A Conceptual Design for An Exoplanet Imager (PDF)
ABSTRACT
This paper reports the results of a design study for an exoplanet imaging system. The design team consisted of the students in the “Electromagnetic Sensing for Space-Bourne Imaging” class taught by the principal author in the Spring, 2005 semester. The design challenge was to devise a space system capable of forming 10X10 pixel images of terrestrial-class planets out to 10 parsecs, observing in the 9.0 to 17.0 microns range. It was presumed that this system would operate after the Terrestrial Planet Finder had been deployed and had identified a number of planetary systems for more detailed imaging.

The design team evaluated a large number of tradeoffs, starting with the use of a single monolithic telescope, versus a truss-mounted sparse aperture, versus a formation of free-flying telescopes. Having selected the free-flyer option, the team studied a variety of sensing technologies, including amplitude interferometry, intensity correlation imaging ( ICI , based on the Brown-Twiss effect and phase retrieval), heterodyne interferometry and direct electric field reconstruction. Intensity correlation imaging was found to have several advantages. It does not require combiner spacecraft, nor nanometer-level control of the relative positions, nor diffraction-limited optics. Orbit design, telescope design, spacecraft structural design, thermal management and communications architecture trades were also addressed. A six spacecraft design involving non-repeating baselines was selected. By varying the overall scale of the baselines it was found possible to unambiguously characterize an entire multi-planet system, to image the parent star and, for the largest base scales, to determine 10X10 pixel images of individual planets.

Calculation of Signal-to-Noise Ratio for Image Formation Using Multispectral Intensity Correlation (PDF)
ABSTRACT
In previous work, we explored the possibility of using intensity correlation techniques, based upon the Hanbury Brown-Twiss effect to perform fine resolution imaging in the service of exoplanet astronomy. Here we consider a multi-spectral variant of the Hanbury Brown-Twiss technique. At each of a number of independent, light-gathering telescopes photodetection data encompassing each of a set of frequency channels are obtained and then are communicated to some convenient computational station. At the computational station, the correlations among the photodetections in each of the frequency bands are time averaged and then further averaged over the various frequency channels to arrive at measurements of the mutual coherence magnitude for each pair of telescopes. From these statistics, imaging data are, in turn, computed via phase retrieval techniques. Here, within a modern quantum optics framework, we examine the signal-to-noise characteristics of the coherence estimates obtained in this way under a variety of non-ideal conditions. We provide step-by-step derivations of the statistical quantities needed in a largely self-contained treatment. In particular, we examine the effects of partial coherence on a scene typical of exoplanet imaging and show how partial coherence can be used to greatly attenuate the parent star. We find that the multispectral version of intensity interferometry greatly improves the signal-to-noise ratio in general and dramatically so for exoplanet detection. The results also extend the analysis of signal-to-noise to a wider variety of practical conditions and provide the basis for multispectral intensity correlation imaging system design.

Efficient, Passively Orbiting Constellations for High Resolution Imaging of Geosynchronous Objects (PDF)
ABSTRACT
Over the past several years, much progress has been made in the development of the Intensity Correlation Imaging approach to ultra-fine resolution imaging. In this paper, we consider the design of a LEO-based observatory of small telescopes using the Intensity Correlation Imaging technology to achieve 1 cm resolution imaging of objects in geosynchronous orbit. We formulate the system Modulation Transfer Function (MTF) and then seek to optimize u-v plane coverage by the design of passive, LEO orbits. An adaptive random search technique is used to find constellation designs that offer twice the rate of u-v coverage as earlier results.