Fabrication and test of silicon grisms for JWST-NIRCam

We report on the design, fabrication, and evaluation of a set of silicon grisms for the NIRCam instrument on NASA's James Webb Space Telescope. The primary purpose of these devices is to aid in the alignment of JWST's deployable primary mirror. The grisms will also offer opportunities for slitless astronomical spectroscopy. The design of the grisms was driven by a need to fit into a constrained space, by a need for high resolving power across a broad spectral band, and by the need to survive the cosmic ray dosage to which the instrument will be subjected. The University of Texas Silicon Grating Laboratory is fabricating four identical grisms to cover 2.5-5 μm with a resolving power of 1770 at the blaze wavelength. There will be two grisms, with dispersion axes oriented at 90°, in each arm of the NIRCam long-wavelength camera. We pattern the gratings lithographically onto high resistivity float-zone silicon prisms following the recipe developed for the recently completed grism suite for the FORCAST camera on SOFIA. We discuss the design and production of the NIRCam devices and present the results of the optical testing of the grating surfaces showing that the devices will likely exceed their performance requirements.