Grisms Developed for FOCAS

TL;DR

This paper details the development and testing of various grisms for the FOCAS instrument on the Subaru Telescope, including their design, materials, and performance in laboratory and astronomical observations.

Contribution

It introduces new VPH and replica surface-relief grisms for FOCAS, with detailed performance evaluation and application examples.

Findings

Grisms achieve resolving powers from 280 to 8200.

Laboratory and test observations confirm grism performance.

Successful application in astronomical spectroscopy demonstrated.

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 X 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.