Wavefront Sensing and Control on the James Webb Space Telescope

To obtain the 6.5 m diffraction-limited performance, it is necessary to co-phase the segments. This involves two main steps, each of which uses dedicated hardware within the Near-Infrared Camera (NIRCam). These steps are called coarse phasing and fine phasing.

Coarse phasing uses dispersed Hartmann sensing. This process uses grisms to create wavelength-dependent interferograms based on the light reflected by adjacent mirror segments. This allows identification of large piston errors between segments, which would then be corrected.

Fine phasing is done using a set of weak lenses, which can be chosen to yield images with five defocus settings: –4, –8, 4, 8 and 12 waves at 2.12 micron. Images thus obtained can be analyzed with focus-diverse phase retrieval algorithms, which yield a map of the optical path difference (OPD) over the telescope pupil. The specific algorithm used for Webb is the “hybrid diversity algorithm” (HDA), which is a variation of the well-known Gerchberg-Saxton scheme, with an additional outer loop in the process to provide phase-unwrapping and robustness even with large phase errors.

The phase-retrieval process requires amplitude information for the pupil, which is also obtained using dedicated NIRCam hardware. A Pupil Imaging Lens (PIL) provides the required imaging. The resulting data can also give insight into vignetting or other alignment problems.

Once an OPD map has been determined from NIRCam imagery, its low-spatial frequency content is corrected by moving actuators appropriately to reposition the segments. Mid- and high-spatial-frequency figure errors, which may be caused by manufacturing errors within a segment, cannot be corrected by this process. Nevertheless, these errors are bounded by robust error budgets in the manufacturing process, which ensures diffraction-limit performance. Furthermore, these uncorrectable errors are stable and can be accurately characterized during telescope integration and testing (I&T). This enables accurate prediction and characterization of the post-commissioning PSF.