SSTL with University of Leicester
Development of Grism Technology
A study has been performed by Surrey Satellite Technology Ltd (SSTL) in partnership with the University of Leicester (UL) Space Research Centre on the specification and design of imaging spectrometer instruments using immersed diffraction gratings – called “grisms”. The essential advantage of grisms over conventional diffraction gratings, particularly in the context of space-based instrumentation, is that they provide higher spectral dispersion. Larger dispersion angles imply that grating sizes can be smaller, and the optics associated with the dispersing element can also be smaller. Grism designs can therefore offer imaging spectrometers of acceptable size, where conventional grating designs tend to be excessively large.
Grism imaging spectrometers will be applied particularly where there is a need for very fine spectral resolution over narrow spectral bands; the main area of interest for the study has been monitoring of atmosphere chemistry from space, by measuring the spectral absorption of selected gas species. High resolution is needed in the short-wave infrared (SWIR) band for measurements of concentrations of greenhouse gases CO2 and CH4, and the air quality pollutant CO. High-resolution measurements in the visible region are needed on the oxygen A-band for cloud and pressure measurements, to provide data to interpret measurements in other spectral ranges. Representative spectrometer requirements have been investigated and defined by UL, using inputs from sources such as the CAPACITY study report which informs requirements for the ESA Sentinel 4 and 5 missions. Whilst the CEOI study has concentrated particularly on CO2 absorption bands and the Oxygen A band, the requirements for measurements of other species are similar, so that the results of the study have broad relevance, particularly in the SWIR band.
Instrument requirements have been refined by radiometric analysis to calculate the optical apertures and system sizes needed to achieve target signal-to-noise ratios. A baseline low-earth orbit (LEO) mission scenario has been selected for detailed design of instruments. Observation from LEO will offer global coverage over a 2-day period at spatial resolution around 6 km; a constellation of small satellites may be used to provide more frequent observations. A grism spectrometer system designed for GEO would offer more-frequent coverage of part of Earth surface at coarser resolution.
Initial spectrometer designs have provided target specifications for the key grism components, used to initiate an investigation of grism technology, including both theoretical computation of grism efficiencies (as a function of grism materials, grating angles and profile shapes) and discussions with the most likely manufacturer. The investigation has clarified the feasible designs and potential performance capabilities of immersed gratings. The final task has been to revise spectrometer system designs and to confirm the performance that can be achieved. In conclusion, grism designs can meet the requirements of high-resolution spectrometers for remote sensing of atmosphere chemistry in the visible and SWIR bands. The designs have very significant advantages in terms of compactness of optics and structures, making them ideally suited for deployment on Earth orbiting satellites. Similar designs can be considered for use on airborne platforms and at ground level. The work is led by Dan Lobb, SSTL with University of Leicester