  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.
