Alignment and testing of the NIRSpec filter and grating wheel assembly

In order to perform spectrometric measurements, the Near Infrared Spectrometer (NIRSpec) aboard the James Webb Space Telescope (JWST) needs the ability to select various spectral band widths and split these up into its comprised wavelengths. These functions are achieved by the Filter Wheel Assembly (FWA) and the Grating Wheel Assembly (GWA). The filters of the FWA select a different bandwidth of the spectrum each while the gratings on the GWA yield specific diffractive characteristic for spectral segmentation. A high spectral sensitivity as well as the ability to detect the spectra of various objects at the same time result in high requirements regarding the positioning accuracy of the optics of both mechanisms in order to link the detected spectra to the 2-dimensional images of the observed objects. The NIRSpec mechanism including FWA and GWA will operate at temperature levels below 42K which are established during testing inside of a cryostat. However the alignment and testing of these mechanisms requires a lot of thought since there is very limited access to the item under test within such a device. Alignment needs to be preloaded based on simulations and testing is reduced to optical methods and evaluation of electrical signals. This paper describes the methods used for the various alignment steps, the corresponding tests and their precision of measurement as well as the achieved accuracies in the mechanism performance.