Development of Volume Phase Holographic (VPH) Grism for Visible to Near Infrared Instruments of the 8.2m Subaru Telescope

FOCAS of an optical instrument for the 8.2m Subaru telescope is planned to provide nine grisms. The five grisms out of nine commissioned ones have been made by a replication method. FOCAS will employ volume phase holographic (VPH) grisms for the residual four. A very high dispersion grism of the resolving power over 10,000 which is consisted a higher order (Echelle type) VPH grating sandwiched between two high index prisms, is also developing. The VPH grisms of FOCAS for high dispersion spectroscopic observations is 110 by 106 mm in aperture size and 110 mm in maximum thickness. We applied a photosensitive resin as the recording materials for a tick VPH grating. In this study, we evaluated the efficiency of replica and VPH gratings by means of the analysis program valid for transmission gratings of the rigorous coupled-wave analysis (RCWA) method to know an optimum condition for manufacture, and characterized the performance of fabricated grisms. The VPH grating with a size of 50 by 50 mm was experimentally fabricated by means of a two-wave interference exposure at λ=532nm. Measured diffraction efficiency of the VPH grating is 88 % at λ = 400nm. We performed spectroscopic observations of Leonid meteors by using an image intensified CCD video camera and the experimental VPH grism as a objective dispersion element at Nobeyama, National Astronomical Observatory of Japa