8,803 research outputs found
The IPAC Image Subtraction and Discovery Pipeline for the intermediate Palomar Transient Factory
We describe the near real-time transient-source discovery engine for the
intermediate Palomar Transient Factory (iPTF), currently in operations at the
Infrared Processing and Analysis Center (IPAC), Caltech. We coin this system
the IPAC/iPTF Discovery Engine (or IDE). We review the algorithms used for
PSF-matching, image subtraction, detection, photometry, and machine-learned
(ML) vetting of extracted transient candidates. We also review the performance
of our ML classifier. For a limiting signal-to-noise ratio of 4 in relatively
unconfused regions, "bogus" candidates from processing artifacts and imperfect
image subtractions outnumber real transients by ~ 10:1. This can be
considerably higher for image data with inaccurate astrometric and/or
PSF-matching solutions. Despite this occasionally high contamination rate, the
ML classifier is able to identify real transients with an efficiency (or
completeness) of ~ 97% for a maximum tolerable false-positive rate of 1% when
classifying raw candidates. All subtraction-image metrics, source features, ML
probability-based real-bogus scores, contextual metadata from other surveys,
and possible associations with known Solar System objects are stored in a
relational database for retrieval by the various science working groups. We
review our efforts in mitigating false-positives and our experience in
optimizing the overall system in response to the multitude of science projects
underway with iPTF.Comment: 66 pages, 21 figures, 7 tables, accepted by PAS
The 1998 November 14 Occultation of GSC 0622-00345 by Saturn. I. Techniques for Ground-Based Stellar Occultations
On 1998 November 14, Saturn and its rings occulted the star GSC 0622-00345.
We observed atmospheric immersion with NSFCAM at the National Aeronautics and
Space Administration's Infrared Telescope Facility on Mauna Kea, Hawaii.
Immersion occurred at 55.5\circ S planetocentric latitude. A 2.3 {\mu}m,
methane-band filter suppressed reflected sunlight. Atmospheric emersion and
ring data were not successfully obtained. We describe our observation,
light-curve production, and timing techniques, including improvements in
aperture positioning, removal of telluric scintillation effects, and timing.
Many of these techniques are known within the occultation community, but have
not been described in the reviewed literature. We present a light curve whose
signal-to-noise ratio per scale height is 267, among the best ground-based
signals yet achieved, despite a disadvantage of up to 8 mag in the stellar flux
compared to prior work.Comment: LaTeX/emulateapj, 6 pages, 3 figures. Online items: The FITS-format
light curve and the IDL code for the timing model are available from ApJ or
the lead autho
The XMM-Newton serendipitous ultraviolet source survey catalogue
The XMM-Newton Serendipitous Ultraviolet Source Survey (XMM-SUSS) is a
catalogue of ultraviolet (UV) sources detected serendipitously by the Optical
Monitor (XMM-OM) on-board the XMM-Newton observatory. The catalogue contains
ultraviolet-detected sources collected from 2,417 XMM-OM observations in 1-6
broad band UV and optical filters, made between 24 February 2000 and 29 March
2007. The primary contents of the catalogue are source positions, magnitudes
and fluxes in 1 to 6 passbands, and these are accompanied by profile
diagnostics and variability statistics. The XMM-SUSS is populated by 753,578 UV
source detections above a 3 sigma signal-to-noise threshold limit which relate
to 624,049 unique objects. Taking account of substantial overlaps between
observations, the net sky area covered is 29-54 square degrees, depending on UV
filter. The magnitude distributions peak at 20.2, 20.9 and 21.2 in UVW2, UVM2
and UVW1 respectively. More than 10 per cent of sources have been visited more
than once using the same filter during XMM-Newton operation, and > 20 per cent
of sources are observed more than once per filter during an individual visit.
Consequently, the scope for science based on temporal source variability on
timescales of hours to years is broad. By comparison with other astrophysical
catalogues we test the accuracy of the source measurements and define the
nature of the serendipitous UV XMM-OM source sample. The distributions of
source colours in the UV and optical filters are shown together with the
expected loci of stars and galaxies, and indicate that sources which are
detected in multiple UV bands are predominantly star-forming galaxies and stars
of type G or earlier.Comment: Accepted for publication in MNRA
Implementation of robust image artifact removal in SWarp through clipped mean stacking
We implement an algorithm for detecting and removing artifacts from
astronomical images by means of outlier rejection during stacking. Our method
is capable of addressing both small, highly significant artifacts such as
cosmic rays and, by applying a filtering technique to generate single frame
masks, larger area but lower surface brightness features such as secondary
(ghost) images of bright stars. In contrast to the common method of building a
median stack, the clipped or outlier-filtered mean stacked point-spread
function (PSF) is a linear combination of the single frame PSFs as long as the
latter are moderately homogeneous, a property of great importance for weak
lensing shape measurement or model fitting photometry. In addition, it has
superior noise properties, allowing a significant reduction in exposure time
compared to median stacking. We make publicly available a modified version of
SWarp that implements clipped mean stacking and software to generate single
frame masks from the list of outlier pixels.Comment: PASP accepted; software for download at
http://www.usm.uni-muenchen.de/~dgruen
- …