6,446 research outputs found
Homogeneous Photometry for Star Clusters and Resolved Galaxies. II. Photometric Standard Stars
Stars appearing in CCD images obtained over 224 nights during the course of
69 observing runs have been calibrated to the Johnson/Kron-Cousins BVRI
photometric system defined by the equatorial standards of Landolt (1992, AJ,
104, 340). More than 15,000 stars suitable for use as photometric standards
have been identified, where "suitable" means that the star has been observed
five or more times during photometric conditions and has a standard error of
the mean magnitude less than 0.02 mag in at least two of the four bandpasses,
and shows no significant evidence of intrinsic variability. Many of these stars
are in the same fields as Landolt's equatorial standards or Graham's (1982,
PASP, 94, 244) southern E-region standards, but are considerably fainter. This
enhances the value of those fields for the calibration of photometry obtained
with large telescopes. Other standards have been defined in fields containing
popular objects of astrophysical interest, such as star clusters and famous
galaxies, extending Landolt-system calibrators to declinations far from the
equator and to stars of sub-Solar chemical abundances. I intend to continue to
improve and enlarge this set of photometric standard stars as more observing
runs are reduced. The full current database of photometric indices is being
made freely available via a site on the World-Wide Web, or by direct request to
the author. Although the contents of the database will evolve in detail, at any
given time it should represent the largest sample of precise BVRI broad-band
photometric standards available anywhere.Comment: Accepted for July 2000 PAS
Laser speckle technique for burner liner strain measurements
Thermal and mechanical strains were measured on samples of a common material used in jet engine burner liners, which were heated from room temperature to 870 C and cooled back to 220 C, in a laboratory furnance. The physical geometry of the sample surface was recorded at selected temperatures by a set of 12 single exposure speckle-grams. Sequential pairs of specklegrams were compared in a heterodyne interferometer which give high precision measurement of differential displacements. Good speckle correlation between the first and last specklegrams is noted which allows a check on accumulate errors
Johnson-Cousins magnitudes of comparison stars in the fields of ten Seyfert galaxies
We present UBVRcIc magnitudes of 49 comparison stars in the fields of the
Seyfert galaxies Mrk 335, Mrk 79, Mrk 279, Mrk 506, 3C 382, 3C 390.3, NGC 6814,
Mrk 304, Ark 564, and NGC 7469 in order to facilitate the photometric
monitoring of these objects; 36 of the stars have not been calibrated before.
The comparison stars are situated in 5x5 arcmin fields centred on the Seyfert
galaxies, their V band flux ranges from 11.7 to 18.2 mag with a median value of
16.3 mag, and their B-V colour index ranges from 0.4 to 1.6 mag with a median
value of 0.8 mag. The median errors of the calibrated UBVRcIc magnitudes are
0.08, 0.04, 0.03, 0.04, and 0.06 mag, respectively. Comparison stars were
calibrated for the first time in three of the fields (Mrk 506, 3C 382, and Mrk
304). The comparison sequences in the other fields were improved in various
aspects. Extra stars were calibrated in four fields (Mrk 335, Mrk 79, NGC 6814,
and NGC 7469) - most of these stars are fainter and are situated closer to the
Seyfert galaxies compared to the existing comparison stars. The passband
coverage of the sequences in five fields (Mrk 335, Mrk 79, Mrk 279, NGC 6814,
and Ark 564) was complemented with U band.Comment: Accepted for publication in Astronomische Nachrichten, the article
title shortene
Demonstration of laser speckle system on burner liner cyclic rig
A demonstration test was conducted to apply speckle photogrammetry to the measurement of strains on a sample of combustor liner material in a cyclic fatigue rig. A system for recording specklegrams was assembled and shipped to the NASA Lewis Research Center, where it was set up and operated during rig tests. Data in the form of recorded specklegrams were sent back to United Technologies Research Center for processing to extract strains. Difficulties were found in the form of warping and bowing of the sample during the tests which degraded the data. Steps were taken by NASA personnel to correct this problem and further tests were run. Final data processing indicated erratic patterns of strain on the burner liner sample
Risk, Return and Social Impact: Demystifying the Law of Mission Investing by U.S. Foundations
Discusses in detail the legal aspects of mission-related investing, including federal and state fiduciary laws, foundations' fiduciary responsibility, and emerging practices, and makes recommendations. Includes examples of investments and case studies
A Brief Guide to the Law of Mission Investing for U.S. Foundations
U.S foundations have considerable freedom to invest their assets in ways that further their missions, even at greater risk or lower financial return. The legal framework that governs the investment of foundation assets is both complex and ambiguous, however, with the result that many foundation leaders and investment advisors are unclear about what is legally permissible. Anne Stetson and Mark Kramer of FSG have prepared two reports, in consultation with nationally-recognized legal experts and senior foundation officers, analyzing the federal tax and state fiduciary laws as they apply to US foundations. In addition to legal analysis, the reports provide practical recommendations as to how foundations can best navigate these laws in making mission-related or program-related investments. A Brief Guide to the Law of Mission Investing for U.S. Foundations is a short 18 page booklet, suitable for foundation staff and boards, as well as their advisors, explaining in non-technical language the factors foundations must consider in making mission investments
Arcfinder: An algorithm for the automatic detection of gravitational arcs
We present an efficient algorithm designed for and capable of detecting
elongated, thin features such as lines and curves in astronomical images, and
its application to the automatic detection of gravitational arcs. The algorithm
is sufficiently robust to detect such features even if their surface brightness
is near the pixel noise in the image, yet the amount of spurious detections is
low. The algorithm subdivides the image into a grid of overlapping cells which
are iteratively shifted towards a local centre of brightness in their immediate
neighbourhood. It then computes the ellipticity for each cell, and combines
cells with correlated ellipticities into objects. These are combined to graphs
in a next step, which are then further processed to determine properties of the
detected objects. We demonstrate the operation and the efficiency of the
algorithm applying it to HST images of galaxy clusters known to contain
gravitational arcs. The algorithm completes the analysis of an image with
3000x3000 pixels in about 4 seconds on an ordinary desktop PC. We discuss
further applications, the method's remaining problems and possible approaches
to their solution.Comment: 12 pages, 12 figure
Discovery of a rapidly pulsating subdwarf B star candidate in omega Cen
We report the discovery of the first variable extreme horizontal branch star
in a globular cluster (omega Cen). The oscillation uncovered has a period of
114 s and an amplitude of 32 mmags. A comparison between horizontal branch
models and observed optical colours indicates an effective temperature of
31,500+-6,300 K for this star, placing it within the instability strip for
rapidly oscillating B subdwarfs. The time scale and amplitude of the pulsation
detected are also in line with what is expected for this type of variable, thus
strengthening the case for the discovery of a new subdwarf B pulsator.Comment: 5 pages, 7 figures; accepted for publication in A&
Study of application of space telescope science operations software for SIRTF use
The design and development of the Space Telescope Science Operations Ground System (ST SOGS) was evaluated to compile a history of lessons learned that would benefit NASA's Space Infrared Telescope Facility (SIRTF). Forty-nine specific recommendations resulted and were categorized as follows: (1) requirements: a discussion of the content, timeliness and proper allocation of the system and segment requirements and the resulting impact on SOGS development; (2) science instruments: a consideration of the impact of the Science Instrument design and data streams on SOGS software; and (3) contract phasing: an analysis of the impact of beginning the various ST program segments at different times. Approximately half of the software design and source code might be useable for SIRTF. Transportability of this software requires, at minimum, a compatible DEC VAX-based architecture and VMS operating system, system support software similar to that developed for SOGS, and continued evolution of the SIRTF operations concept and requirements such that they remain compatible with ST SOGS operation
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