We present recent results from the Wide-Field Imaging Interferometry
Testbed (WIIT). Using a multi-pixel detector for spatial multiplexing, WIIT has demonstrated the ability to acquire wide-field imaging interferometry data. Specifically, these are "double Fourier"
data that cover a field of view much larger than the subaperture diffraction spot size. This ability is of great import for a number of proposed missions, including the Space Infrared Interferometric Telescope (SPIRIT), the Submillimeter Probe of the Evolution of Cosmic Structure (SPECS), and the Terrestrial Planet Finder (TPF-I)/DARWIN. The recent results are discussed and analyzed, and future study directions are described.
We discuss the procedure used to characterize the Wide-Field Imaging
Interferometry Testbed (WIIT) components and system, including
spectral transmission, throughput, wavefront quality, mechanical and
thermal stability, and susceptibility to turbulence. The sources of
uncertainty and visibility loss are identified and evaluated, and we
briefly discuss measures taken to mitigate these effects. We further
discuss calibration techniques which can be used to compensate for
visibility loss factors, and describe the applicability of these
calibration techniques to the future space-based far-IR interferometry
missions SPIRIT (Space Infrared Interferometric Telescope) and SPECS
(Submillimeter Probe of the Evolution of Cosmic Structure).
A new optical encoder which measures absolute, true-Cartesian displacement with ultra-high sensitivity and linearity has been developed at NASA's Goddard Space Flight Center. The device is the two-dimensional analog of recently developed linear and rotary encoders based on optical pattern recognition. In this encoder, a glass scale carrying absolute Cartesian position information travels with the payload in an X-Y motion system. Because the scale comprises the entire measurement coordinate system in a monolithic form, motion control axes can be skew to one another to an arbitrary degree and can exhibit substantial lateral drift with no effect on the correctness of X-Y readout, thus eliminating challenges of orthogonal mounting for motion axes and challenges of mounting independent encoders parallel to the directions of travel for each constituent X and Y axis. Prototype devices with ranges of 30 x 30 mm and 150 x 150 mm with 5 nm and 50 nm resolutions, respectively, have been built in the laboratory. Performance data from the Cartesian encoder in the Point Target Assembly for the optical calibration stimulus for Hubble Space Telescope's Wide Field Camera 3 are presented.