  We report on the design, construction, and commissioning of a prototype
aperture masking technology implemented at the Keck OSIRIS Imager: the
holographic aperture mask. Holographic aperture masking (HAM) aims at (i)
increasing the throughput of sparse aperture masking (SAM) by selectively
combining all subapertures across a telescope pupil in multiple interferograms
using a phase mask, and (ii) adding low-resolution spectroscopic capabilities.
Using liquid-crystal geometric phase patterns, we manufacture a HAM mask that
uses an 11-hole SAM design as the central component and a holographic component
comprising 19 different subapertures. Thanks to a multilayer liquid-crystal
implementation, the mask has a diffraction efficiency higher than 96% from 1.1
to 2.5 micron. We create a pipeline that extracts monochromatic closure phases
from the central component as well as multiwavelength closure phases from the
holographic component. We test the performance of the HAM mask in the
laboratory and on-sky. The holographic component yields 26 closure phases with
spectral resolutions between R$\sim$6.5 and R$\sim$15. On April 19, 2019, we
observed the binary star HDS 1507 in the Hbb filter ($\lambda_0 = 1638$ nm and
$\Delta \lambda = 330$ nm) and retrieved a constant separation of 120.9 $\pm
0.5$ mas for the independent wavelength bins, which is in excellent agreement
with literature values. For both the laboratory measurements and the
observations of unresolved reference stars, we recorded nonzero closure phases
-- a potential source of systematic error that we traced to polarization
leakage of the HAM optic. We propose a future upgrade that improves the
performance, reducing this effect to an acceptable level. Holographic aperture
masking is a simple upgrade of SAM with increased throughput and a new
capability of simultaneous low-resolution spectroscopy that provides new
differential observables.
