Astronomical instrumentation

Abstract:

As detailed instrument design progresses, judgements have to be made as to what changes to allow and when models such as thermal, stray-light and mechanical structure analysis have to be re-run. Starting from a well-founded preliminary design, and using good engineering design when incorporating changes, the design detailing and re-run of the models should bring no surprises. Nevertheless there are issues for maintaining the design and model configuration to a reasonably concurrent level. Using modern modeling software packages and foresight in setting up the models the process is made efficient, but at the same time the level of detail and number of cases now needed for instrument reviews is also large in order to minimise risks. We describe examples from the detailed instrument design of the VISTA IR Camera to illustrate these aspects and outline the design and analysis methods used.

Abstract:

This paper describes the image analysis algorithm developed for VISTA to recover wavefront information from curvature wave front sensor images. This technique is particularly suitable in situations where the defocused images have a limited number of pixels and the intrinsic or null aberrations contribute significantly to distort the images. The algorithm implements the simplex method of Neider and Mead. The simplex algorithm generates trial wavefront coefficients that are fed into a ray tracing algorithm which in turn produces a pair of defocused images. These trial defocused images are then compared against the images obtained from a sensor, using a fitness function. The value returned from the fitness function is fed back to the simplex algorithm, which then decides how the next set of trial coefficients is produced.

Abstract:

We present the detection of nuclear stellar discs in the low-luminosity elliptical galaxies, NGC 4458 and 4478, which are known to host a kinematically decoupled core. Using archival Hubble Space Telescope imaging, and available absorption line-strength index data based on ground-based spectroscopy, we investigate the photometric parameters and the properties of the stellar populations of these central structures. Their scalelength, h, and face-on central surface brightness, μ0c, fit on the μ0c-h relation for galaxy discs. For NGC 4458, these parameters are typical for nuclear discs, while the same quantities for NGC 4478 lie between those of nuclear discs and the discs of discy ellipticals. We present Lick/Image Dissector Scanner (IDS) absorption line-strength measurements of Hβ, Mg b and (Fe) along the major and minor axes of the galaxies. We model these data with simple stellar populations that account for the α/Fe overabundance. The counter-rotating central disc of NGC 4458 is found to have similar properties to the decoupled cores of bright ellipticals. This galaxy has been found to be uniformly old despite being counter-rotating. In contrast, the cold central disc of NGC 4478 is younger, richer in metals and less overabundant than the main body of the galaxy. This points to a prolonged star formation history, typical of an undisturbed disc-like, gas-rich (possibly pre-enriched) structure.

Abstract:

We present a re-analysis of the European Large Area Infrared Space Observatory (ISO) Survey (ELAIS) 90-μm observations carried out with ISOPHOT, an instrument on board the ISO of the European Space Agency. With more than 12 deg2, the ELAIS survey is the largest area covered by ISO in a single programme and is about one order of magnitude deeper than the IRAS 100-μm survey. The data analysis is presented and was mainly performed with the PHOT interactive analysis software but using the pairwise method of Stickel et al. for signal processing from edited raw data to signal per chopper plateau. The ELAIS 90-μm catalogue contains 237 reliable sources with fluxes larger than 70 mJy and is available in the electronic version of this article. Number counts are presented and show an excess above the no-evolution model prediction. This confirms the strong evolution detected at shorter (15 μm) and longer (170 μm) wavelengths in other ISO surveys. The ELAIS counts are in agreement with previous works at 90 μm and in particular with the deeper counts extracted from the Lockman hole observations. Comparison with recent evolutionary models show that the models of Franceschini et al. and Guiderdoni et al. (which includes a heavily extinguished population of galaxies) give the best fit to the data. Deeper observations are nevertheless required to discriminate better between the model predictions in the far-infrared, and are scheduled with the Spitzer Space Telescope, which has already started operating, and will also be performed by ASTRO-F.