Astronomical instrumentation

Abstract:

Near-infrared bandpass filters are commonly designed & manufactured using vacuum-evaporated films of Silicon and Silicon Monoxide. However the transparency of these filters is limited by optical absorption in the films when producing filters for wavelengths below 1200nm approximately. This work reports improvements in NIR filter transparency achieved by exploiting recent advances in magnetron sputtering technology. Sputtered silicon compound films have been used to demonstrate efficient bandpass filters for astronomy applications at wavelengths below 1000nm. This process technology allows a new selection of film materials to be used in design of NIR bandpass filters, with transmission and thermal drift characteristics which differ from conventional evaporated coatings. The spectral location of the bandpass is controlled by a non-optical method, which avoids the complex optical monitoring configurations normally required. The speed and flexibility of this process also offers a potential solution for projects which require small batches of custom NIR optical filters. Highly durable filters are obtained without elevated process temperature, which would otherwise be required in conventional evaporation processes. This avoids heating filter substrates which may be sensitive to thermal cycling effects. Attenuation of sidebands to T<0.0001 is reported across the spectral range of common sensor devices. The thermal sensitivity for cryostat applications is characterised and compared to conventional evaporated optical coatings. This method has been applied to 975nm & 985nm bandpass filters for use on VISTA project instrumentation. It also offers improvements for filters at longer wavelengths in the range 1000nm-5000nm. Some examples are reported in this region.

Abstract:

We present the manufacturing and first results from testing and characterising the SWIFT image slicer. The SWIFT image slicer design is based on the MPE-3D and SPIFFI image slicers. It uses plane mirrors to slice the input field but through a novel, de-magnifying design, using a mosaic of spherical lenses, it achieves a considerable de-magnification. Classical polishing techniques can be applied to manufacture both plane and spherical surfaces with very high surface accuracy and quality reducing aberrations and scattered light. The SWIFT image slicer was manufactured over a 18 months period and was delivered to Oxford in September 2007. The commissioning of the SWIFT instrument will take place in August/September 2008.

Abstract:

We present the properties of the discrete X-ray sources detected in our monitoring program of the 'typical' elliptical galaxy, NGC 3379, observed with Chandra ACIS-S in five separate pointings, resulting in a co-added exposure of 324 ks. From this deep observation, 132 sources have been detected within the region overlapped by all observations, 98 of which lie within the D25 ellipse of the galaxy. These 132 sources range in LX from 6 × 1035 erg s-1 (with 3 σ upper limit ≤4 × 1036 erg s-1) to ∼2 × 1039 erg s -1, including one source with LX > 1 × 10 39 erg s-1, which has been classified as a ULX. From optical data, 10 X-ray sources have been determined to be coincident with a globular cluster, these sources tend to have high X-ray luminosity, with three of these sources exhibiting LX > 1 × 1038 erg s-1. From X-ray source photometry, it has been determined that the majority of the 132 sources that have well constrained colors, have values that are consistent with typical LMXB spectra. In addition to this, a subpopulation of 10 sources has been found to exhibit very hard spectra and it is expected that most of these sources are absorbed background AGN. There are 64 sources in this population that exhibit long-term variability, indicating that they are accreting compact objects. Five of these sources have been identified as transient candidates, with a further 3 possible transients. Spectral variations have also been identified in the majority of the source population, where a diverse range of variability has been identified, indicating that there are many different source classes located within this galaxy. © 2008. The American Astronomical Society. All rights reserved.

Abstract:

CℓOVER is a multi-frequency experiment optimised to measure the Cosmic Microwave Background (CMB) polarization, in particular the B-mode component. CℓOVER comprises two instruments observing respectively at 97 GHz and 150/225 GHz. The focal plane of both instruments consists of an array of corrugated feed-horns coupled to TES detectors cooled at 100 mK. The primary science goal of CℓOVER is to be sensitive to gravitational waves down to r ∼ 0.03 (at 3σ) in two years of operations.

Abstract:

We present theoretical models for the formation and evolution of populations of low-mass X-ray binaries (LMXBs) in the two elliptical galaxies NGC 3379 and NGC 4278. The models are calculated with the recently updated StarTrack code, assuming only a primordial galactic field LMXB population. StarTrack is an advanced population synthesis code that has been tested and calibrated using detailed binary star calculations and incorporates all the important physical processes of binary evolution. The simulations are targeted to modeling and understanding the origin of the X-ray luminosity functions (XLFs) of point sources in these galaxies. For the first time we explore the population XLF in luminosities below 1037 ergs s-1, as probed by the most recent observational results. We consider models for the formation and evolution of LMXBs in galactic fields with different CE efficiencies, stellar wind prescriptions, magnetic braking laws, and IMFs. We identify models that produce XLFs consistent with the observations both in shape and absolute normalization, suggesting that a primordial galactic field LMXB population can make a significant contribution to the total population of an elliptical galaxy. We also find that the treatment of the outburst luminosity of transient systems remains a crucial factor for the determination of the XLF, since the modeled populations are dominated by transient X-ray systems. © 2008. The American Astronomical Society. All rights reserved.