All 17 observing modes of the James Webb Space Telescope undergo routine performance monitoring and calibration. This month, while performing calibration by comparing the brightness of standard stars that have been well-cataloged by other observatories to what Webb’s Mid-Infrared Instrument (MIRI) was receiving, team members noticed a discrepancy in the data.
Further analysis of MIRI’s Medium Resolution Spectroscopy (MRS) mode revealed that at the longest wavelengths, the throughput, or the amount of light that is ultimately registered by MIRI’s sensors, has decreased since commissioning last year. No effect has been seen for MIRI imaging, and there is no risk to the instrument. All other observation modes – within MIRI and each of Webb’s other scientific instruments – remain unaffected.
NASA and its partners are developing a systematic plan to approach, analyze, and then explore the issue. The Webb team will continue MIRI observations as planned. The team will gather all relevant ground test and flight data to fully assess MRS performance. Further test observations will be taken to completely characterize the nature of the issue using this particular mode of observation. Next, a plan for long term-monitoring will be enacted, while the team continues to investigate the cause, identify risks, and explore mitigations that would potentially improve performance. One possible mitigation strategy includes taking slightly longer exposures at the affected wavelengths to increase the signal to noise.