JWST Wavefront Sensing and Control
The precise optical alignment of the telescope optics for JWST is achieved and maintained using wavefront sensing imagery from the science instruments, particularly NIRCam, as well as actuators on the back of each mirror segment (including the secondary mirror).
Upon launch and deployment of the observatory, Optical Telescope Element (OTE) commissioning proceeded through several stages of iterative sensing and alignment correction over several months to establish the initial best on-orbit alignments. Once commissioned, the telescope is routinely monitored and corrections to maintain the mirror alignment are issued only occasionally.
Active optics system overview
Because of the unique circumstances of the stable space environment, the wavefront sensing system architecture on JWST is different from large active telescopes on the ground. Most significantly, JWST is free from atmospheric disturbances and gravity-induced deformations, which are the dominant factors requiring rapid correction for active and adaptive telescopes on Earth. Instead, JWST only needs corrections for wavefront aberrations that change much more slowly than the durations of typical science observations. In particular, the need for wavefront corrections during science operations is mostly due to temperature changes that cause slight thermal expansion and contraction of portions of the observatory, typically on timescales of several days. This allows the use of the science instrument imaging detectors for periodic measurements, rather than requiring dedicated wavefront sensor detectors or continuously active segment edge sensors.
NIRCam is the primary wavefront sensor for JWST and, because of its importance to overall observatory operations, is comprised of 2 fully redundant modules. Both NIRCam modules contain several components in its pupil and filter wheels that are used to measure the wavefront (e.g., weak lenses, Dispersed Hartmann sensors). In general, wavefront information is retrieved using focus-diverse phase retrieval, where the focus diversity is provided by the use of a combination of weak lenses (although during commissioning the multi-instrument/field diversity was provided by moving the secondary mirror). Analysis and determination of the wavefront error is performed on the ground using downlinked image data. The necessary mirror commands are then uplinked back to JWST to correct the alignments. Other instruments were used during OTE commissioning to assess the multi-field wavefront and optimize optical performance across the field of view.
Each primary mirror segment has actuators on its back that provide 6 degrees of freedom, as well as control over the radius of curvature. The secondary mirror is also controlled in its 6 degrees of freedom. Thus, there are a total of 132 degrees of freedom in the telescope that need alignment, plus the focus mechanisms in each of the science instruments apart from MIRI. Other alignments, such as the tertiary and fine steering mirror, were established during observatory assembly on the ground and are sufficiently rigid to not need correction in-flight.
After launch and deployment, the primary mirror segments, secondary, and science instruments were misaligned relative to each other by a small amount. An iterative process using several types of wavefront sensing and control was used to bring these mirrors into alignment within tens of nanometers. The large dynamic range (millimeters to nanometers) means that several distinct stages and types of sensing were necessary. This commissioning process was necessarily iterative, due to finite sensing precision and also to mechanism uncertainties inherent to the coarse stage actuator design. As a result, OTE commissioning was iterative at both small scales (a given step may need to be performed several times to converge) and at much larger scales (mechanism uncertainties required looping back to repeat entire sections of the commissioning plan).
The deployment of the secondary mirror, the two 3-mirror folded wing sections of the primary mirror, and initial deployments of segments from their launch restraints took place starting around 16 days after launch. The wavefront sensing and correction process began as soon as the telescope and instruments had cooled sufficiently close to their operating temperatures. This process interspersed individual wavefront sensing and control activities, initial activation and checkouts of the science instruments, and observatory-level calibration tasks that involve many subsystems across the whole observatory, such as the guider and attitude control system.
The main stages of OTE commissioning were (1) segment location and identification, (2) segment-level wavefront control, (3) segment co-phasing, and (4) multi-instrument (multi-field) sensing and control (see Figure 1 below). Because NIRCam is the main wavefront sensing sensor, high quality images were first achieved on NIRCam prior to any of the other instruments, around about halfway through telescope commissioning, then the multi-instrument sensing process adjusted the secondary mirror alignment to optimize image quality over the full instrument suite. The OTE commissioning lasted several months and was completed around mid-April 2022; it was followed by a thermal stability characterization assessment aimed at measuring the observatory's response to changes in spacecraft attitude with respect to the Sun. Continued quantification of stability in flight will better inform ongoing wavefront maintenance. OTE commissioning was followed by the commissioning of the science instruments, during which period the OTE wavefront was routinely measured and corrected, as needed.
During science operations
The OTE has been in routine maintenance mode since the end of OTE commissioning. In this mode, the wavefront is measured every 2 days using NIRCam weak lenses and alignment corrections are made as needed. This 2-day sensing cadence has a loose tolerance; the goal is to schedule wavefront sensing observations so as to accommodate any time-critical observations and to not disrupt long mosaic or time-series observations. Corrections, on the other hand, are expected to be relatively infrequent, typically no more often than every 2 weeks. Altogether, the sensing and control observations will take about 1%–2% of the total observatory time, which is accounted for as part of the observatory calibration overheads.
Note that the intent of corrections is to maintain the telescope alignment, not to intentionally change it. That is, the goal of corrections are to bring the OTE back to the nominal aligned state that it had at the end of the commissioning period, and ensure it continually remains near that state. There is no plan for "campaign" style observations in which the OTE would be temporarily optimized for one instrument over another. Nor is there any need for observers to request scheduling their observations with any particular timing constraints relative to wavefront sensing. However, to mitigate any possible impacts of thermal changes to the point spread function during certain high-contrast imaging observations, Cycle 1 users were directed to force back-to-back observations of science targets and PSF reference star observations. Pending ongoing assessment of in-flight performance, this restriction may be relaxed in future cycles. Moreover, the cadence of sensing and control measurements may be adjusted in later cycles based on achieved performances.
The wavefront sensing image data from NIRCam and the derived wavefront maps are delivered to the MAST Archive, similar to other calibration program data, and are accessible to the community. Examples of trending of the observatory optical performance are illustrated below, where the wavefront error (WFE) is shown as a function of time since the start of the Cycle 1 (mid-July 2022). Other trending capabilities are also offered as part of the WebbPSF Python software (e.g., https://webbpsf.readthedocs.io/en/stable/jwst_measured_opds.html).
Acton, D. S., et al. 2022, SPIE 12180, 121800U
Phasing the Webb Telescope
Acton, D. S., et al. 2012, SPIE 8442, 84422H
Wavefront sensing and controls for the James Webb Space Telescope
Perrin, M. D., et al. 2016, SPIE 9904, 99040F
Preparing for JWST wavefront sensing and control operations