James Webb Space Telescope segment phasing using differential optical transfer functions

TL;DR

This paper proposes using differential optical transfer functions (dOTF) for wavefront sensing to achieve precise segment phasing of the James Webb Space Telescope, enabling faster, safer, and more versatile alignment procedures.

Contribution

It introduces two methods for pupil modification in JWST for dOTF-based wavefront sensing, including a novel actuator displacement approach applicable to multiple instruments.

Findings

Actuator-based pupil modification enables fast, safe segment phasing.

Method allows calibration of non-common-path aberrations.

Multiple line-of-sight aberration measurements possible.

Abstract

Differential Optical Transfer Function (dOTF) is an image-based, non-iterative wavefront sensing method that uses two star images with a single small change in the pupil. We describe two possible methods for introducing the required pupil modification to the JWST, one using a small (<lambda/4) displacement of a single segment's actuator and another that uses small misalignments of NIRCam's filter wheel. While both methods should work with NIRCam, the actuator method will allow both MIRI and NIRISS to be used for segment phasing, which is new functionality. Since the actuator method requires only small displacements, it should provide a fast and safe phasing alternative that reduces mission risk and can be performed frequently for alignment monitoring and maintenance. Since a single actuator modification can be seen by all three cameras, it should be possible to calibrate the non-common-path aberrations between them. Large segment discontinuities can be measured using dOTFs in two filter bands. Using two images of a star field, aberrations along multiple lines of sight through the telescope can be measured simultaneously. Also, since dOTF gives the pupil field amplitude as well as phase, it could provide a first approximation or constraint to the planned iterative phase retrieval algorithms.