  The Stroke Minimization algorithm developed at the Princeton High Contrast
Imaging Laboratory has proven symmetric dark hole generation using minimal
stroke on two deformable mirrors (DM) in series. The windowed approach to
Stroke Minimization has proven symmetric dark holes over small bandwidths by
using three wavelengths to define the bandwidth of correction in the
optimization problem. We address the relationship of amplitude and phase
aberrations with wavelength, how this changes with multiple DMs, and the
implications for simultaneously correcting both to achieve symmetric dark
holes. Operating Stroke Minimization in the windowed configuration requires
multiple wavelength estimates. To save on exposures, a single estimate is
extrapolated to bounding wavelengths using the established relationship in
wavelength to produce multiple estimates of the image plane electric field.
Here we demonstrate better performance by improving this extrapolation of the
estimate to other wavelengths. The accuracy of the functional relationship will
ultimately bound the achievable bandwidth, therefore as a metric these results
are also compared to estimating each wavelength separately. In addition to
these algorithm improvements, we also discuss a laboratory upgrade and how it
can better simulate broadband starlight. We also discuss the possibility of
leveraging two DMs in series to directly estimate the electric field over a
narrow bandwidth and the challenges associated with it.
