We have developed an approach for systematically investigating the optical throughput performance of the different segments of a Michelson stellar interferometer, and applied it to the characterization of the Navy Prototype Optical Interferometer (NPOI). We report the results of the first phase of throughput measurements on NPOI, as well as some of the lessons learned.
Since the current generation of ground-based optical interferometers all suffers from varying degree of throughput degradation while the dominant causes for throughput loss are expected to vary for each individual instrument, the methodologies and approaches developed here could be of general use for the quantitative characterization of the throughput performance of the different optical interferometers, a prerequisite for its ultimate improvement.
The Fizeau Interferometer Testbed (FIT) is a ground-based laboratory experiment at Goddard Space Flight Center (GSFC) designed to develop and test technologies that will be needed for future interferometric spacecraft missions. Specifically, the research from this experiment is a proof-of-concept for optical accuracy and stability, closed-loop control algorithms, optimal sampling methodology of the Fourier UV-plane, computational models for system performance, and image synthesis techniques for a sparse array of 7 to 30 mirrors. It will assess and refine the technical requirements on hardware, control, and imaging algorithms for the Stellar Imager (SI), its pathfinder mission, and other sparse aperture and interferometric imaging mission concepts. This ground-based optical system is a collaborative effort between NASA's GSFC, Sigma Space Corporation, the Naval Research Laboratory, and the University of Maryland. We present an overview of the FIT design goals and explain their associated validation methods. We further document the design requirements and provide a status on their completion. Next, we show the overall FIT design, including the optics and data acquisition process. We discuss the technologies needed to insure success of the testbed as well as for an entire class of future mission concepts. Finally, we compare the expected performance to the actual performance of the testbed using the initial array of seven spherical mirrors. Currently, we have aligned and phased all seven mirrors, demonstrated excellent system stability for extended periods of time, and begun open-loop operations using "pinhole" light sources. Extended scenes and calibration masks are being fabricated and will shortly be installed in the source module. Installation of all the different phase retrieval/diversity algorithms and control software is well on the way to completion, in preparation for future tests of closed-loop operations.
Stellar Imager (SI) is a potential NASA space-based UV imaging interferometer to resolve the stellar disks of nearby stars. SI would consist of 20 - 30 separate spacecraft flying in formation at the Earth-Sun L2 libration point. Onboard wavefront control would be required to initially align the formation and maintain alignment during science observations and after array reconfiguration. The Fizeau Interferometry Testbed (FIT) is a testbed currently under development at the NASA/Goddard Space Flight Center to develop and study the wavefront control methodologies for Stellar Imager and other large, sparse aperture telescope systems. FIT consists of 7 articulated spherical mirrors in a Golay pattern, expandable up to 30 elements, and reconfigurable into multiple array patterns. FIT’s purpose is to demonstrate image quality versus array configuration and to develop and advance the wavefront control for SI. FIT uses extended scene wavelength, focus and field diversity to estimate the wavefront across the set of apertures. The recovered wavefront is decomposed into the eigenmodes of the control matrix and actuators are moved to minimize the wavefront piston, tip and tilt. Each mirror’s actuators are 3 degrees of freedom, however, they do not move each of the mirrors about a point on each mirrors surface, thus the mapping from wavefront piston, tip/tilt to mirror piston, tip/tilt is not diagonal. We initially estimate this mapping but update it as part of wavefront sensing and control process using system identification techniques. We discuss the FIT testbed, wavefront control methodology, and show initial results from FIT.
The Wide-Field Imaging Interferometry Testbed (WIIT) will provide valuable information for the development of space-based interferometers. This laboratory instrument operates at optical wavelengths and provides the ability to test operational
algorithms and techniques for data reduction of interferometric
data. Here we present some details of the system design and
implementation, discuss the overall performance of the system to
date, and present our plans for future development of WIIT. In
order to make best use of the interferometric data obtained with
this system, it is critical to limit uncertainties within the
system and to accurately understand possible sources of error. The
WIIT design addresses these criteria through a number of ancillary
systems. The use of redundant metrology systems is one of the most
important features of WIIT, and provides knowledge of the delay
line position to better than 10 nm. A light power detector is used
to monitor the brightness of our light sources to ensure that small
fluctuations in brightness do not affect overall performance. We
have placed temperature sensors on critical components of the
instrument, and on the optical table, in order to assess environmental effects on the system. The use of these systems provides us with estimates of the overall system uncertainty, and allows an overall characterization of the results to date. These estimates allow us to proceed forward with WIIT, adding rotation stages for 2-D interferometry. In addition, they suggest possible avenues for system improvement. The possibility exists to place WIIT inside an environmentally controlled chamber within the Diffraction Grating Evaluation Facility (DGEF) at Goddard in order to provide maximum control over environmental conditions. Funding for WIIT is provided by NASA Headquarters through the ROSS/SARA Program and by the Goddard Space Flight Center through the IR&D Program.
The Wide-field Imaging Interferometry Testbed was designed to validate, experiment with, and refine the technique of wide field mosaic imaging for optical/IR interferometers. We offer motivation for WIIT, present the testbed design, and describe algorithms that can be used to reduce the data from a spatial and spectral Michelson interferometer. A conventional single-detector Michelson interferometer operating with narrow bandwidth at center wavelength lc is limited in its field of view to the primary beam of the individual telescope apertures, or ~λc/dtel radians, where dtel is the telescope diameter. Such a field is too small for many applications; often one wishes to image extended sources. We are developing and testing techniques analogous to the mosaicing method employed in millimeter and radio astronomy, but applicable to optical/IR Michelson interferometers, in which beam combination is done in the pupil plane. An Npix × Npix array detector placed in the image plane of the interferometer is used to record simultaneously the fringe patterns from many contiguous telescope fields, effectively multiplying the field size by Npix/2, where the factor 2 allows for Nyquist sampling. This technique will be especially valuable for interferometric space observatories, such as the Space Infrared Interferometric Telescope and the Submillimeter Probe of the Evolution of Cosmic Structure.
In order for data products from WIIT to be as robust as possible, the alignment and mechanical positions of source, receiver, and detector components must be controlled and measured with extreme precision and accuracy, and the ambient environment must be monitored to allow environmental effects to be correlated with even small perturbations to fringe data. Relevant detailed anatomy of many testbed components and assemblies are described. The system of displacement measuring interferometers (DMI), optical encoders, optical alignment tools, optical power monitors, and temperature sensors implemented for control and monitoring of the testbed is presented.
The Stellar Imager (SI) is envisioned as a space-based, UV-optical interferometer composed of 10 or more one-meter class
elements distributed with a maximum baseline of 0.5 km. It is designed to image stars and binaries with sufficient resolution to enable long-term studies of stellar magnetic activity patterns,
for comparison with those on the sun. It will also support asteroseismology (acoustic imaging) to probe stellar internal structure, differential rotation, and large-scale circulations.
SI will enable us to understand the various effects of the magnetic fields of stars, the dynamos that generate these fields, and the internal structure and dynamics of the stars. The ultimate goal of the mission is to achieve the best-possible forecasting of solar activity as a driver of climate and space weather on time scales ranging from months up to decades, and an understanding of the impact of stellar magnetic activity on life in the Universe. In this paper we describe the scientific goals of the mission, the performance requirements needed to address these goals, the "enabling technology" development efforts being pursued, and the design concepts now under study for the full mission and a possible pathfinder mission.
Future NASA missions will require wide field of view interferometric imaging in order to obtain high angular resolution over large fields of view. In particular, far-infrared and submillimeter missions will require interferometry because the long wavelengths drive large baselines in order to achieve reasonable spatial resolution and because the scientific motivations require large fields of view/ However, the requirement for a direct detection interferometer to cover a wide field of view over a wide spectral band has not been demonstrated. Because of this, we are developing a testbed for demonstrating wide field imaging interferometry algorithms that will allow us to evaluate the system issues and algorithms associated with this type of observatory. This paper will describe the drivers for this testbed, the design of this testbed, and the tests and algorithms we plan to run and demonstrate.
Far infrared interferometers in space would enable extraordinary measurements of the early universe, the formation of galaxies, stars, and planets, and would have great discovery potential. Since half the luminosity of the universe and 98% of the photons released since the Big Bang are now observable at far IR wavelengths (40 - 500 micrometers ), and the Earth's atmosphere prevents sensitive observations from the ground, this is one of the last unexplored frontiers of space astronomy. We present the engineering and technology requirements that stem from a set of compelling scientific goals and discuss possible configurations for two proposed NASA missions, the Space Infrared Interferometric Telescope and the Submillimeter Probe of the Evolution of Cosmic Structure.
We discuss concepts for deploying direct-detection interferometers in space which are optimized for the wavelength range 40 micrometers to 500 micrometers . In particular, we introduce two missions in NASA's current strategic plan: SPIRIT (SPace InfraRed Interferometric Telescope) and SPECS (Submillimeter Probe of the Evolution of Cosmic Structure).
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