NIRCam Wavefront Sensing
JWST NIRCam wavefront sensing..............
NIRCam will also be used for wave front sensing to assure perfect alignment and shape of the different primary mirror segments so that their wavefronts match properly, creating a diffraction-limited 6.6 m telescope, rather than overlapping images from 18 individual 1.3m telescopes. Each imaging module has a pupil wheel with extra optics and pupil analyzers for wave front sensing. The wave front sensing capability is provided fully redundant in both imaging modules because the mission depends critically on its functionality.
The wavefront sensing (WFS) activities occur in two distinct phases of the mission.
During commisioning NIRCam will be used to align all 18 primary mirror segments so that the primary mirror would function as though it were a monolithic mirror. The mirror segments must be positioned so their wavefronts are phased.
Steps in the wave-front sensing and control process during JWST commissioning are shown in the figure below. NIRCam-specific activities are highlighted in yellow. Optional or possible execution paths are shown as dashed red lines.
- WFS observations will be executed every two days to measure the relative alignment of the components of the Optical Telescope Element (OTE), especially the segments of the primary and the six degrees of freedom of the secondary.
- Weak lenses will be used to image a bright target star using several defocus settings.
- Following downlink to the ground, these images will be analyzed at STScI using specialized software under development by Ball Aerospace. The software will perform focus-diverse phase retrieval to determine the optical path difference (OPD) map over the telescope exit pupil.
- In turn, this will be used to determine the mirror actuator moves necessary to optimize the JWST image quality. Such wavefront control is expected to happen no more frequently than once every two weeks under normal operations.
Wavefront Sensing Optics
- The pupil and filter wheels contain the bulk of the elements required for wavefront sensing and control (WFS&C) functions for JWST.
- They are predominantly in the SW channel and include two arrays of grisms for Dispersed Hartman Sensing (DHS), used during the coarse phasing of the primary mirror segments (and in conjunction with the F150W2 WFS passband filter in the filter wheel), and weak lenses to be used during fine phasing of the mirror segments and in their routine adjustment.
- Two other long-wavelength grisms (dispersion directions perpendicular to each other) are included for dispersed fringe sensing to increase the coarse-phasing capture range beyond what can be accommodated by the short wavelength grisms, and may be employed during commissioning if necessary.
- The two DHS devices in each SW channel are rotated 60° relative to one each other, and each samples 10 inter-segment edges. This allows coarse-phasing of segments in a pair-wise fashion, while ultimately achieving phasing of the entire segment array.
The pupil wheels include a pupil alignment pinhole projector assembly (PAPPA). The SW PAPPA has on both sides (inward and outward) a pattern of 18 pinholes associated with the primary mirror segments. The PAPPA will be used in conjunction with the pupil imaging lens (PIL) located in the short wavelength channel. Images of a bright star taken using the PIL will map the OTE pupil, permitting us to determine whether NIRCam is properly aligned with the OTE pupil, and allow adjustments to be made (if necessary) using the FAM. A second purpose of PIL is to measure the OTE illumination pattern of the NIRCam pupil, needed as an input to the phase-diversity calculations used during fine phasing of the mirror segments.