Webster Cash

Overview of the Far Ultraviolet Spectroscopic Explorer Mission

The Far Ultraviolet Spectroscopic Explorer satellite observes light in the far-ultraviolet spectral region, 905-1187 Angstrom, with a high spectral resolution. The instrument consists of four co-aligned prime-focus telescopes and Rowland spectrographs with microchannel plate detectors. Two of the telescope channels use Al :LiF coatings for optimum reflectivity between approximately 1000 and 1187 Angstrom, and the other two channels use SiC coatings for optimized throughput between 905 and 1105 Angstrom. The gratings are holographically ruled to correct largely for astigmatism and to minimize scattered light. The microchannel plate detectors have KBr photocathodes and use photon counting to achieve good quantum efficiency with low background signal. The sensitivity is sufficient to examine reddened lines of sight within the Milky Way and also sufficient to use as active galactic nuclei and QSOs for absorption-line studies of both Milky Way and extragalactic gas clouds. This spectral region contains a number of key scientific diagnostics, including O VI, H I, D I, and the strong electronic transitions of H-2 and HD.

Detection of Earth-like planets around nearby stars using a petal-shaped occulter

Direct observation of Earth-like planets is extremely challenging, because their parent stars are about 1010 times brighter but lie just a fraction of an arcsecond away. In space, the twinkle of the atmosphere that would smear out the light is gone, but the problems of light scatter and diffraction in telescopes remain. The two proposed solutions—a coronagraph internal to a telescope and nulling interferometry from formation-flying telescopes—both require exceedingly clean wavefront control in the optics. An attractive variation to the coronagraph is to place an occulting shield outside the telescope, blocking the starlight before it even enters the optical path. Diffraction and scatter around or through the occulter, however, have limited effective suppression in practically sized missions. Here I report an occulter design that would achieve the required suppression and can be built with existing technology. The compact mission architecture of a coronagraph is traded for the inconvenience of two spacecraft, but the daunting optics challenges are replaced with a simple deployable sheet 30 to 50 m in diameter. When such an occulter is flown in formation with a telescope of at least one metre aperture, terrestrial planets could be seen and studied around stars to a distance of ten parsecs.

Optical constants for thin films of C, diamond, Al, Si, and CVD SiC from 24 Å to 1216 Å

A method for deriving optical constants from reflectance vs angle of incidence measurements using a nonlinear least-squares curve-fitting technique based on the chi(2) test of fit is presented and used to derive optical constants for several thin-film materials. The curve-fitting technique incorporates independently measured values for the film surface roughness, film thickness, and incident beam polarization. The technique also provides a direct method for estimating probable errors in the derived optical constants. Data are presented from 24 A to 1216 A for thin-film samples of C, synthetic diamond, Al, Si, and CVD SiC. Auger electron spectroscopy depth profiling measurements were performed on some of the samples to characterize sample composition including oxidation and contamination.

Laboratory detection of X-ray fringes with a grazing-incidence interferometer

Starting with Galileos observations of the Solar System, improvements of an order of magnitude in either the sensitivity or resolution of astronomical instruments have always brought revolutionary discoveries. The X-ray band of the spectrum, where exotic objects can have extremely high surface brightness, is ideally suited for significant improvements in imaging, but progress has been impeded by a lack of optics of sufficiently high sensitivity and quality. Here we present an X-ray interferometer design that is practical for adaptation to astronomical observatories. Our prototype interferometer, having just under one millimetre of baseline, creates fringes at 1.25 keV with an angular resolution of 100 milliarcseconds. With a larger version in orbit it will be possible to resolve X-ray sources at 10-7 arcseconds, three orders of magnitude better than the finest-resolution images ever achieved on the sky (in the radio part of the spectrum) and over one million times better than the current best X-ray images. With such resolutions, we can study the environments of pulsars, resolve and then model relativistic blast waves, image material falling into a black hole, watch the physical formation of astrophysical jets, and study the dynamos of stellar coronae.

Optical constants for thin films of Ti, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Hf, Ta, W, Re, Ir, Os, Pt, and Au from 24 A to 1216 A

Reflectance vs incidence angle measurements have been performed from 24 A to 1216 A on electron-beam evaporated samples of Ti, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Hf, Ta, W, Re, Os, Ir, Pt, and Au, and using a nonlinear least-squares curve-fitting technique, the optical constants have been determined. Independently measured values of the incident beam polarization, film thicknesses, and film surface roughnesses are incorporated into the derivation of the optical constants. Additionally, Auger electron spectroscopy depth profiling measurements have been performed on each sample to characterize sample composition including oxidation and contamination.

The X-ray superbubble in Cygnus

We show that Cyg X-6 and Cyg X-7 are part of a large X-ray ring 13 /sup 0/ in diameter. At a distance of 2 kpc this X-ray source lies behind the Great Rift of Cygnus and as such has radiation from its control region absorbed. The diameter of the emitting region is 450 pc. The observed X-ray luminosity of 5 x 10/sup 36/ ergs s/sup -1/ at about 2 x 10/sup 6/ K. The emitting electron density in the shell is 0.02 cm/sup -3/ which implies a total thermal energy content greater than 6 x 10/sup 51/ ergs.

Echelle spectrographs at grazing incidence.

It is shown that by using the conical diffraction mount existing echelle gratings can be used at grazing incidence to achieve high spectral resolution in the extreme UV and soft x rays. Design considerations for grazing incidence echelle spectrographs are examined, and two sample designs are discussed. The first, for use in the extreme UV has a primary mirror and an entrance slit to the spectrograph. The system has resolution of 10(4), operates at any wavelength longward of 100 A, and covers 30% of the spectrum at a single setting. The x-ray spectrograph uses objective gratings to obtain spectral resolution of 2.8 x 10(4) over any factor of 2 in wavelength. It operates to wavelengths as short as 4 A.

X-rays from RS Canum Venaticorum systems - A HEAO 1 survey and the development of a coronal model

The paper presents HEAO 1 low-energy X-ray observations of 59 known or suspected RS CVn systems cited in the lists of Hall (1976), Eggen (1978), and the circulars of the Working Group on RS CVn systems of IAU Commission 42. The data are used to argue against the validity of the minimum flux coronal models. A coronal loop model is used to derive expressions for the loop parameters in terms of observable quantities, and find acceptable solutions for RS CVn systems. It is concluded that the difference between solar activity and that observed in RS CVn systems may be merely a matter of scale.

X-ray optics. 2: A technique for high resolution spectroscopy

A novel combination of optical elements and properties is combined to achieve high spectral resolution using grazing incidence optics of modest quality. We show through analysis and ray tracing of examples that using radial groove gratings at high blaze angles in the manner of an echelle spectrograph can provide high spectral resolution. We compare this arrangement to the conventional in-plane designs and show the off-plane to be superior in nearly every respect. Cross dispersion can be provided by the energy resolution of a CCD detector. Then, we show how additional resolution can be squeezed from the system by strategic placement of gratings to take advantage of the azimuthal response of a Wolter x-ray optic. With a telescope that has only 30-sec of arc imaging we are able to support resolution lambda/deltalambda of 3000, while a conventional design at the same graze angle supports resolution of only 200.

CONSTRUCTION OF AN EARTH MODEL: ANALYSIS OF EXOPLANET LIGHT CURVES AND MAPPING THE NEXT EARTH WITH THE NEW WORLDS OBSERVER

The orbital light curve of a terrestrial exoplanet will likely contain valuable information about the surface and atmospheric features of the planet, both in its overall shape and hourly variations. We have constructed an empirically based code capable of simulating observations of the Earth from any orientation, at any time of year with continuously updated cloud and snow coverage with a New Worlds Observatory. By simulating these observations over a full orbital revolution at a distance of 10 pc we determine that the detection of an obliquity or seasonal terrain change is possible at low inclinations. In agreement with other studies, a 4 m New Worlds Observer can accurately determine the rotation rate of the planet at a success rate from ~30% to 80% with only 5 days of observations depending on the signal to noise of the observations. We also attempt simple inversions of these diurnal light curves to sketch a map of the reflecting planets surface features. This mapping technique is only successful with highly favorable systems and in particular requires that the cloud coverage must be lower than the Earths average. Our test case of a 2 M ⊕ planet at 7 pc distance with low exo-zodiacal light and 25% cloud coverage produced crude, but successful results. Additionally, with these highly favorable systems NWO may be able to discern the presence of liquid surface water (or other smooth surfaces) though it requires a complex detection available only at crescent phases in high inclination systems.