| Popis: |
A number of Earth-observation missions, particularly those aimed at monitoring atmosphere composition, require hyperspectral instruments measuring Earth reflectance and emission at very fine spectral resolution. Examples include instruments for ESA’s Sentinel 5 mission, operating in the short-wave IR (SWIR) spectral region to quantify greenhouse gases, and also in the near-IR region (Oxygen-A band) to provide data on clouds, aerosols and atmosphere pressure. Conventional dispersive spectrometers, typically using diffraction gratings, are needed to provide data at moderate spatial resolution over large swath widths. Fine spectral resolution, together with the system aperture needed for radiometric resolution, tends to demand large beam diameters at gratings; this can lead to excessively large spectrometers, driven by the apertures of gratings and the associated collimators and camera lenses. This presents problems for accommodation of space-based spectrometers, particularly on small platforms. Apertures, and complete spectrometer sizes, are reduced if the gratings provide high angular spectral dispersion (radians/nm) – this is a property of “immersed” gratings 1,2. An immersed grating is a reflecting grating formed on a prism of refracting material, in which the incident and diffracted beams are both in the refracting medium. Instruments will typically use immersed gratings in silicon for short-wave IR bands, and silica for near-IR bands. This paper describes example designs for spectrometers using immersed gratings, and early results from a current development of immersed gratings are outlined. |
