Wavefront Sensing and Control on the James Webb Space Telescope

Most WFS&C steps are performed at a single field point in NIRCam. During the initial alignment, however, multi-field, multi-instrument wavefront sensing is also performed in order to remove ambiguities that might arise when using NIRCam alone. In principle, these WFS&C steps can be repeated as necessary during the Webb mission.

After OTE commissioning has been completed, a maintenance process of WFS&C executes for the duration of the mission. In its cooled, equilibrium state, the primary mirror experiences a large temperature gradient (some 20–30 degrees over the 6.5 m aperture) in the direction away from the sunshield. This gradient is stable, and any distortion that it induces can be corrected during OTE commissioning. However, as the telescope is operated and pointed at different positions within the field of regard, small temperature changes can occur at the level of tenths of a degree. These changes induce changes in the mirror figures, which must be controlled to maintain image quality.

During regular operations, NIRCam observations for WFS&C are executed every two days to monitor the Webb image quality. The weak lenses are used to image a bright target star using several defocus settings. The implied wavefront errors are corrected if they are deemed unacceptable, which is not expected to occur more frequently than every two weeks.

The software modules to analyze WFS data and control the mirror actuators are being developed by Ball Aerospace. The Institute is incorporating these modules into the WFS&C executive software, which handles the interfaces with other aspects of the Webb Science and Operations Center (such as the archive, proposal planning, and flight operations). The entire WFS&C software subsystem (WSS) recently passed its critical design audit.

The functionality of the WFS&C algorithms has been verified on the test bed telescope (TBT), a 1/6 scale model of the Webb OTE, at Ball Aerospace. Additional verification and validation activities continue in the coming years. The NIRCam WFS&C hardware has recently been tested with good results in a cryogenic and vacuum environment (cryo-vac), using the NIRCam engineering test unit. Later tests include cryo-vac testing at Johnson Space Center of the integrated OTE and ISIM.

OPD maps inferred from WFS data are delivered to the Webb archive and are available to astronomers. Software that calculates PSFs, by combining OPD data with optics models and I&T data, is planned for development at the Institute. This is akin to Hubble’s TinyTim software, but deals with the many optical complexities specific to Webb. Software with more limited functionality, called jwpsf, is already available here. This software produces calculations based on realizations of the wavefront error budget, rather than hardware characterizations. However, it is useful for prospective observers to assess the expected image quality as function of instrument, detector, wavelength, and filter.