3,487 research outputs found
Photometric observations of selected, optically bright quasars for Space Interferometry Mission and other future celestial reference frames
Photometric observations of 235 extragalactic objects that are potential
targets for the Space Interferometry Mission (SIM) are presented. Mean B, V, R,
I magnitudes at the 5% level are obtained at 1 - 4 epochs between 2005 and 2007
using the 1-m telescopes at Cerro Tololo Inter-American Observatory and Naval
Observatory Flagstaff Station. Of the 134 sources which have V magnitudes in
the Veron & Veron-Cetty catalog a difference of over 1.0 mag is found for the
observed-catalog magnitudes for about 36% of the common sources, and 10 sources
show over 3 mag difference. Our first set of observations presented here form
the basis of a long-term photometric variability study of the selected
reference frame sources to assist in mission target selection and to support in
general QSO multi-color photometric variability studies.Comment: 40 pages, 13 figures, 4 table
Comparing Tycho-2 Astrometry with UCAC1
The Tycho-2 Catalogue, released in February 2000, is based on the ESA
Hipparcos space mission data and various ground-based catalogs for proper
motions. An external comparison of the Tycho-2 astrometry is presented here
using the first U.S. Naval Observatory CCD Astrograph Catalog (UCAC1). The
UCAC1 data were obtained from observations performed at CTIO between February
1998 and November 1999, using the 206 mm aperture 5-element lens astrograph and
a 4k x 4k CCD. Only small systematic differences in position between Tycho-2
and UCAC1 up to 15 milliarcseconds (mas) are found, mainly as a function of
magnitude. The standard deviations of the distributions of the position
differences are in the 35 to 140 mas range, depending on magnitude. The
observed scatter in the position differences is about 30% larger than expected
from the combined formal, internal errors, also depending on magnitude. The
Tycho-2 Catalogue has the more precise positions for bright stars (V <= 10 mag)
while the UCAC1 positions are significantly better at the faint end (11 mag <=
V <= 12.5 mag) of the magnitude range in common. UCAC1 goes much fainter (to
R=16) than Tycho-2; however complete sky coverage is not expected before mid
2003.Comment: LaTeX, 8 pages, 3 PS figures, accepted by AJ (Aug 2000) see also
http://ad.usno.navy.mil/ad/ucac/ request for UCAC1 CD-ROM: e-mail to
[email protected] request for Tycho-2 CD-ROM: e-mail to
[email protected] or [email protected]
The PPMXL catalog of positions and proper motions on the ICRS. Combining USNO-B1.0 and 2MASS
USNO-B1.0 and 2MASS are the most widely used full-sky surveys. However, 2MASS
has no proper motions at all, and USNO-B1.0 published only relative, not
absolute (i.e. on ICRS) proper motions. We performed a new determination of
mean positions and proper motions on the ICRS system by combining USNO-B1.0 and
2MASS astrometry. This catalog is called PPMXL {VO-access to the catalog is
possible via http://vo.uni-hd.de/ppmxl}, and it aims to be complete from the
brightest stars down to about full-sky. PPMXL contains about 900
million objects, some 410 million with 2MASS photometry, and is the largest
collection of ICRS proper motions at present. As representative for the ICRS we
chose PPMX. The recently released UCAC3 could not be used because we found
plate-dependent distortions in its proper motion system north of -20
declination. UCAC3 served as an intermediate system for . The resulting typical individual mean errors of the proper motions
range from 4 mas/y to more than 10 mas/y depending on observational history.
The mean errors of positions at epoch 2000.0 are 80 to 120 mas, if 2MASS
astrometry could be used, 150 to 300 mas else. We also give correction tables
to convert USNO-B1.0 observations of e.g. minor planets to the ICRS system.Comment: 9 pages, 9 figure
Pilot/vehicle model analysis of visual and motion cue requirements in flight simulation
The optimal control model (OCM) of the human operator is used to predict the effect of simulator characteristics on pilot performance and workload. The piloting task studied is helicopter hover. Among the simulator characteristics considered were (computer generated) visual display resolution, field of view and time delay
Coronary artery grafting in infants
Coronary artery bypass grafting (CABG) with cardiac vale repair is an
uncommon surgery in infants. CABG is technically demanding in infants due
to the small size not only of the coronary arteries but also the potential graft
arteries. The short and long-term outcome of surgery is not known and thus
has largely been avoided. This article reports the case histories of two infants in whom CABG was undertaken successfully as a life-saving measure.peer-reviewe
Efficient Computation of the Nonlinear Schrödinger Equation with Time-Dependent Coefficients
open access articleMotivated by the limited work performed on the development of computational techniques for solving the nonlinear Schrödinger equation with time-dependent coefficients, we develop a modified Runge-Kutta pair with improved periodicity and stability characteristics. Additionally, we develop a modified step size control algorithm, which increases the efficiency of our pair and all other pairs included in the numerical experiments. The numerical results on the nonlinear Schrödinger equation with periodic solution verified the superiority of the new algorithm in terms of efficiency. The new method also presents a good behaviour of the maximum absolute error and the global norm in time, even after a high number of oscillations
The second US Naval Observatory CCD Astrograph Catalog (UCAC2)
The second USNO CCD Astrograph Catalog, UCAC2 was released in July 2003.
Positions and proper motions for 48,330,571 sources (mostly stars) are
available on 3 CDs, supplemented with 2MASS photometry for 99.5% of the
sources. The catalog covers the sky area from -90 to +40 degrees declination,
going up to +52 in some areas; this completely supersedes the UCAC1 released in
2001. Current epoch positions are obtained from observations with the USNO
8-inch Twin Astrograph equipped with a 4k CCD camera. The precision of the
positions are 15 to 70 mas, depending on magnitude, with estimated systematic
errors of 10 mas or below. Proper motions are derived by utilizing over 140
ground-and space-based catalogs, including Hipparcos/Tycho, the AC2000.2, as
well as yet unpublished re-measures of the AGK2 plates and scans from the NPM
and SPM plates. Proper motion errors are about 1 to 3 mas/yr for stars to 12th
magnitude, and about 4 to 7 mas/yr for fainter stars to 16th magnitude. The
observational data, astrometric reductions, results, and important information
for the users of this catalog are presented.Comment: accepted by AJ, AAS LaTeX, 14 figures, 10 table
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