I recently completed a study I’m very excited about using a non-redundant mask like the one on JWST’s NIRISS instrument for wavefront sensing. Our method could help measure misalignments in the primary and secondary mirrors on JWST. While NIRISS’s mask will help to study the evolution of protoplanetary systems, how AGN fuel their central engines, and other exciting astrophysical phenomena, it can also be used to measure misalignments of the optics on JWST.
A little bit of background:
The James Webb Space Telescope has a segmented primary mirror and each segment can be controlled in several ways — up and down, side to side, tilted, and curved. When JWST begins setting up, before any science can begin, these mirrors must be aligned. There is a suite of dedicated wavefront sensing hardware in the JWST instrument NIRCam (the Near InfraRed Camera). NIRCam aligns the mirror segments first in big steps, then in small steps. But to make sure the whole field is well corrected (that the secondary mirror is also aligned) requires the use of JWST’s other instruments at different locations in the field.
The problem with phase retrieval:
An image alone is not necessarily enough to retrieve the wavefront in the pupil since features in image intensity do not all map uniquely to phases in the pupil. Successful methods must introduce some “diversity” to solve the problem uniquely.
What did we do?
Diversity is often accomplished by introducing defocus (as with NIRCam’s weak lenses, or moves of the secondary mirror for other instruments). In my study, instead, we use images from a non-redundant mask to first estimate the phase in the pupil. This phase estimate can move a phase retrieval algorithm using a full pupil image in the right direction. In the end the wavefront is measured with just two in-focus images, one with the NRM, one with the full pupil. No moving mirrors or extra hardware necessary.
You can find our published paper here: https://www.osapublishing.org/oe/abstract.cfm?URI=oe-24-14-15506
Alexandra Greenbaum, PhD 