A new technique is presented for measuring the phase map due to aberrations in the wavefront-sensor non-common path
of an adaptive optics system, for the common architecture in which wavefront-sensor and science paths are split at a
dichroic. The underlying technique was originally developed to give absolute surface metrology with laboratory phaseshifting
interferometers, correcting in that case for the unknown phase corruption due to imperfections in a transmission
flat that contains the reference surface. As applied to an adaptive optics system, the technique makes use of simple
mechanical actuation, normally available for pupil alignment, to isolate errors up- and down-stream of the dichroic.
With auxiliary phase-diversity data from the science path, flexure-induced phase errors within both non-common paths
may be characterized, yielding information about the temporally persistent speckles they are thought to produce.
E. E. Bloemhof, E. E. Bloemhof,
"A novel means of measuring non-common path errors in an adaptive optics system", Proc. SPIE 9148, Adaptive Optics Systems IV, 91485E (1 July 2014); doi: 10.1117/12.2056413; https://doi.org/10.1117/12.2056413