514 research outputs found
Representations of world coordinates in FITS
The initial descriptions of the FITS format provided a simplified method for
describing the physical coordinate values of the image pixels, but deliberately
did not specify any of the detailed conventions required to convey the
complexities of actual image coordinates. Building on conventions in wide use
within astronomy, this paper proposes general extensions to the original
methods for describing the world coordinates of FITS data. In subsequent
papers, we apply these general conventions to the methods by which spherical
coordinates may be projected onto a two-dimensional plane and to
frequency/wavelength/velocity coordinates.Comment: 15 Pages, 1 figure, LaTex with Astronomy & Astrophysics macro
package, submitted to A&A, related papers at
http://www.aoc.nrao.edu/~egreise
Representations of celestial coordinates in FITS
In Paper I, Greisen & Calabretta (2002) describe a generalized method for
assigning physical coordinates to FITS image pixels. This paper implements this
method for all spherical map projections likely to be of interest in astronomy.
The new methods encompass existing informal FITS spherical coordinate
conventions and translations from them are described. Detailed examples of
header interpretation and construction are given.Comment: Consequent to Paper I: "Representations of world coordinates in
FITS". 45 pages, 38 figures, 13 tables, aa macros v5.2 (2002/Jun). Both
papers submitted to Astronomy & Astrophysics (2002/07/19). Replaced to try to
get figure and table placement right (no textual changes
Anomalous radio emission from dust in the Helix
A byproduct of experiments designed to map the CMB is the recent detection of
a new component of foreground Galactic emission. The anomalous foreground at ~
10--30 GHz, unexplained by traditional emission mechanisms, correlates with
100um dust emission. We report that in the Helix the emission at 31 GHz and
100um are well correlated, and exhibit similar features on sky images, which
are absent in H\beta. Upper limits on the 250 GHz continuum emission in the
Helix rule out cold grains as candidates for the 31 GHz emission, and provide
spectroscopic evidence for an excess at 31 GHz over bremsstrahlung. We estimate
that the 100um-correlated radio emission, presumably due to dust, accounts for
at least 20% of the 31 GHz emission in the Helix. This result strengthens
previous tentative interpretations of diffuse ISM spectra involving a new dust
emission mechanism at radio frequencies. Very small grains have not been
detected in the Helix, which hampers interpreting the new component in terms of
spinning dust. The observed iron depletion in the Helix favors considering the
identity of this new component to be magnetic dipole emission from hot
ferromagnetic grains. The reduced level of free-free continuum we report also
implies an electronic temperature of Te=4600\pm1200K for the free-free emitting
material, which is significantly lower than the temperature of 9500\pm500K
inferred from collisionally-excited lines (abridged).Comment: Accepted for publication in Ap
A simple and low-power optical limiter for multi-GHz pulse trains
We study the limiting-amplification capability of a saturated Semiconductor Optical Amplifier (SOA) followed by an optical band-pass filter. We experimentally demonstrate that this simple optical circuit can be effectively exploited to realize a low-power optical limiter for amplitude-modulated pulse trains at multi-GHz repetition rate. We report very large amplitude-modulation-reduction factors for the case of 20 and 40 GHz pulse trains that are super-imposed with modulating frequencies ranging from 100kHz to several GHz. (C) 2007 Optical Society of America
A simple and low-power optical limiter for multi-GHz pulse trains
We study the limiting-amplification capability of a saturated
Semiconductor Optical Amplifier (SOA) followed by an optical band-pass filter.
We experimentally demonstrate that this simple optical circuit can be effectively exploited to realize a low-power optical limiter for amplitudemodulated pulse trains at multi-GHz repetition rate.
We report very large amplitude-modulation-reduction factors for the case of 20 and 40 GHz pulse trains that are super-imposed with modulating frequencies ranging from 100 kHz to several GHz
A New Look at the Large-Scale HI Structure of the LMC
We present a Parkes multibeam \HI survey of the Large Magellanic Cloud (LMC).
This survey, which is sensitive to spatial structure in the range 200 pc to 10
kpc, complements the Australia Telescope Compact survey, which is sensitive to
structure in the range 15 pc to 500 pc. With an rms column density sensitivity
of 8 x 10^16/cm^2 for narrow lines and 4 x 10^17/cm^2 for typical linewidths of
40 km/s, emission is found to be extensive well beyond the main body of the
LMC. Arm-like features extend from the LMC to join the Magellanic Bridge and
the Leading Arm, a forward counterpart to the Magellanic Stream. These
features, whilst not as dramatic as those in the SMC, appear to have a common
origin in the Galactic tidal field, in agreement with recent 2MASS and DENIS
results for the stellar population. The diffuse gas which surrounds the LMC,
particularly at pa's 90 to 330 deg, appears to be loosely associated with tidal
features, but loosening by the ram pressure of tenuous Galactic halo gas
against the outer parts of the LMC cannot be discounted. High-velocity clouds,
which lie between the Galaxy and the LMC in velocity and which appear in the UV
spectra of some LMC stars, are found to be associated with the LMC if their
heliocentric velocity exceeds about +100 km/s. They are possibly the product of
energetic outflows from the LMC disk. The HI mass of the LMC is found to be
(4.8+/-0.2) x 10^8 Msun (for an assumed distance of 50 kpc), substantially more
than previous recent measurements.Comment: Accepted by MNRAS, 19 pages, 13 figurs, 21 eps files, full resolution
paper (1.4 M) available at
http://www.atnf.csiro.au/~lstavele/papers/lmc_mb_paper.ps.g
- âŠ