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The Prime Focus Spectrograph (PFS) is a major instrument under developmentfor the 8.2 m Subaru telescope. Four identical spectrograph modules are locatedin a room above one Nasmyth focus. A 55~m fiber optic cable feeds light to thespectrographs from a robotic positioner at the prime focus, behind thewide-field corrector developed for Hyper Suprime-Cam. The positioner contains2400 fibers and covers a 1.3~degree hexagonal field of view.  The spectrograph optical design consists of a Schmidt collimator, twodichroic beamsplitters to split the light into three channels, and for eachchannel a volume phase holographic (VPH) grating and a dual-corrector, modifiedSchmidt reimaging camera. This design provides a 275~mm collimated beamdiameter, wide simultaneous wavelength coverage from 380~nm to 1.26~\textmu m,and good imaging performance at the fast f/1.05 focal ratio required from thecameras to avoid oversampling the fibers. The three channels are designated asthe blue, red, and near-infrared (NIR), and cover the bandpasses 380--650~nm(blue), 630--970~nm (red), and 0.94--1.26~\textmu m (NIR). A mosaic of twoHamamatsu 2k$\times$4k, 15~\textmu m pixel CCDs records the spectra in the blueand red channels, while the NIR channel employs a 4k$\times$4k,substrate-removed HAWAII-4RG array from Teledyne, with 15~\textmu m pixels anda 1.7~\textmu m wavelength cutoff.  VPH gratings were an obvious choice for PFS and a set of three prototype VPHgratings (one each of the blue, red, and NIR designs) was ordered and has beenrecently delivered. In this paper we present the design and specifications forthe PFS gratings, the plan and setups used for testing both the prototype andfinal gratings, and results from recent optical testing of the prototypegrating set.

Volume phase holographic gratings for the Subaru Prime Focus Spectrograph: performance measurements of the prototype grating set