Interferometer

An interferometer of relatively simple design which is tilt compensated, and which facilitates adjustment of the path lengths of split light beams is described. The interferometer includes a pair of plate-like elements with a dielectric coating and an oil film between them, that forms a beamsplitter interface, and with a pair of reflector surfaces at the ends of the plates. A pair of retroreflectors are positioned so that each split beam component is directed by a retroreflector onto one of the reflector surfaces and is then returned to the beamsplitter interface, so that the reflector surfaces tilt in a direction and amount that compensates for tilting of the beamsplitter interface

Interferometer

A high resolution interferometer is described. The interferometer is insensitive to slight misalignment of its elements, avoids channeling in the spectrum, generates a maximum equal path fringe contrast, produces an even two sided interferogram without critical matching of the wedge angles of the beamsplitter and compensator wedges, and is optically phase tunable. The interferometer includes a mirror along the path of each beam component produced by the beamsplitter, for reflecting the beam component from the beamsplitter, for reflecting the beam component from the beamsplitter to a corresponding retroreflector and for reflecting the beam returned by the retroreflector back to the beamsplitter. A wedge located along each beam component path, is large enough to cover the retroreflector, so that each beam component passes through the wedge during movement towards the retroreflector and away therefrom

Polarization Effects in Reflecting Coronagraphs for White Light Applications in Astronomy

The properties of metal thin films have been largely overlooked in discussions of the technical limitations and problems that arise in the field of direct detection of exoplanets. Here, polarization properties and anisotropy properties of highly reflecting thin metal films are examined within the context of the requirements for the ultra-low scattered-light system performance of coronagraphs applied to space and ground-based high-contrast, white-light astronomy. Wavelength-dependent optical constants for highly reflecting thin metal films, taken from the literature are used to calculate the polarization-dependent transmissivity of a typical coronagraph. The effects of degraded performance on the astronomical science are examined. Suggestions are made for future work.Comment: 14 Pages, 7 Figures, Accepted to Astrophysical Journa

Detection of Earth-like Planets Using Apodized Telescopes

The mission of NASA's Terrestrial Planet Finder (TPF) is to find Earth-like planets orbiting other stars and characterize the atmospheres of these planets using spectroscopy. Because of the enormous brightness ratio between the star and the reflected light from the planet, techniques must be found to reduce the brightness of the star. The current favorite approach to doing this is with interferometry: interfering the light from two or more separated telescopes with a $\pi$ phase shift, nulling out the starlight. While this technique can, in principle, achieve the required dynamic range, building a space interferometer that has the necessary characteristics poses immense technical difficulties. In this paper, we suggest a much simpler approach to achieving the required dynamic range. By simply adjusting the transmissive shape of a telescope aperture, the intensity in large regions around the stellar image can be reduced nearly to zero. This approach could lead to construction of a TPF using conventional technologies, requiring space optics on a much smaller scale than the current TPF approach.Comment: Accepted for publication in ApJ Letters, 9 pages, 6 figure

Probing Interstellar Dust With Space-Based Coronagraphs

We show that space-based telescopes such as the proposed Terrestrial Planet Finder Coronagraph will be able to detect the light scattered by the interstellar grains along lines of sight passing near stars in our Galaxy. The relative flux of the scattered light within one arcsecond of a star at 100 pc in a uniform interstellar medium of 0.1 H atoms cm^-3 is about 10^-7. The halo increases in strength with the distance to the star and is unlikely to limit the coronagraphic detection of planets around the nearest stars. Grains passing within 100 AU of Sun-like stars are deflected by radiation, gravity and magnetic forces, leading to features in the scattered light that can potentially reveal the strength of the stellar wind, the orientation of the stellar magnetic field and the relative motion between the star and the surrounding interstellar medium.Comment: Accepted for publication in ApJ Supplement

Report of the Secretary of War: Wagon Road Routes in Utah Territory

Report of the secretary of war, communicating, in compliance with a resolution of the Senate, Captain Simpson\u27s report and map of wagon road routes in Utah Territory