Development of volume phase holographic (VPH) grism for visible to near-infrared instruments of 8.2-m Subaru telescope

Abstract

Faint Object Camera and Spectrograph (FOCAS) is a versatile open use optical instrument of the 8.2m Subaru Telescope for the enabling imaging and spectroscopic observations. A suite of nine grisms optimized for different resolving powers and appropriate wavelength ranges have been planned for this instrument. Five grisms among the nine were fabricated by a replication method and four additional grisms with the resolving power of 5,000 are under fabrication using the volume phase holographic (VPH) gratings. A very high dispersion Echelle grism with the resolving power over 10,000 is also developing with a VPH grating sandwiched between two high index prisms. The high dispersion VPH grisms are 110 by 106 mm in aperture size and 110 mm in maximum thickness. We employed a photosensitive resin as the recording material for a thick VPH grating. In the present paper, we report the result of evaluation of the diffraction efficiency of the replica grisms and the VPH gratings by means of the rigorous coupled-wave analysis (RCWA) method to derive the optimum design parameters. An optimized VPH grating with a size of 50 by 50 mm was experimentally fabricated by means of a two-wave interference exposure at 532nm. The measured diffraction efficiency of this VPH grating is 88% at 400nm. We performed spectroscopic observations of Leonid meteors by using an image intensified CCD video camera and an experimentally fabricated VPH grism as an objective dispersion element at Nobeyama, National Astronomical Observatory of Japan in November, 2001. Consequently, we successfully obtained numerous high-quality spectroscopic data of meteors.