11 research outputs found
Measuring and Correcting Wind-Induced Pointing Errors of the Green Bank Telescope Using an Optical Quadrant Detector
Wind-induced pointing errors are a serious concern for large-aperture
high-frequency radio telescopes. In this paper, we describe the implementation
of an optical quadrant detector instrument that can detect and provide a
correction signal for wind-induced pointing errors on the 100m diameter Green
Bank Telescope (GBT). The instrument was calibrated using a combination of
astronomical measurements and metrology. We find that the main wind-induced
pointing errors on time scales of minutes are caused by the feedarm being blown
along the direction of the wind vector. We also find that wind-induced
structural excitation is virtually non-existent. We have implemented offline
software to apply pointing corrections to the data from imaging instruments
such as the MUSTANG 3.3 mm bolometer array, which can recover ~70% of
sensitivity lost due to wind-induced pointing errors. We have also performed
preliminary tests that show great promise for correcting these pointing errors
in real-time using the telescope's subreflector servo system in combination
with the quadrant detector signal.Comment: 17 pages, 11 figures; accepted for publication in PAS
Status on Iterative Transform Phase Retrieval Applied to the GBT Data
This slide presentation reviews the use of iterative transform phase retrieval in the analysis of the Green Bank Radio Telescope (GBT) Data. It reviews the NASA projects that have used phase retrieval, and the testbed for the algorithm to be used for the James Webb Space Telescope. It shows the comparison of phase retrieval with an interferometer, and reviews the two approaches used for phase retrieval, iterative transform (ITA) or parametric (non-linear least squares model fitting). The concept of ITA Phase Retrieval is reviewed, and the application to Radio Antennas is reviewed. The presentation also examines the National Radio Astronomy Observatory (NRAO) data from the GBT, and the Fourier model that NRAO uses to analyze the data. The challenge for ITA phase retrieval is reviewed, and the coherent approximation for incoherent data is shown. The validity of the approximation is good for a large tilt. There is a review of the proof of concept of the Phase Review simulation using the input wavefront, and the initial sampling parameters estimate from the focused GBT data
Holographic Measurement and Improvement of the Green Bank Telescope Surface
We describe the successful design, implementation, and operation of a 12 GHz
holography system installed on the Robert C. Byrd Green Bank Telescope (GBT).
We have used a geostationary satellite beacon to construct high-resolution
holographic images of the telescope mirror surface irregularities. These images
have allowed us to infer and apply improved position offsets for the 2209
actuators which control the active surface of the primary mirror, thereby
achieving a dramatic reduction in the total surface error (from 390 microns to
~240 microns, rms). We have also performed manual adjustments of the corner
offsets for a few panels. The expected improvement in the radiometric aperture
efficiency has been rigorously modeled and confirmed at 43 GHz and 90 GHz. The
improvement in the telescope beam pattern has also been measured at 11.7 GHz
with greater than 60 dB of dynamic range. Symmetric features in the beam
pattern have emerged which are consistent with a repetitive pattern in the
aperture due to systematic panel distortions. By computing average images for
each tier of panels from the holography images, we confirm that the magnitude
and direction of the panel distortions, in response to the combination of
gravity and thermal gradients, are in general agreement with finite-element
model predictions. The holography system is now fully integrated into the GBT
control system, and by enabling the telescope staff to monitor the health of
the individual actuators, it continues to be an essential tool to support
high-frequency observations.Comment: Accepted for publication in PASP. Contains 28 pages with 2 tables and
9 figures (several at reduced quality). The full resolution version is
available at http://wwwlocal.gb.nrao.edu/ptcs/papers/Hunter2011/gbtholo.ps.gz
(34MB gzip file unpacks to 134MB postscript
Cold atom Clocks and Applications
This paper describes advances in microwave frequency standards using
laser-cooled atoms at BNM-SYRTE. First, recent improvements of the Cs
and Rb atomic fountains are described. Thanks to the routine use of a
cryogenic sapphire oscillator as an ultra-stable local frequency reference, a
fountain frequency instability of where
is the measurement time in seconds is measured. The second advance is a
powerful method to control the frequency shift due to cold collisions. These
two advances lead to a frequency stability of at 7\times 10^{-16}^{87}^{133}$Cs fountains.
Finally we give an update on the cold atom space clock PHARAO developed in
collaboration with CNES. This clock is one of the main instruments of the
ACES/ESA mission which is scheduled to fly on board the International Space
Station in 2008, enabling a new generation of relativity tests.Comment: 30 pages, 11 figure
Actual and undiagnosed HIV prevalence in a community sample of men who have sex with men in Auckland, New Zealand
<p>Abstract</p> <p>Background</p> <p>The prevalence of HIV infection and how this varies between subgroups is a fundamental indicator of epidemic control. While there has been a rise in the number of HIV diagnoses among men who have sex with men (MSM) in New Zealand over the last decade, the actual prevalence of HIV and the proportion undiagnosed is not known. We measured these outcomes in a community sample of MSM in Auckland, New Zealand.</p> <p>Methods</p> <p>The study was embedded in an established behavioural surveillance programme. MSM attending a gay community fair day, gay bars and sex-on-site venues during 1 week in February 2011 who agreed to complete a questionnaire were invited to provide an anonymous oral fluid specimen for analysis of HIV antibodies. From the 1304 eligible respondents (acceptance rate 48.5%), 1049 provided a matched specimen (provision rate 80.4%).</p> <p>Results</p> <p>HIV prevalence was 6.5% (95% CI: 5.1-8.1). After adjusting for age, ethnicity and recruitment site, HIV positivity was significantly elevated among respondents who were aged 30-44 or 45 and over, were resident outside New Zealand, had 6-20 or more than 20 recent sexual partners, had engaged in unprotected anal intercourse with a casual partner, had had sex with a man met online, or had injected drugs in the 6 months prior to survey. One fifth (20.9%) of HIV infected men were undiagnosed; 1.3% of the total sample. Although HIV prevalence did not differ by ethnicity, HIV infected non-European respondents were more likely to be undiagnosed. Most of the small number of undiagnosed respondents had tested for HIV previously, and the majority believed themselves to be either "definitely" or "probably" uninfected. There was evidence of continuing risk practices among some of those with known HIV infection.</p> <p>Conclusions</p> <p>This is the first estimate of actual and undiagnosed HIV infection among a community sample of gay men in New Zealand. While relatively low compared to other countries with mature epidemics, HIV prevalence was elevated in subgroups of MSM based on behaviour, and diagnosis rates varied by ethnicity. Prevention should focus on raising condom use and earlier diagnosis among those most at risk, and encouraging safe behaviour after diagnosis.</p
A blind transform based approach for the detection of isolated astrophysical pulses
his paper presents a blind algorithm for the automatic detection of isolated astrophysical pulses. The detection algorithm is applied to spectrograms (also known as “filter bank data” or “the (t,f) plane”). The detection algorithm comprises a sequence of three steps: (1) a Radon transform is applied to the spectrogram, (2) a Fourier transform is applied to each projection parametrized by an angle, and the total power in each projection is calculated, and (3) the total power of all projections above 90° is compared to the total power of all projections below 90° and a decision in favor of an astrophysical pulse present or absent is made. Once a pulse is detected, its Dispersion Measure (DM) is estimated by fitting an analytically developed expression for a transformed spectrogram containing a pulse, with varying value of DM, to the actual data. The performance of the proposed algorithm is numerically analyzed
Disassortative Age-Mixing Does Not Explain Differences in HIV Prevalence between Young White and Black MSM: Findings from Four Studies
Viral suppression and HIV transmission in serodiscordant male couples: an international, prospective, observational, cohort study
Background: Evidence on viral load and HIV transmission risk in HIV-serodiscordant male homosexual couples is limited to one published study. We calculated transmission rates in couples reporting condomless anal intercourse (CLAI), when HIV-positive partners were virally suppressed, and daily pre-exposure prophylaxis (PrEP) was not used by HIV-negative partners. Methods: In the Opposites Attract observational cohort study, serodiscordant male homosexual couples were recruited from 13 clinics in Australia, one in Brazil, and one in Thailand. At study visits, HIV-negative partners provided information on sexual behaviour and were tested for HIV and sexually transmitted infections; HIV-positive partners had HIV viral load tests, CD4 cell count, and sexually transmitted infection tests done. Viral suppression was defined as less than 200 copies per mL. Linked within-couple HIV transmissions were identified with phylogenetic analysis. Incidence was calculated per couple-year of follow-up, focusing on periods with CLAI, no use of daily PrEP, and viral suppression. One-sided upper 95% CI limits for HIV transmission rates were calculated with exact Poisson methods. Findings: From May 8, 2012, to March 31, 2016, in Australia, and May 7, 2014, to March 31, 2016, in Brazil and Thailand, 358 couples were enrolled. 343 couples had at least one follow-up visit and were followed up for 588·4 couple-years. 258 (75%) of 343 HIV-positive partners had viral loads consistently less than 200 copies per mL and 115 (34%) of 343 HIV-negative partners used daily PrEP during follow-up. 253 (74%) of 343 couples reported within-couple CLAI during follow-up, with a total of 16 800 CLAI acts. Three new HIV infections occurred but none were phylogenetically linked. There were 232·2 couple-years of follow-up and 12 447 CLAI acts in periods when CLAI was reported, HIV-positive partners were virally suppressed, and HIV-negative partners did not use daily PrEP, resulting in an upper CI limit of 1·59 per 100 couple-years of follow-up for transmission rate. Interpretation: HIV treatment as prevention is effective in men who have sex with men. Increasing HIV testing and linking to immediate treatment is an important strategy in HIV prevention in homosexual men. Funding: National Health and Medical Research Council; amfAR, The Foundation for AIDS Research; ViiV Healthcare; and Gilead Sciences
Viewing Pornography Depicting Unprotected Anal Intercourse: Are There Implications for HIV Prevention Among Men Who Have Sex with Men?
New cosmological constraints on the variation of fundamental constants: the fine structure constant and the Higgs vacuum expectation value
Aims. We study the time variation of the fine structure constant, α, and the Higgs vacuum expectation value v, during the Big Bang nucleosynthesis (BBN).
Methods. We computed primordial abundances of light nuclei produced during the BBN stage by including resonances in the leading reaction rates which reduce the primordial abundance of beryllium. We performed this calculation considering that α and v may vary during the BBN. Using observable data on deuterium, 4He, and 7Li, we set constraints on the variation of the fundamental constants.
Results. Results indicate a null variation of α and v, while the best-fit value for the baryon-to-photon ratio agrees well with the WMAP value.
Conclusions. We found that the variation of α is null within 3σ, the variation of v is null within 6σ, and the preferred value of the baryon-to-photon ratio is in good agreement, within 3σ, with the value extracted using the WMAP data. We improve the fits respect to previous works