122 research outputs found
Measurement of Antenna Surfaces from In- and Out-Of-Focus Beam Maps using Astronomical Sources
We present a technique for the accurate estimation of large-scale errors in
an antenna surface using astronomical sources and detectors. The technique
requires several out-of-focus images of a compact source and the
signal-to-noise ratio needs to be good but not unreasonably high. For a given
pattern of surface errors, the expected form of such images can be calculated
directly. We show that it is possible to solve the inverse problem of finding
the surface errors from the images in a stable manner using standard numerical
techniques. To do this we describe the surface error as a linear combination of
a suitable set of basis functions (we use Zernike polynomials). We present
simulations illustrating the technique and in particular we investigate the
effects of receiver noise and pointing errors. Measurements of the 15-m James
Clerk Maxwell telescope made using this technique are presented as an example.
The key result is that good measurements of errors on large spatial scales can
be obtained if the input images have a signal-to-noise ratio of order 100 or
more. The important advantage of this technique over transmitter-based
holography is that it allows measurements at arbitrary elevation angles, so
allowing one to characterise the large scale deformations in an antenna as a
function of elevation.Comment: 6 pages, 5 figures (accepted by Astronomy & Astrophysics
Non-Thermal Continuum toward SGRB2(N-LMH)
An analysis of continuum antenna temperatures observed in the Green Bank
Telescope (GBT) spectrometer bandpasses is presented for observations toward
SgrB2(N-LMH). Since 2004, we have identified four new prebiotic molecules
toward this source by means of rotational transitions between low energy
levels; concurrently, we have observed significant continuum in the GBT
spectrometer bandpasses centered at 85 different frequencies in the range of 1
to 48 GHz. The continuum heavily influences the molecular spectral features
since we have observed far more absorption lines than emission lines for each
of these new molecular species. Hence, it is important to understand the
nature, distribution, and intensity of the underlying continuum in the GBT
bandpasses for the purposes of radiative transfer, i.e. the means by which
reliable molecular abundances are estimated. We find that the GBT spectrometer
bandpass continuum is consistent with optically-thin, non thermal (synchrotron)
emission with a flux density spectral index of -0.7 and a Gaussian source size
of ~143" at 1 GHz that decreases with increasing frequency as nu^(-0.52). Some
support for this model is provided by high frequency Very Large Array (VLA)
observations of SgrB2.Comment: Accepted for Publication in the Astrophysical Journal Letter
Out-Of-Focus Holography at the Green Bank Telescope
We describe phase-retrieval holography measurements of the 100-m diameter
Green Bank Telescope using astronomical sources and an astronomical receiver
operating at a wavelength of 7 mm. We use the technique with parameterization
of the aperture in terms of Zernike polynomials and employing a large defocus,
as described by Nikolic, Hills & Richer (2006). Individual measurements take
around 25 minutes and from the resulting beam maps (which have peak signal to
noise ratios of 200:1) we show that it is possible to produce low-resolution
maps of the wavefront errors with accuracy around a hundredth of a wavelength.
Using such measurements over a wide range of elevations, we have calculated a
model for the wavefront-errors due to the uncompensated gravitational
deformation of the telescope. This model produces a significant improvement at
low elevations, where these errors are expected to be the largest; after
applying the model, the aperture efficiency is largely independent of
elevation. We have also demonstrated that the technique can be used to measure
and largely correct for thermal deformations of the antenna, which often exceed
the uncompensated gravitational deformations during daytime observing.
We conclude that the aberrations induced by gravity and thermal effects are
large-scale and the technique used here is particularly suitable for measuring
such deformations in large millimetre wave radio telescopes.Comment: 10 pages, 7 figures (accepted by Astronomy & Astrophysics
A search for ortho-benzyne (o-C6H4) in CRL 618
Polycyclic aromatic hydrocarbons (PAHs) have been proposed as potential
carriers of the unidentified infrared bands (UIRs) and the diffuse interstellar
bands (DIBs). PAHs are not likely to form by gas-phase or solid-state
interstellar chemistry, but rather might be produced in the outflows of
carbon-rich evolved stars. PAHs could form from acetylene addition to the
phenyl radical (C6H5), which is closely chemically related to benzene (C6H6)
and ortho-benzyne (o-C6H4). To date, circumstellar chemical models have been
limited to only a partial treatment of benzene-related chemistry, and so the
expected abundances of these species are unclear. A detection of benzene has
been reported in the envelope of the proto-planetary nebula (PPN) CRL 618, but
no other benzene-related species has been detected in this or any other source.
The spectrum of o-C6H4 is significantly simpler and stronger than that of C6H5,
and so we conducted deep Ku-, K- and Q-band searches for o-C6H4 with the Green
Bank Telescope. No transitions were detected, but an upper limit on the column
density of 8.4x10^13 cm^-2 has been determined. This limit can be used to
constrain chemical models of PPNe, and this study illustrates the need for
complete revision of these models to include the full set of benzene-related
chemistry.Comment: 13 pages, 4 figures, to be published in The Astrophysical Journal
Letter
A comparison of radome- and astrodome-enclosed large radio telescopes at millimeter wavelengths: The Large Millimeter Telescope
We present a systematic comparison of the main figures of merit for an open-air radio telescope and two different types of enclosed antennas: (1) an ordinary radome, with a metal space frame providing the required mechanical rigidity and a dielectric membrane, and (2) an âastrodome,â i.e., a corotating rigid dome with a large window covered by a tensile membrane structure. The analysis is limited to submillimeter and millimeter wavelengths and large (âł30 m) antenna/enclosure systems, where the window tensile structure is very unlikely to be removable and is supported by either a metal space frame or cable networks. As compared with previous studies of this type, here we concentrate on the specific effects that these large metallic support structures have on sensitive astronomical observations. In particular, we critically discuss how the wind-induced random motions of the metal space frame can limit the sensitivity of continuum observations, as a result of fluctuating shadowing and spillover effects combined with various beam-chopping techniques. Using the Large Millimeter Telescope as a benchmark, we provide baselines for future projects where a similar comparison is needed
Terahertz epsilon-near-zero graded-index lens
An epsilon-near-zero graded-index converging lens with planar
faces is proposed and analyzed. Each perfectly-electric conducting (PEC)
waveguide comprising the lens operates slightly above its cut-off frequency
and has the same length but different cross-sectional dimensions. This
allows controlling individually the propagation constant and the normalized
characteristic impedance of each waveguide for the desired phase front at
the lens output while Fresnel reflection losses are minimized. A complete
theoretical analysis based on the waveguide theory and Fermatâs principle
is provided. This is complemented with numerical simulation results of
two-dimensional and three-dimensional lenses, made of PEC and
aluminum, respectively, and working in the terahertz regime, which show
good agreement with the analytical work.Effort sponsored by Spanish Government under contracts Consolider âEngineering
Metamaterialsâ CSD2008-00066 and TEC2011-28664-C02-01. P.R.-U. is sponsored by the
Government of Navarra under funding program âFormaciĂłn de tecnĂłlogosâ 055/01/11. M.N.-
C. is supported by the Imperial College Junior Research Fellowship. M. B. acknowledges
funding by the Spanish Government under the research contract program Ramon y Cajal
RYC-2011-08221. N.E. acknowledges the support from the US Office of Naval Research
(ONR) Multidisciplinary University Research Initiatives (MURI) grant number N00014-10-1-
0942
First observations with CONDOR, a 1.5 THz heterodyne receiver
The THz atmospheric windows centered at roughly 1.3 and 1.5~THz, contain
numerous spectral lines of astronomical importance, including three high-J CO
lines, the N+ line at 205 microns, and the ground transition of para-H2D+. The
CO lines are tracers of hot (several 100K), dense gas; N+ is a cooling line of
diffuse, ionized gas; the H2D+ line is a non-depleting tracer of cold (~20K),
dense gas. As the THz lines benefit the study of diverse phenomena (from
high-mass star-forming regions to the WIM to cold prestellar cores), we have
built the CO N+ Deuterium Observations Receiver (CONDOR) to further explore the
THz windows by ground-based observations. CONDOR was designed to be used at the
Atacama Pathfinder EXperiment (APEX) and Stratospheric Observatory For Infrared
Astronomy (SOFIA). CONDOR was installed at the APEX telescope and test
observations were made to characterize the instrument. The combination of
CONDOR on APEX successfully detected THz radiation from astronomical sources.
CONDOR operated with typical Trec=1600K and spectral Allan variance times of
30s. CONDOR's first light observations of CO 13-12 emission from the hot core
Orion FIR4 (= OMC1 South) revealed a narrow line with T(MB) = 210K and
delta(V)=5.4km/s. A search for N+ emission from the ionization front of the
Orion Bar resulted in a non-detection. The successful deployment of CONDOR at
APEX demonstrates the potential for making observations at THz frequencies from
ground-based facilities.Comment: 4 pages + list of objects, 3 figures, to be published in A&A special
APEX issu
The Optical Design and Characterization of the Microwave Anisotropy Probe
The primary goal of the MAP satellite, now in orbit, is to make high fidelity
polarization sensitive maps of the full sky in five frequency bands between 20
and 100 GHz. From these maps we will characterize the properties of the cosmic
microwave background (CMB) anisotropy and Galactic and extragalactic emission
on angular scales ranging from the effective beam size, <0.23 degree, to the
full sky. MAP is a differential microwave radiometer. Two back-to-back shaped
offset Gregorian telescopes feed two mirror symmetric arrays of ten corrugated
feeds. We describe the prelaunch design and characterization of the optical
system, compare the optical models to the measurements, and consider multiple
possible sources of systematic error.Comment: ApJ in press; 22 pages with 11 low resolution figures; paper is
available with higher quality figures at
http://map.gsfc.nasa.gov/m_mm/tp_links.htm
Physical studies of Centaurs and Trans-Neptunian Objects with the Atacama Large Millimeter Array
Once completed, the Atacama Large Millimeter Array (ALMA) will be the most
powerful (sub)millimeter interferometer in terms of sensitivity, spatial
resolution and imaging. This paper presents the capabilities of ALMA applied to
the observation of Centaurs and Trans-Neptunian Objects, and their possible
output in terms of physical properties. Realistic simulations were performed to
explore the performances of the different frequency bands and array
configurations, and several projects are detailed along with their feasibility,
their limitations and their possible targets. Determination of diameters and
albedos via the radiometric method appears to be possible on ~500 objects,
while sampling of the thermal lightcurve to derive the bodies' ellipticity
could be performed at least 30 bodies that display a significant optical
lightcurve. On a limited number of objects, the spatial resolution allows for
direct measurement of the size or even surface mapping with a resolution down
to 13 milliarcseconds. Finally, ALMA could separate members of multiple systems
with a separation power comparable to that of the HST. The overall performance
of ALMA will make it an invaluable instrument to explore the outer solar
system, complementary to space-based telescopes and spacecrafts.Comment: Accepted for publication in Icarus (23 pages, 7 figures
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