425 research outputs found
High resolution imaging at Palomar
For the last two years we have embarked on a program of understanding the ultimate limits of ground-based optical imaging. We have designed and fabricated a camera specifically for high resolution imaging. This camera has now been pressed into service at the prime focus of the Hale 5 m telescope. We have concentrated on two techniques: the Non-Redundant Masking (NRM) and Weigelt's Fully Filled Aperture (FFA) method. The former is the optical analog of radio interferometry and the latter is a higher order extension of the Labeyrie autocorrelation method. As in radio Very Long Baseline Interferometry (VLBI), both these techniques essentially measure the closure phase and, hence, true image construction is possible. We have successfully imaged binary stars and asteroids with angular resolution approaching the diffraction limit of the telescope and image quality approaching that of a typical radio VLBI map. In addition, we have carried out analytical and simulation studies to determine the ultimate limits of ground-based optical imaging, the limits of space-based interferometric imaging, and investigated the details of imaging tradeoffs of beam combination in optical interferometers
Noise in optical synthesis images. I. Ideal Michelson interferometer
We study the distribution of noise in optical images produced by the aperture synthesis technique, in which the principal source of noise is the intrinsic shot noise of photoelectric detection. The results of our analysis are directly applicable to any space-based optical interferometer. We show that the signal-to-noise ratio of images synthesized by such an ideal interferometric array is essentially independent of the details of the beam-combination geometry, the degree of array redundancy, and whether zero-spatial-frequency components are included in image synthesis. However, the distribution of noise does depend on the beam-combination geometry. A highly desirable distribution, one of uniform noise across the entire image, is obtained only when the beams from the n primary apertures are subdivided and combined pairwise on n(n - 1)/2 detectors
The unblinking eye on the sky
From near-Earth asteroids to superluminous supernovae and counterparts to
gravitational wave sources, the Zwicky Transient Facility will soon scan the
night sky for transient phenomena.Comment: Author's version of "Mission Control" profile published in Nature
Astronomy. 3 pages, 1 figure. https://www.nature.com/articles/s41550-017-007
Noise in optical synthesis images. II. Sensitivity of an ^nC_2 interferometer with bispectrum imaging
We study the imaging sensitivity of a ground-based optical array of n apertures in which the beams are combined pairwise, as in radio-interferometric arrays, onto n(n - 1)/2 detectors, the so-called ^nC_2 interferometer. Groundbased operation forces the use of the fringe power and the bispectrum phasor as the primary observables rather than the simpler and superior observable, the Michelson fringe phasor. At high photon rates we find that bispectral imaging suffers no loss of sensitivity compared with an ideal array (space based) that directly uses the Michelson fringe phasor. In the opposite limit, when the number of photons per spatial coherence area per coherence time drops below unity, the sensitivity of the array drops rapidly relative to an ideal array. In this regime the sensitivity is independent of n, and hence it may be efficient to have many smaller arrays, each operating separately and simultaneously
Deep narrow band imagery of the diffuse ISM in M33
Very deep narrow band images were obtained for several fields in the local group spiral galaxy M33 using a wide field reimaging Charge Coupled Device (CCD) camera on the 1.5 m telescope at Palomar Observatory. The reimaging system uses a 306 mm collimator and a 58 mm camera lens to put a 16 minute by 16 minute field onto a Texas Instruments 800 x 800 pixel CCD at a resolution of 1.2 arcseconds pixel (-1). The overall system is f/1.65. Images were obtained in the light of H alpha (S II) lambda lambda 6717, 6731, (O III) lambda 5007, and line-free continuum bands 100A wide, centered at 6450A and 5100A. Assuming a distance of 600 kpc to M33 (Humphreys 1980, Ap. J., 241, 587), this corresponds to a linear scale of 3.5 pc pixel (-1), and a field size of 2.8 kpc x 2.8 kpc. Researchers discuss the H alpha imagery of a field centered approx. equal to 8 minutes NE of the nucleus, including the supergiant HII region complex NGC 604. Two 2000 second H alpha images and two 300 second red continuum images were obtained of two slightly offset fields. The fields were offset to allow for discrimination between real emission and possible artifacts in the images. All images were resampled to align them with one of the H alpha frames. The continuum images were normalized to the line images using the results of aperture photometry on a grid of stars in the field, then the rescaled continuum data were directly subtracted from the line data
M51: Molecular spiral arms, GMAs and superclouds
Researchers present an aperture synthesis image of M51 in the CO 1 to 0 line at 9 seconds x 7 seconds resolution made with the Owens Valley Millimeter Interferometer. The image is a mosaic of 30 one-arcminute fields. The image shows narrow spiral arms which are coincident with the optical dust lanes and non-thermal radio emission, but are offset from the ridges of H alpha emission. Many dense concentrations of CO emission, termed Giant Molecular Associations (GMAs), are seen both along and between the arms. The typical GMA mass is about 3 times 10(exp 7) solar mass. Most of the on-arm GMAs appear to be gravitationally bound. These GMAs consist of several spectral components (Molecular Superclouds) with typical mass 10(exp 7) solar mass, which also appear to be bound. The observed streaming motions in the GMAs are consistent with density wave theory. The interarm GMAs are not gravitationally bound, and are likely to be due to a secondary compression of the density wave
Carbon Nanoelectronic Heterodyne Sensors : A New Paradigm for Chemical and Biological Detection.
In 1959, in his famous talk ‘There is plenty of room at the bottom’, physicist Richard Feynman had envisaged a new era of science where one could build electronic systems which would sense and interact with a world only a few atoms in size. To build such systems we not only need new materials but also new transduction strategies. The hunt for new materials has led us back to carbon, a material known since antiquity. Carbon nanotube and graphene-two allotropes of carbon, possess structural, electronic, optical and mechanical properties perfect for building fast, robust and sensitive nano-systems. However, the available sensing technologies are still incapable of high fidelity detection critical for studying nanoscale events in complex environments like ligand-receptor binding, molecular adsorption/desorption, π-π stacking, catalysis, etc.
In this thesis, I first introduce a fundamentally new nanoelectronic sensing technology based on heterodyne mixing to investigate the interaction between charge density fluctuations in a nanoelectronic sensor caused by oscillating dipole moment of molecule and an alternating current drive voltage which excites it. By detecting molecular dipole instead of associated charge, we address the limitations of conventional charge-detection based nanoelectronic sensing techniques.
In particular, using a carbon nanotube heterodyne platform, I demonstrate for the first time, biological detection in high ionic background solutions where conventional charge-detection based techniques fail due to fundamental Debye screening effect. Next, we report the first graphene nanoelectronic heterodyne vapor sensors which can detect a plethora of vapor molecules with high speed (~ 0.1 second) and high sensitivity (< 1 part per billion) simultaneously; recording orders-of-magnitude improvement over existing nanoelectronic sensors which suffer from fundamental speed-sensitivity tradeoff issue.
Finally, we use heterodyne detection as a probe to quantify the fundamental non-covalent binding interaction between small molecules and graphene by analyzing the real-time molecular desorption kinetics. More importantly, we demonstrate for the first time, electrical tuning of molecule-graphene binding kinetics by electrostatic control of graphene work function signifying the ability to tailor chemical interactions on-demand.
Our work not only lays a foundation for next-generation of rapid and sensitive nanoelectronic detectors, but also provides an insight into the fundamental molecule-nanomaterial interaction.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111436/1/girishsk_1.pd
Optical identification of binary pulsars: Implications for magnetic field decay in neutron stars
We report the discovery of the optical counterparts of two binary pulsar systems-0655+64 and 0820+02. In accordance with stellar evolution scenarios we find the optical counterparts to be white dwarfs. The existence of a cool and therefore old white dwarf in the 0655+64 system contradicts the standard hypothesis of exponential decay of magnetic fields in neutron stars. We resolve this contradiction by hypothesizing that the magnetic field in neutron stars consists of two components-an exponentially decaying field and a steady field. The former probably resides in the crust and the latter perhaps in the core: The consequences of this hypothesis for the lifetimes of millisecond pulsars, binary pulsars, and galactic bulge X-ray sources are discussed
Asteroids in GALEX: Near-ultraviolet photometry of the major taxonomic groups
We present ultraviolet photometry (NUV band, 180--280 nm) of 405 asteroids
observed serendipitously by the Galaxy Evolution Explorer (GALEX) from
2003--2012. All asteroids in this sample were detected by GALEX at least twice.
Unambiguous visible-color-based taxonomic labels (C type versus S type) exist
for 315 of these asteroids; of these, thermal-infrared-based diameters are
available for 245. We derive NUV-V color using two independent models to
predict the visual magnitude V at each NUV-detection epoch. Both V models
produce NUV-V distributions in which the S types are redder than C types with
more than 8-sigma confidence. This confirms that the S types' redder spectral
slopes in the visible remain redder than the C types' into the NUV, this
redness being consistent with absorption by silica-containing rocks. The GALEX
asteroid data confirm earlier results from the International Ultraviolet
Explorer, which two decades ago produced the only other sizeable set of UV
asteroid photometry. The GALEX-derived NUV-V data also agree with previously
published Hubble Space Telescope (HST) UV observations of asteroids 21 Lutetia
and 1 Ceres. Both the HST and GALEX data indicate that NUV band is less useful
than u band for distinguishing subgroups within the greater population of
visible-color-defined C types (notably, M types and G types).Comment: 13 pages, 11 figures, accepted 2015-May-6 to The Astrophysical
Journal. Includes one machine-readable table of NUV asteroid detections.
Version 2 includes a corrected citation to Waszczak et al. (2015) arXiv
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The Highest L(sub X)/L(sub opt) Sources in the ROSAT All-Sky Survey
The purpose of our program was to identify new and interesting soft X-ray sources from the Bright Source Catalog of the ROSAT All-Sky Survey. Our intent was to use XMM to observe a sample of BSC objects that had been identified as less than 10% likely to be associated with any object in the USNO-A2.0 catalog (Rutledge et al. 2000). We requested a single 5-ks pointing for each of 32 sources in this category in order to make a systematic examination of the properties of these sources. Ultimately, we hoped to identify new isolated neutron stars from within this population. Our requested observations were scheduled for execution at the end of Cycle 2, for two of our targets only (selected by sky position). A two-target sample does not allow for the statistical investigation that we originally proposed; however, based on the identification of the BSC object in the XMM data, an improved position, and reevaluation of likely off-band counterparts, it does enable a source-by-source evaluation of whether the BSC object is an isolated neutron star (INS)
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