Astronomical instrumentation

Abstract:

We describe the design and performance of an easy to machine horn which exhibits excellent beam circularity and low cross polarisation over a relatively large bandwidth. No grooves are machined into the horn walls but, alternatively, flare angle discontinuities are generated along the horn profile. In other words, the horns will have several flare angles or sections instead of one. For example, if the horn consists of two flare angles, it could then be considered as a conventional Potter horn. As can be seen below, even with this simple design, excellent radiation patterns can be obtained over 15% bandwidth. The bandwidth could be further increased by adding more subsections with 30% bandwidth obtained when the profile is based on 4 sections. The operation of the horn is based on generating higher order modes at the correct amplitude ratio and phase with respect to the incident TE11 mode in the circular waveguide, which is achieved by accurate determination of the magnitude and location of the flare steps. This in turn yields a field distribution at the horn aperture that has low sidelobes and cross polarization in the radiation pattern. A key component in the design package is the optimization software that searches for the correct magnitude and location of the flare discontinuities. We have generated a software package based on the combination of modal matching, genetic algorithm (GA) and simplex optimization. The genetic code is first used to locate the proximity of the global minimum. The set of parameters obtained are then used as a starting point for the simplex method, which refines the parameters to the required accuracy. We shall illustrate our method by showing radiation patterns using two and three step discontinuities and also patterns for a spline profiled horn based on work by other investigators who used different optimization techniques.

Abstract:

We describe the objectives, design and predicted performance of Clover, a fully-funded, UK-led experiment to measure the B-mode polarisation of the Cosmic Microwave Background (CMB). Three individual telescopes will operate at 97, 150 and 225 GHz, each populated by up to 256 horns. The detectors, TES bolometers, are limited by unavoidable photon noise, and coupled to an optical design which gives very low systematic errors, particularly in cross-polarisation. The telescopes will sit on three-axis mounts on a site in the Atacama Desert. The angular resolution of around 8 ́ and sky coverage of around 1000 deg2 provide multipole coverage of 20<ℓ<1000. Combined with the high sensitivity, this should allow the B-mode signal to be measured (or constrained) down to a level corresponding to a tensor-to-scalar ratio of r = 0.01, providing the emission from polarised foregrounds can be subtracted. This in turn will allow constraints to be placed on the energy scale of inflation, providing an unprecedented insight into the early history of the Universe.

Abstract:

Mid-infrared polarimetry remains an underexploited technique; where available it is limited in spectral coverage from the ground, and conspicuously absent from both the Spitzer and JWST instrument suites. The unique characteristics of SOFIA affords unprecedented spectral coverage and sensitivity in the mid-infrared waveband, offering new vistas in the exploration of astrophysical objects, including (a) galaxies and AGN, (b) star formation regions and (c) debris disks. Furthering the existing 5-40μm imaging and spectroscopic capabilities of SOFIA, and the University of Florida's mid-IR imagers, spectrometer and Polarimeter designs of T-ReCS and CanariCam, we present an overview of science highlights that could be performed from a ∼5-40μm imaging- and spectro-polarimeter on SOFIA. A secondary science driver is the inclusion of low- to moderate- resolution (total flux) spectroscopy at these wavelengths. Such an instrument concept would plug an unfilled area of both SOFIA and space-based instrumentation, providing SOFIA with unique and exciting science capabilities.

Abstract:

We present the results of the Gemini Deep Planet Survey, a near-infrared adaptive optics search for giant planets and brown dwarfs around 85 nearby young stars. The observations were obtained with the Altair adaptive optics sys- tem at the Gemini North telescope, and angular differential imaging was used to suppress the speckle noise of the central star. Typically, the observations are sensitive to angular separations beyond 0.5″ with 5 σ contrast sensitivities in magnitude difference at 1.6 μm of 9.5 at 0.5″, 12.9 at 1″, 15.0 at 2″, and 16.5 at 5″. These sensitivities are sufficient to detect planets more massive than 2 MJ with a projected separation in the range 40-200 AU around a typical target. Second-epoch observations of 48 stars with candidates (out of 54) have confirmed that all candidates are unrelated background stars. A detailed statistical analysis of the survey results is presented. Assuming a planet mass distribution dn/dm oc m-1.2 anda semimajor-axis distribution dn/da ∝ a-1, the 95% credible upper limits on the fraction of stars with at least one planet of mass 0.5-13 MJ are 0.28 for the range 10-25 AU, 0.13 for 25-50 AU, and 0.093 for 50-250 AU; this result is weakly dependent on the semimajor-axis distribution power-law index. The 95% credible interval for the fraction of stars with at least one brown dwarf companion having a semimajor axis in the range 25-250 AU is 0.019-0.015+0.083 irrespective of any assumption on the mass and semimajor-axis distributions. The observations made as part of this survey have resolved the stars HD 14802, HD 166181, and HD 213845 into binaries for the first time. © 2007. The American Astronomical Society. All rights reserved.

Abstract:

We present a differential wavefront sampling method for the efficient alignment of centred optical systems. Using the inter-element effects reported in our previous study, this method generates a linear symmetric matrix that relates the optical wavefront to misalignments within the system. The solution vector of this matrix equation provides a unique description of decentre and tilt misalignments of the system. We give a comparison of this approach to the existing method in the first case study and then illustrate characteristics of the new approach using the subsequent four case studies and Monte-Carlo alignment simulations. The results reveal superiority of the method over the existing one in misalignment estimation accuracy and demonstrate the practical feasibility and robustness. © 2007 Optical Society of America.