Créidhe O'Sullivan

Measurements of Structure in the Cosmic Background Radiation with the Cambridge Cosmic Anisotropy Telescope

We have observed a 2° × 2° area of sky at frequencies of 15.5 and 16.5 GHz with the Cambridge Cosmic Anisotropy Telescope (CAT). Comparison with earlier measurements at 13.5 GHz shows that the observed structure at 16.5 GHz arises predominantly from the cosmic microwave background. The broadband power, averaged over spherical harmonic multipole orders between 330 and 680, is (ΔT/T) = 2.0+ 0.4−0.4 × 10-5, which is consistent with the predictions of a standard COBE-normalized, cold dark matter model.

Absolute Tilt from a Laser Guide Star: A First Experiment

Absolute tip–tilt recovery using a tilt signal measured on a Laser Guide Star is a central problem in the framework of the development of Adaptive Optics Systems reaching full sky coverage down to visible wavelengths. In the past few years, various techniques aimed at solving this problemhave been proposed. However only a couple of these has been recentlytested in practice.We report about an experiment aimed at evaluating the performance of one of these techniques called the ‘Elongation Perspective’ technique. Our experiment has been performed using the ALFA system inCalar-Alto (Spain) and involves the simultaneous operation of the 3.6 m and the 2.2 m telescopes at the Observatory.This article describes the telescope configuration used, as well as the datareduction process carried out in order to estimate the scientific objecttilt. The technique performances are discussed in terms of the residualtilt error variance and related correlation coefficient. The analysisshows that, despite the low SNR of our measurements, the atmospheric tiltvariance is reduced to 80% of its initial value corresponding to acorrelation coefficient of about 0.6. To get a betterestimate of the performance achievable using this technique,the tilt error variance due to photon noise in the laser measurementis estimated and removed from the obtained tilt error variance.When this correction is done, thisvariance is reduced to about 50% of its initial value, showingthat the use of this technique can give rise to a significant reduction of the scientific object image motion.

Laser guide star: monitoring and light pollution

This paper summarizes three years of observations of the resonant optical backscatter of laser, used to produce a mesospheric sodium-layer laser guide star for an adaptive optics system. Observations were obtained from a neighboring telescope. The aim of this work was two-fold: to study the Na plume (altitude and profile variations) and to study the Rayleigh cone in order to achieve scattering measurements relevant to the light pollution created by a sodium laser guide star. We report on the short-term characteristics of the sodium layer and stress the consequences of these variations for Laser Guide Star Adaptive Optics System operations. From the measurements of the background intensity measured while observing the laser guide star and the top of the Rayleigh cone, we can derive information on the light pollution produced by the laser as well as the resulting implications for an observatory laser management policy. Information on the laser intensity, size and shape along the Rayleigh cone are also presented.