867 research outputs found
Astrometry and photometry with HST-WFC3. I. Geometric distortion corrections of F225W, F275W, F336W bands of the UVIS-channel
An accurate geometric distortion solution for the Hubble Space Telescope
UVIS-channel of Wide Field Camera 3 is the first step towards its use for high
precision astrometry. In this work we present an average correction that
enables a relative astrometric accuracy of ~1 mas (in each axis for well
exposed stars) in three broad-band ultraviolet filters (F225W, F275W, and
F336W). More data and a better understanding of the instrument are required to
constrain the solution to a higher level of accuracy.Comment: 20 pages, 7 figures (3 in low resolution), 3 tables. Accepted for
publication in PASP on October 16 200
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
Ground-Based CCD Astrometry with Wide Field Imagers. I. [Observations just a few years apart allow decontamination of field objects from members in two Globular clusters.]
This paper is the first of a series of papers in which we will apply the
methods we have developed for high-precision astrometry (and photometry) with
the Hubble Space Telescope to the case of wide-field ground-based images. In
particular, we adapt the software originally developed for WFPC2 to
ground-based, wide field images from the WFI at the ESO 2.2m telescope. In this
paper, we describe in details the new software, we characterize the WFI
geometric distortion, discuss the adopted local transformation approach for
proper-motion measurements, and apply the new technique to two-epoch archive
data of the two closest Galactic globular clusters: NGC 6121 (M4) and NGC 6397.
The results of this exercise are more than encouraging. We find that we can
achieve a precision of ~7 mas (in each coordinate) in a single exposure for a
well-exposed star, which allows a very good cluster-field separation in both
M4, and NGC 6397, with a temporal baseline of only 2.8, and 3.1 years,
respectively.Comment: 1 pages, 17 figures, 1 table. Accepted for publication in A&A, on
April 15 2006. For high resolution version:
http://www.eso.org/~lbedin/WFI_method/ms.ps.g
A PSF-based approach to Kepler/K2 data. I. Variability within the K2 Campaign 0 star clusters M 35 and NGC 2158
Kepler and K2 data analysis reported in the literature is mostly based on
aperture photometry. Because of Kepler's large, undersampled pixels and the
presence of nearby sources, aperture photometry is not always the ideal way to
obtain high-precision photometry and, because of this, the data set has not
been fully exploited so far. We present a new method that builds on our
experience with undersampled HST images. The method involves a point-spread
function (PSF) neighbour-subtraction and was specifically developed to exploit
the huge potential offered by the K2 "super-stamps" covering the core of dense
star clusters. Our test-bed targets were the NGC 2158 and M 35 regions observed
during the K2 Campaign 0. We present our PSF modeling and demonstrate that, by
using a high-angular-resolution input star list from the Asiago Schmidt
telescope as the basis for PSF neighbour subtraction, we are able to reach
magnitudes as faint as Kp~24 with a photometric precision of 10% over 6.5
hours, even in the densest regions. At the bright end, our photometric
precision reaches ~30 parts-per-million. Our method leads to a considerable
level of improvement at the faint magnitudes (Kp>15.5) with respect to the
classical aperture photometry. This improvement is more significant in crowded
regions. We also extracted raw light curves of ~60,000 stars and detrended them
for systematic effects induced by spacecraft motion and other artifacts that
harms K2 photometric precision. We present a list of 2133 variables.Comment: 27 pages (included appendix), 2 tables, 25 figures (5 in low
resolution). Accepted for publication in MNRAS on November 05, 2015. Online
materials will be available on the Journal website soo
Ground-based CCD astrometry with wide field imagers. IV. An improved Geometric Distortion Correction for the Blue prime-focus Camera at the LBT
High precision astrometry requires an accurate geometric distortion solution.
In this work, we present an average correction for the Blue Camera of the Large
Binocular Telescope which enables a relative astrometric precision of ~15 mas
for the B_Bessel and V_Bessel broad-band filters. The result of this effort is
used in two companion papers: the first to measure the absolute proper motion
of the open cluster M67 with respect to the background galaxies; the second to
decontaminate the color-magnitude diagram of M67 from field objects, enabling
the study of the end of its white dwarf cooling sequence. Many other
applications might find this distortion correction useful.Comment: 13 pages, 11 figures, 4 tables. accepted for publication on Astronomy
and Astrophysic
The Puzzling White Dwarf Cooling Sequence in NGC6791: A Simple Solution
In this paper we demonstrate that the puzzling bright peak in the luminosity
function of the white dwarf (WD) cooling sequence of NGC6791 can be naturally
accounted for if ~34% of the observed WDs are WD+WD binary systems.Comment: 12 pages, 3 figures. Accepted (April 9th 2008) on ApJ Lette
Accurate Internal Proper Motions of Globular Clusters
We have undertaken a long term program to measure high precision proper
motions of nearby Galactic globular cluster (GC) stars using multi-epoch
observations with the WFPC2 and the ACS cameras on-board the Hubble Space
Telescope. The proper motions are used to study the internal cluster
kinematics, and to obtain accurate cluster distances. In this paper, we also
show how the proper motions of the field stars projected in the direction of
the studied clusters can be used to set constraints on the Galaxy kinematics.Comment: 4 pages, 2 figures, A.S.P. Conf. Ser., in press in Vol. 296, 200
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