Operability assessment concept for the JWST/NIRSpec micro-shutter array (MSA)

JWST/NIRSpec will include the first space-borne multi-object spectrograph, comprising a micro-shutter array (MSA) of a quarter of a million closable apertures that can be individually addressed to select up to a couple of hundred objects within a ~3.2x3.4 arcmin field of view. Although more than ~85% of the unvignetted shutters are fully operational, the high degree of mechanical movement combined with complex circuitry on a small scale, inevitably leads to some non-operable shutters. In this paper we present an overview of the operability assessment concept for the MSA, employed during both ground tests and in flight. We describe the procedures used to detect, mitigate against, and even repair the non-operable shutters, and show the effect upon the multiplexing capability and output data from NIRSpec. We also present the operability trending results from ground tests, and discuss the probable impact on nominal operations after launch.