The Vector Vortex Coronagraph: Laboratory Results and First Light at Palomar Observatory

High-contrast coronagraphy will be needed to image and characterize faint extra-solar planetary systems. Coronagraphy is a rapidly evolving field, and many enhanced alternatives to the classical Lyot coronagraph have been proposed in the past ten years. Here, we discuss the operation of the vector vortex coronagraph, which is one of the most efficient possible coronagraphs. We first present recent laboratory results, and then first light observations at the Palomar observatory. Our near-infrared H-band (centered at ~ 1.65 microns) and K-band (centered at ~ 2.2 microns) vector vortex devices demonstrated excellent contrast results in the lab, down to ~ 1e-6 at an angular separation of 3 lb/d. On sky, we detected a brown dwarf companion 3000 times fainter than its host star (HR 7672) in the Ks band (centered at ~2.15 microns), at an angular separation of ~ 2.5 lb/d. Current and next-generation high-contrast instruments can directly benefit from the demonstrated capabilities of such a vector vortex: simplicity, small inner working angle, high optical throughput (>90%), and maximal off-axis discovery space

A dim candidate companion to \epsilon Cephei

Using a vector vortex coronagraph behind the 1.5-m well-corrected subaperture (WCS) at Palomar, we detected a second object very close to \epsilon Cephei, a \delta Scuti F0 IV star. The candidate companion, ~50 times fainter than \epsilon Cephei, if physically associated, is a late-type K or early M star, and lies at an angular separation of 330 mas, or 1.1 \lambda/D for the WCS, making it the smallest angle detection ever realized with a coronagraph in terms of \lambda/D units. The projected separation of the putative companion is ~8.6 AU, most likely on a highly eccentric orbit. The recently detected near-infrared excess is thus likely not due to hot dust. Moreover, we also show that the previously reported IRAS 60 \mu m excess was due to source confusion on the galactic plane.Comment: 5 pages, 3 figure

Calibration of planetary brightness temperature spectra at near-millimeter and submillimeter wavelengths with a Fourier-transform spectrometer

A medium-resolution Fourier-transform spectrometer for ground-based observation of astronomical sources at near-millimeter and submillimeter wavelengths is described. The steps involved in measuring and calibrating astronomical spectra are elaborated. The spectrometer is well suited to planetary spectroscopy, and initial measurements of the intrinsic brightness temperature spectra of Uranus and Neptune at wavelengths of 1.0 to 1.5 mm are presented

Measurement, modeling, and adjustment of the 10.4-m-diameter Leighton telescopes

The design of the Leighton telescopes and the unique techniques used in their fabrication make these telescopes particularly amenable to precise modeling and measurement of their performance. The surface is essentially a continuous membrane supported at 99 uniformly distributed nodes by a pin joint triangular grid space frame. This structure can be accurately modeled and the surface can be adjusted using low- resolution maps. Holographic measurements of the surface figure of these telescopes at the Caltech Submillimeter Observatory (CSO) and the Owens Valley Radio Observatory (OVRO) have been made over several epochs with a repeatability of 5 - 10 micrometer over the zenith angle range from 15 to 75 degrees. The measurements are consistent with the calculated gravitational distortions. Several different surface setting strategies are evaluated and the 'second order deviation from homology,' Hd, is introduced as a measure of the gravitational degradation that can be expected for an optimally adjusted surface. Hd is defined as half of the RMS difference between the deviations from homology for the telescope pointed at the extremes of its intended sky coverage range. This parameter can be used to compare the expected performance of many different types of telescopes, including off-axis reflectors and slant-axis or polar mounts as well as standard alt-az designs. Subtle asymmetries in a telescope's structure are shown to dramatically affect its performance. The RMS surface error of the Leighton telescope is improved by more than a factor of two when optimized over the positive zenith angle quadrant compared to optimization over the negative quadrant. A global surface optimization algorithm is developed to take advantage of the long term stability and understanding of the Leighton telescopes. It significantly improves the operational performance of the telescope over that obtained using a simple 'rigging angle' adjustment. The surface errors for the CSO are now less than 22 micrometer RMS over most of the zenith angle range and the aperture efficiency at 810 GHz exceeds 33%. This illustrates the usefulness of the global surface optimization procedure

Improved High Contrast Imaging with On-Axis Telescopes using a Multi-Stage Vortex Coronagraph

The vortex coronagraph is one of the most promising coronagraphs for high contrast imaging because of its simplicity, small inner working angle, high throughput, and clear off-axis discovery space. However, as with most coronagraphs, centrally-obscured on-axis telescopes degrade contrast. Based on the remarkable ability of vortex coronagraphs to move light between the interior and exterior of pupils, we propose a method, based on multiple vortices, that, without sacrificing throughput, reduces the residual light leakage to (a/A)^n, with n >=4, and a and A being the radii of the central obscuration and primary mirror, respectively. This method thus enables high contrasts to be reached even with an on-axis telescope.Comment: 3 pages, 2 figure

Submillimeter Imaging of NGC 891 with SHARC

The advent of submillimeter wavelength array cameras operating on large ground-based telescopes is revolutionizing imaging at these wavelengths, enabling high-resolution submillimeter surveys of dust emission in star-forming regions and galaxies. Here we present a recent 350 micron image of the edge-on galaxy NGC 891, which was obtained with the Submillimeter High Angular Resolution Camera (SHARC) at the Caltech Submillimeter Observatory (CSO). We find that high resolution submillimeter data is a vital complement to shorter wavelength satellite data, which enables a reliable separation of the cold dust component seen at millimeter wavelengths from the warmer component which dominates the far-infrared (FIR) luminosity.Comment: 4 pages LaTeX, 2 EPS figures, with PASPconf.sty; to appear in "Astrophysics with Infrared Surveys: A Prelude to SIRTF

Surface figure measurements of radio telescopes with a shearing interferometer

A new technique for determining the surface figure of large submillimeter wavelength telescopes is presented, which is based on measuring the telescope’s focal plane diffraction pattern with a shearing interferometer. In addition to the instrumental theory, results obtained using such an interferometer on the 10.4-m diam telescope of the Caltech Submillimeter Observatory are discussed. Using wavelengths near 1 mm, a measurement accuracy of 9 µm, or λ/115, has been achieved, and the rms surface accuracy has been determined to be just under 30 µm. The distortions of the primary reflector with changing elevation angle have also been measured and agree well with theoretical predictions of the dish deformation

Dual Fabry-Perot filter for measurement of CO rotational spectra: design and application to the CO spectrum of Venus

We present the design of a harmonic resonant filter that can be used with a Fourier transform spectrometer (FTS) for simultaneous measurement of a series of lines in the CO rotational ladder. To enable studies of both broad CO absorptions in Venus and modestly red-shifted CO emission from external galaxies, relatively broad (approximately 10-30-GHz FWHM) transmission passbands are desirable. Because a single low-finesse Fabry Perot (FP) etalon has insufficient interline rejection, a dual-FP etalon was considered. Such a design provides significantly better interband rejection and somewhat more flattopped transmission spikes. A prototype filter of this type, made of two thin silicon disks spaced by an air gap, has been constructed and used with our FTS at the Caltech Submillimeter Observatory for simultaneous measurement of the four submillimeter CO transitions in the atmosphere of Venus that are accessible from the ground

A Radio Polarimetric Study of the Galactic Center Threads

Multi-frequency, polarimetric VLA observations of the non-thermal filaments (NTF's), G0.08+0.15, and G359.96+0.09, also known as the Northern and Southern Threads are presented at 20, 6, 3.6 and 2 cm, with high enough spatial resolution to be resolved for the first time at 6 and 3.6 cm. The 20 cm image reveals a wealth of new detail in the radio sources lying within the inner 60 pc of the Galaxy. The Southern Thread has a prominent split along its length, similar to splitting at the ends of previously studied NTF's. With resolutions as fine as 2'', the 3.6 and 6 cm images reveal a high degree of continuity and little substructure internal to the filament. The spectral index of the Northern Thread has been determined over a broad range of frequencies. Its flux density falls with frequency, alpha=-0.5 between 90 and 6 cm, and becomes much steeper (alpha=-2.0) between 6 and 2 cm. The spectral index does not vary significantly along the length of the Northern Thread, which implies either that the diffusion timescale for the emitting electrons is less than their synchrotron lifetime, or that the emitting electrons are reaccelerated continuously at multiple positions along the filament. Because of the lack of spectral index variation, we have not located the source of relativistic electrons. Polarization observations at 6 and 3.6 cm confirm the non-thermal nature of the emission from the Northern Thread. The fractional polarization in the Northern Thread reaches 70% in some regions, although the polarized emission is patchy. Large rotation measures (RM > 2000 rad/m2) have been observed with irregular variations across the filament.The intrinsic magnetic field in the Northern Thread is predominantly aligned along its long axis.Comment: 19 pages, incl. 24 figs; to appear in the Astrophysical Journa

Imaging faint brown dwarf companions close to bright stars with a small, well-corrected telescope aperture

We have used our 1.6 m diameter off-axis well-corrected sub-aperture (WCS) on the Palomar Hale telescope in concert with a small inner-working-angle (IWA) phase-mask coronagraph to image the immediate environs of a small number of nearby stars. Test cases included three stars (HD 130948, HD 49197 and HR7672) with known brown dwarf companions at small separations, all of which were detected. We also present the initial detection of a new object close to the nearby young G0V star HD171488. Follow up observations are needed to determine if this object is a bona fide companion, but its flux is consistent with the flux of a young brown dwarf or low mass M star at the same distance as the primary. Interestingly, at small angles our WCS coronagraph demonstrates a limiting detectable contrast comparable to that of extant Lyot coronagraphs on much larger telescopes corrected with current-generation AO systems. This suggests that small apertures corrected to extreme adaptive optics (ExAO) levels can be used to carry out initial surveys for close brown dwarf and stellar companions, leaving followup observations for larger telescopes.Comment: accepted for publication in the Astrophysical Journa