Rather than using an adaptive optics (AO) system to correct a telescope's entire pupil, it can instead be used to more finely correct a smaller subaperture. Indeed, existing AO systems can be used to correct a subaperture 1 3 to 1 2 the size of a 5Y10 m telescope to extreme adaptive optics (ExAO) levels. We discuss the potential performance of a clear off-axis well-corrected subaperture (WCS), and describe our initial imaging results with a 1.5 m diameter WCS on the Palomar Observatory's Hale Telescope. These include measured Strehl ratios of 0.92Y0.94 in the infrared (2.17 m) and %0.12 in the B band, the latter allowing a binary of separation 0.34 00 to be easily resolved in the blue. Such performance levels enable a variety of novel observational modes, such as infrared ExAO, visible-wavelength AO, and high-contrast coronagraphy. One specific application suggested by the high Strehl ratio stability obtained (1%) is the measurement of planetary transits and eclipses. Also described is a simple ''dark hole'' experiment carried out on a binary star, in which a comatic phase term was applied directly to the deformable mirror, in order to shift the diffraction rings to one side of the point-spread function.
The presence of highly aspheric wave fronts in an interferometer leads to a need for system calibration, and this calibration requirement affects the design of the interferometer. Dynamic range, vignetting, and the ability to characterize components all must be considered during the design stages. The interferometer must be designed with respect to wave-front propagation as opposed to reference sphere aberrations. A nonnull interferometer for measurement of aspheric transmitted wave fronts has been developed, and the design process is described. Transmitted wave fronts for a conformal window and a progressive-addition bifocal lens have been measured to demonstrate the applicability of the system to aspheric testing.
We demonstrate the modal filtering properties of newly developed single mode silver halide fibers for use at midinfrared wavelengths, centered at 10.5 microm. The goal was to achieve a suppression of nonfundamental modes greater than a factor of 300 to enable the detection and characterization of Earthlike exoplanets with a space-based nulling interferometer. Fiber segments of 4.5 cm, 10.5 cm, 15 cm, and 20 cm lengths were tested. We find that the performance of the fiber was limited not by the modal filtering properties of the core but by the unsuppressed cladding modes present at the output of the fiber. In 10.5 cm and longer sections, this effect can be alleviated by properly aperturing the output. Exclusive of coupling losses, the fiber segments of 10.5-20 cm length can provide power suppression of undesirable components of the input field by a factor of 15,000 at least. The demonstrated performance thus far surpasses our requirements, such that even very short sections of fiber provide adequate modal filtering for exoplanet characterization.
ACCESS is one of four medium-class mission concepts selected for study in 2008-9 by NASA's Astrophysics Strategic Mission Concepts Study program. ACCESS evaluates a space observatory designed for extreme high-contrast imaging and spectroscopy of exoplanetary systems. An actively-corrected coronagraph is used to suppress the glare of diffracted and scattered starlight to contrast levels required for exoplanet imaging. The ACCESS study considered the relative merits and readiness of four major coronagraph types, and modeled their performance with a NASA medium-class space telescope. The ACCESS study asks: What is the most capable medium-class coronagraphic mission that is possible with telescope, instrument, and spacecraft technologies available today? Using demonstrated high-TRL technologies, the ACCESS science program surveys the nearest 120+ AFGK stars for exoplanet systems, and surveys the majority of those for exozodiacal dust to the level of 1 zodi at 3 AU. Coronagraph technology developments in the coming year are expected to further enhance the science reach of the ACCESS mission concept.
This paper provides an overview of technology development for the Terrestrial Planet Finder Interferometer (TPF-I). TPF-I is a mid-infrared space interferometer being designed with the capability of detecting Earth-like planets in the habitable zones around nearby stars. The overall technology roadmap is presented and progress with each of the testbeds is summarized.
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