).
Table 1: Summary of basic instrument specifications
Figure 1: Photo of the SHIMMER instrument (image credit: NASA, NRL)
Figure 2: Schematic illustration of SHIMMER measurement technique (image credit: NRL)
Figure 3: SHIMMER block diagram and optical layout (image credit: NRL)
The camera also includes a shutter mechanism, thermoelectric coolers, cooling fins, and fans for convectively dispelling heat into the cabin air.
The SHIMMER middeck instrument was controlled by a laptop. As a secondary payload on STS-112, SHIMMER was allocated a total of three orbits for observations during the mission, a checkout period prior to ISS docking, and two nominal operation periods following ISS undocking. During each period, the crew maneuvered the Orbiter to an attitude that placed the SHIMMER line-of-sight tangent to the limb at a predetermined optimal altitude while placing the window out of the sun's direct illumination. The instrument was attached to the Orbiter crewhatch using a bracket assembly that utilized threaded studs located around the optical window.
The data from the STS-112 mission indicate that SHIMMER met design goals, producing high spectral resolution solar and OH spectra over a broad altitude range without the use of any moving optical components. The mission has provided a successful and invaluable proof-of-concept of the SHS technology in space-based remote sensing.
Figure 4: Photo of the SHIMMER instrument assembly (image credit: NRL)
Figure 5: Illustration of the SHIMMER instrument (image credit: NRL)
Background on SHS (Spatial Heterodyne Spectroscopy)
Interferometric spectroscopic instruments offer sensitivities typically 100 times those of conventional grating spectroscopic instruments of similar size in many applications. In 1990, Fred Roesler and John Harlander at the Department of Physics of the University of Wisconsin at Madison conceived and developed an unusual and novel interference spectroscopic technique called Spatial Heterodyne Spectroscopy (SHS). It is an interferometric Fourier transform technique, but unlike conventional Fourier transform spectroscopy (FTS) it requires no moving parts for obtaining a spectrum. Moreover it can be field-widened without moving parts to provide additional gains of typically 100. As a result of these gains, the SHS instrument can be made small and still achieve a level of performance equal or superior to grating instruments of practical dimensions. At the same time it avoids many of the mechanical problems associated with conventional field-widened FTS techniques.
SHS provides the first practical approach to extend interference spectroscopy into the FUV (Far Ultraviolet, 1200-2000 Â) spectral range. The technique appears to be offering high-resolution spaceborne spectroscopy applications in astronomy (detection of faint interstellar emission lines, study of the dynamics of hot interstellar gases) as well as in Earth observation. Examples in this field include the measurement of vertical density profiles of the hydroxyl (OH) radical in the middle atmosphere (30-100 km), study of the distribution of aerosols in the mesosphere, and to investigate the role of OH in the photochemistry of water vapor and ozone in the presence of aerosols in the mesosphere. 8) 9) 10) 11)
In the basic SHS design, Fizeau fringes of wavenumber-dependent spatial frequency are produced by a modified Michelson interferometer in which the return mirrors are replaced by conventional blazed diffraction gratings (see Figure 6 part a). The fringes are recorded on a position sensitive detector and Fourier-transformed to recover the spectral content of the source. Zero spatial frequency corresponds to the Littrow wavenumber of the gratings, which can be chosen by adjustment of the interferometer. Since zero spatial frequency corresponds to a finite wavenumber, SHS measures differences between the source and alignment wavelengths, and high resolution spectra over a limited spectral range can be recovered with modest requirements on the spatial resolution of the detector. In this process, no element is mechanically scanned.
Figure 6: Schematic diagram of the SHS configuration
The resolving power of an SHS design is equal to the theoretical resolving power of the dispersive (i.e. grating) system while the field of view of the system is characteristic of interferometric spectrometers (conventional Michelson and Fabry-Perot). The interferometric field of view gives SHS systems a 100-fold gain in sensitivity for diffuse source spectroscopy over diffraction grating spectrometers of the same size and resolving power. Furthermore, field widening techniques can be applied to SHS systems which enable SHS to view even larger fields of view. Gains associated with field widening are typically two orders of magnitude in solid angle over conventional interferometers (104 larger than diffraction grating spectrometers).
Field widening is accomplished by inserting prisms into the arms of the interferometer (see Figure 6 part b). The prism apex angles are chosen so that from a geometrical optics point of view the gratings appear coincident. The geometrical path difference in the system is then near zero for a wide range of input angles and the system behaves much like a conventional Michelson interferometer at zero path difference. Aberrations introduced by the prisms ultimately limit the field of view, but not before large gains can be achieved in many applications.
The SHS optical system is similar to the Twyman-Green interferometer that is often used for optical testing of surfaces. In the SHS system, diffraction gratings replace the test and reference surfaces of the Twyman-Green; hence grating figure errors, if present, distort the Fizeau fringe pattern. Such fringe distortions degrade the instrumental profile and reduce the signal-to-noise ratio in the recovered spectrum. However SHS fringe distortions for monochromatic light provide a measure of the grating figure errors that can be used in software to correct broadband interferograms without loss in signal-to-noise ratio.
Fringe distortions resulting from figure errors and index inhomogeneities in the beam splitter and field widening prisms can be corrected if these elements are of good quality over the area sampled by one spatial resolution element on the detector. In the SHS system, these elements are nearly focused on the detector; hence the area that is sampled at each detector pixel is relatively small. This is to be contrasted with conventional interferometers where a single channel detector collects light over the full aperture of the critical optical components. Figure errors in conventional interferometers will result in a reduction in the contrast of the fringes and a reduced signal-to-noise ratio in the recovered spectrum.
Due to the heterodyne nature of SHS interferograms, figure errors distort the individual (carrier) fringes to a greater extent than the envelope (visibility) of the fringes. The envelope provides physical information on the spectral line shape; so therefore, a correction technique is needed which corrects the carrier fringes without distorting their envelope.
The optical concepts that lead to the predicted high performance for the SHS were also demonstrated and validated by Harlander and Roesler in various field tests.
1) J. M. Harlander,, F. L. Roesler, J. G. Cardon,. C. R. Englert, R. R. Conway, “SHIMMER: A spatial heterodyne spectrometer for remote sensing of the Earth's middle atmosphere, Applied Optics, Vol. 41, pp.1343-1352, 2002
2) Note: The technology of spatial heterodyne spectroscopy was developed by Fred Roesler and John Harlander of UWM in 1990. SHIMMER observations represent the first spaceborne demonstration of this technology.
3) Information provided by Robert R. Conway and Christoph R. Englert of NRL, Washington D. C.
4) J. Harlander, H. T. Tran, F. L. Roesler, K. P. Jaehnig, et al., ”Field-Widened Spatial Heterodyne Spectroscopy: correcting for Optical Defects and New Vacuum Ultraviolet Performance Tests,” EUV, X-Ray and Gamma-Ray Instrumentation for Astronomy V, SPIE Proceedings 1994, Vol. 2280, p. 310-319
5) Joel G. Cardon, Christoph R. Englert, John M. Harlander, Fred L. Roesler, Michael H. Stevens, “SHIMMER on STS-112:: Development and Proof-of-Concept Flight,” Space 2003, Sept. 23-25, 2003, Long Beach, California, AIAA 2003-6224, URL: http://uap-www.nrl.navy.mil/uap/7641/publications/2003_6224.pdf
6) J. G. Cardon, C. R. Englert, M. H. Stevens, R. R. Conway, J. M. Harlander, F. L. Roesler, “New Observations of Hydroxyl from the Space Shuttle by NRL's SHIMMER,” URL: http://www.nrl.navy.mil/content.php?P=04REVIEW172
7) “Expanding the Station Backbone/ Increasing Crew Capabilities,” STS-112/9A Shuttle Press Kit, Sept. 5, 2002, URL: http://www.shuttlepresskit.com/sts-112/spk-112.pdf
8) J. M. Harlander, “Spatial Heterodyne Spectroscopy, Interferometric Performance at any Wavelength without Scanning,” Ph.D. Thesis, University of Wisconsin-Madison, 1991
9) J. Harlander, R. J. Reynolds, F. L. Roesler, “Spatial Heterodyne Spectroscopy for the Exploration of Diffuse Interstellar Emission Lines at Far-Ultraviolet Wavelengths,” The Astrophysical Journal, Vol. 396, 1992, pp. 730-740
10) Note: The invention was patented (US patent No 5059027, issued Oct. 22, 1991), the patent was assigned to the Wisconsin Alumni Research Foundation.
11) Christoph R. Englert, John M. Harlander, Joel G. Cardon, Fred L. Roesler, “Correction of phase distortion in spatial heterodyne spectroscopy,” Applied Optics, Vol. 43,No 36, Dec. 20, 2004, pp. 6680-6687, URL: http://uap-www.nrl.navy.mil/uap/7641/publications/6680.pdf
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.