  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.
