The long process of getting the James Webb Space Telescope ready to begin collecting science data continues, and the Webb team has met another goal with the alignment of three out of its four instruments. The alignment process is a set of careful very small adjustments to each instrument to make sure they are in exactly the right location to receive light from the telescope’s large primary mirror. A few weeks ago the telescope’s mirrors were aligned with its main camera, called NIRCam, and now the telescope’s other instruments are being similarly adjusted.
Webb’s three near-infrared instruments (the Near-Infrared Slitless Spectrograph or NIRISS, the Near-Infrared Spectrometer or NIRSpec, and the Near-Infrared Camera or NIRCam) and its guidance sensor (the Fine Guidance Sensor or FGS) are now all aligned to its mirrors, leaving just the one mid-infrared instrument to go. The mid-infrared instrument (MIRI) takes longer to align because it uses a different type of sensor, which have to be cooled to an extremely low temperature of just 7 degrees kelvin. MIRI is still in the process of being cooled down to its operating temperature, and once it reaches this milestone then it too can be aligned.
The team had planned to make adjustments to the telescope’s secondary mirror — a smaller round mirror on the end of a boom arm — during the alignment process for the first three instruments, called phase six. However, it turned out that their alignments were so accurate that this wasn’t necessary, so they will wait until MIRI is fully cooled before making any final tweaks to the secondary mirror, in phase seven.
“As a general rule, the commissioning process starts with coarse corrections and then moves into fine corrections. The early secondary mirror coarse corrections, however, were so successful that the fine corrections in the first iteration of Phase Six were unnecessary,” said Chanda Walker, Webb wavefront sensing and control scientist at Ball Aerospace, in a NASA blog post. “This accomplishment was due to many years of planning and great teamwork among the wavefront sensing team.”
Once MIRI is cooled and all four instruments are aligned, there will be a second multi-instrument alignment phase to make any final small tweaks or adjustments. With alignment complete, the team will be able to move on to calibrating the instruments, getting them ready to begin science operations this summer.
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