4,067 research outputs found
An Empirical Pixel-Based Correction for Imperfect CTE. I. HST's Advanced Camera for Surveys
We use an empirical approach to characterize the effect of charge-transfer
efficiency (CTE) losses in images taken with the Wide-Field Channel of the
Advanced Camera for Surveys. The study is based on profiles of warm pixels in
168 dark exposures taken between September and October 2009. The dark exposures
allow us to explore charge traps that affect electrons when the background is
extremely low. We develop a model for the readout process that reproduces the
observed trails out to 70 pixels. We then invert the model to convert the
observed pixel values in an image into an estimate of the original pixel
values. We find that when we apply the image-restoration process to science
images with a variety of stars on a variety of background levels, it restores
flux, position, and shape. This means that the observed trails contain
essentially all of the flux lost to inefficient CTE. The Space Telescope
Science Institute is currently evaluating this algorithm with the aim of
optimizing it and eventually providing enhanced data products. The empirical
procedure presented here should also work for other epochs (eg., pre-SM4),
though the parameters may have to be recomputed for the time when ACS was
operated at a higher temperature than the current -81 C. Finally, this
empirical approach may also hold promise for other instruments, such as WFPC2,
STIS, the ACS's HRC, and even WFC3/UVIS.Comment: 86 pages, 25 figures (6 in low resolution). PASP accepted on July 21,
201
Annular Solar Eclipse of 10 May 1994
An annular eclipse of the Sun will be widely visible from the Western Hemisphere on 10 May 1994. The path of the Moon's shadow passes through Mexico, the United States of America, maritime Canada, the North Atlantic, the Azores and Morocco. Detailed predictions for this event are presented and include tables of geographic coordinates of the annular path, local circumstances for hundreds of cities, maps of the path of annular and partial eclipse, weather prospects, and the lunar limb profile
Impact of Inundation and Changes in Garrison Diversion Project Plans on the North Dakota Economy
This report attempts to identify the economic consequences of the Garrison Diversion Unit and North Dakota's Missouri River impoundments from the state's perspective. The authors hope it will assist individuals and groups making decisions affecting the future development of North Dakota's water resources.Resource /Energy Economics and Policy,
Genetic improvement of passionfruit to achieve improved disease resistance
The purpose of this report is to present the final results of all activities conducted under HAL Project PF04001 'Genetic improvement of passionfruit to achieve improved disease resistance'. The report provides a summary of project findings, a description of technology transfer activities, and recommendations arising from the outcomes of the project. The overall objective of this project was to develop improved disease management strategies for the Australian passionfruit industry through breeding and
improvements in our understanding of fungal and viral diseases of passionfruit. This project targeted three areas; 1) breeding of rootstocks and scions with improved virus and cold tolerance, 2) the investigation of defence enzymes in passionfruit and the use of defence activators and 3) understanding the role of infection by single and multiple viruses and examining the use of PWV mild-strain cross-protection
Toward High-Precision Astrometry with WFPC2. I. Deriving an Accurate PSF
The first step toward doing high-precision astrometry is the measurement of
individual stars in individual images, a step that is fraught with dangers when
the images are undersampled. The key to avoiding systematic positional error in
undersampled images is to determine an extremely accurate point-spread function
(PSF). We apply the concept of the {\it effective} PSF, and show that in images
that consist of pixels it is the ePSF, rather than the often-used instrumental
PSF, that embodies the information from which accurate star positions and
magnitudes can be derived. We show how, in a rich star field, one can use the
information from dithered exposures to derive an extremely accurate effective
PSF by iterating between the PSF itself and the star positions that we measure
with it. We also give a simple but effective procedure for representing spatial
variations of the HST PSF. With such attention to the PSF, we find that we are
able to measure the position of a single reasonably bright star in a single
image with a precision of 0.02 pixel (2 mas in WF frames, 1 mas in PC), but
with a systematic accuracy better than 0.002 pixel (0.2 mas in WF, 0.1 mas in
PC), so that multiple observations can reliably be combined to improve the
accuracy by .Comment: 33 pp. text + 15 figs.; accepted by PAS
The ACS Survey of Galactic Globular Clusters. X. New Determinations of Centers for 65 Clusters
We present new measurements of the centers for 65 Milky Way globular
clusters. Centers were determined by fitting ellipses to the density
distribution within the inner 2\arcmin of the cluster center, and averaging
the centers of these ellipses. The symmetry of clusters was also analyzed by
comparing cumulative radial distributions on opposite sides of the cluster
across a grid of trial centers. All of the determinations were done with
stellar positions derived from a combination of two single-orbit ACS images of
the core of the cluster in and . We find that the
ellipse-fitting method provides remarkable accuracy over a wide range of core
sizes and density distributions, while the symmetry method is difficult to use
on clusters with very large cores, or low density. The symmetry method requires
a larger field, or a very sharply peaked density distribution.Comment: 17 pages, 8 figures, Accepted for publication in AJ, supplementary
material will be available upon publicatio
Students\u27 Reflection on Community Service Learning
Internships are a rapidly expanding educational technique to enhance students\u27 ability to learn. Internships permit and encourage students to experience the real world while receiving academic credit. Seekonk High School in Seekonk, Massachusetts, has an internship program for high school seniors called Independent Study. It allows students to leave school one day a week for the school year and volunteer within the community. Students receive academic credit for their participation in community service learning
The M31 Velocity Vector. I. Hubble Space Telescope Proper Motion Measurements
We present the first proper motion measurements for the galaxy M31. We
obtained new V-band imaging data with the HST ACS/WFC and WFC3/UVIS of a
spheroid field near the minor axis, an outer disk field along the major axis,
and a field on the Giant Southern Stream. The data provide 5-7 year time
baselines with respect to pre-existing deep first-epoch observations. We
measure the positions of thousands of M31 stars and hundreds of compact
background galaxies in each field. High accuracy and robustness is achieved by
building and fitting a unique template for each individual object. The average
proper motion for each field is obtained from the average motion of the M31
stars between the epochs with respect to the background galaxies. For the three
fields, the observed proper motions (mu_W,mu_N) are (-0.0458, -0.0376),
(-0.0533, -0.0104), and (-0.0179,-0.0357) mas/yr, respectively. The ability to
average over large numbers of objects and over the three fields yields a final
accuracy of 0.012 mas/yr. The robustness of the proper-motion measurements and
uncertainties are supported by the fact that data from different instruments,
taken at different times and with different telescope orientations, as well as
measurements of different fields, all yield statistically consistent results.
Papers II and III explore the implications for our understanding of the
history, future, and mass of the Local Group. (Abridged)Comment: 42 pages, 13 figures, to be published in ApJ. Version with high
resolution figures and N-body movies available at
http://www.stsci.edu/~marel/M31 . Press materials, graphics, and
visualizations available at
http://hubblesite.org/newscenter/archive/releases/2012/2
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