On-orbit Degradation of the JWST/NIRSpec Micro-Shutter Array: Multiplexing and Target Acquisition

Abstract: A statistical analysis of the impact of the diminishing number of operational shutters experienced by the JWST/NIRSpec Micro Shutter Array since Commissioning is presented. It is shown that the number of high priority science targets that NIRSpec is able to observe simultaneously has so far decreased by 3.1%. Of greater concern, however, is NIRSpec's diminished ability to carry out autonomous MSATA target acquisition, which is more sensitive to the loss of shutters than is the multiplexing. In the flagship case of MSA observations of deep fields, the number of pointings at which it is not possible to reach the required minimum number of 5 Valid Reference Stars has increased from 4.9% to 6.3% and is beginning to become noticeable. Similarly, the number of higher risk target acquisitions that need to be carried out with fewer than the maximum allowed number of 8 Reference Stars has grown from 27% to 31%.