  A rotational modulator (RM) gamma-ray imager is capable of obtaining
significantly better angular resolution than the fundamental geometric
resolution defined by the ratio of detector diameter to mask-detector
separation. An RM imager consisting of a single grid of absorbing slats
rotating ahead of an array of a small number of position-insensitive detectors
has the advantage of fewer detector elements (i.e., detector plane pixels) than
required by a coded aperture imaging system with comparable angular resolution.
The RM therefore offers the possibility of a major reduction in instrument
complexity, cost, and power. A novel image reconstruction technique makes it
possible to deconvolve the raw images, remove sidelobes, reduce the effects of
noise, and provide resolving power a factor of 6 - 8 times better than the
geometric resolution. A 19-channel prototype RM developed in our laboratory at
Louisiana State University features 13.8 deg full-angle field of view, 1.9 deg
geometric angular resolution, and the capability of resolving sources to within
35' separation. We describe the technique, demonstrate the measured performance
of the prototype instrument, and describe the prospects for applying the
technique to either a high-sensitivity standoff gamma-ray imaging detector or a
satellite- or balloon-borne gamma-ray astronomy telescope.
