Grisms Developed for FOCAS

Abstract

Faint Object Camera and Spectrograph (FOCAS) is a versatile common-use optical instrument for the 8.2m Subaru Telescope, offering imaging and spectroscopic observations. FOCAS employs grisms with resolving powers ranging from 280 to 8200 as dispersive optical elements. A grism is a direct-vision grating composed of a transmission grating and prism(s). FOCAS has five grisms with replica surface-relief gratings including an echelle-type grism, and eight grisms with volume-phase holographic (VPH) gratings. The size of these grisms is 110 mm×106 mm in aperture with a maximum thickness of 110 mm. We employ not only the dichromated gelatin, but also the hologram resin as a recording material for VPH gratings. We discuss the performance of these FOCAS grisms measured in the laboratory, and verify it by test observations, and show examples of astronomical spectroscopic observations.

Identification of the progenitors of rich clusters and member galaxies in rapid formation at z > 2

“…It provides a multi-object spectroscopic (MOS) capability at 0.9∼2.5 µm with a 4 ′ ×7 ′ filed of view covered by two HAWAII-2 2048×2048 arrays with the spatial resolution of 0.117 arcsec/pixel. We used a low-resolution grism (HK500: R∼500 for 0.8 arcsec slit width) for 5 masks, and a high resolution grism (VPH-K: R∼1700 for 0.8 arcsec slit width; see Ebizuka et al 2011) for one of the 3 masks for PKS1138. We made more than 15 slits per mask.…”

Simulation and partial prototyping of an eight‐junction holographic spectrum‐splitting photovoltaic module

“…Next, material absorption and Fresnel losses were incorporated. Absorption of DCG was extracted from published internal transmittance data . NOA 88, used to glue the gratings into a stack as well as attaching the superstrate of each grating, is assumed to be index matched.…”

“…Holographic diffraction gratings, on the other hand, can be fabricated in a large area format at high fidelity. Dichromated gelatin (DCG), a common holographic recording medium, is highly transparent in the wavelength range of interest for photovoltaics and despite being hygroscopic can be quite durable under high‐intensity illumination for extended periods when encapsulated …”

Novel gratings for astronomical observations

“…On the other hand, a volume phase holographic (VPH) grating, in which the refractive index is modulated sinusoidally, is able to achieve diffraction efficiency up to 100% for S or P polarization at the first diffraction order [8,9]. VPH gratings are installed in numerous astronomical instruments including the 8.2 m Subaru telescope in order to achieve several times larger angular dispersion as a SR transmission grating [10,11]. However, since the VPH grating has low diffraction efficiency in higher diffraction orders, it is unsuitable for an echelle spectrograph.…”