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Conventional reflection echelle gratings of surface relief (SR) type have significantly different spectral characteristics for s- and p-polarizations. Another problem was the phenomenon of an anomaly, in which the efficiency of p-polarization of a specific diffraction order is greatly reduced due to the influence of surface plasmons. The reflector facet transmission (RFT) grating is a new echelle grating of transmission type in which the incident light from one side facet of the sawtoothshaped grating is reflected by the other side facet, and the diffracted light is exited from the flat back side of the grating. The RFT grating achieves a large angular dispersion, higher diffraction efficiency and wider bandwidth than SR echelle gratings. Furthermore, since the RFT grating can be placed near the collimator and camera (imaging) optical system, the astronomical instrument can be made smaller than the reflective echelle grating. We prototyped a RFT grating with hard resin by ultra-precision cutting using a diamond tool. As a result of measurement of the diffraction efficiency, it demonstrates that a RFT grating is highly efficient and has small scattering losses.
A grism (direct-vision diffraction grating) combines a diffraction grating and prism to make any order and wavelength go straight. By setting a grism into the collimating beam (necessary for an astronomical instrument to place pupil mask, filters, etc.) of an imaging instrument, it becomes possible to easily perform spectroscopic observations. The prism array is a device aimed at increasing the angular dispersion (resolving power) of a grism within the limited size of the collimating beam part of the instrument.
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