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About NUI Galway
About NUI Galway
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Clusters and Variability
As the title of this page indicates, we have fingers in many different pies. The common thread in all our endeavours is the development and use of improved astronomical/imaging technologies, principally in the software domain, to attain a variety of scientific goals. We therefore have considerable experience in:
- The end-to-end process of acquiring (or simulating) and processing astronomical images, including:
- Exposure Time Calculator (ETC) development, to model instrument throughput as a function of wavelength
- Point-Spread-Function (PSF) modelling, including time-variable seeing distributions and spatially-variable PSFs (SV-PSFs)
- Simulating images from various instruments, incorporating high levels of realism in both astrophysical and instrumental inputs
- Design of custom narrowband optical filters, for spectrophotometry
- Deriving schemes for attaining the maximum signal-to-noise ratio in imaging, taking observational duty cycle into account
- Developing automated data sorting and reduction "pipeline" software, with sophisticated exception handling
- Advanced image processing/analysis techniques, such as:
- Deconvolution, with SV-PSFs and subsampling
- Crowded-field photometry – both PSF-fitting (DAOPHOT) and image subtraction (ISIS)
- Time-series photometric analysis
- Astrometric calibration
- Post-exposure imaging sharpening (PEIS) and frame selection (“Lucky
Our research interest in these technologies drives our scientific research programmes – and vice-versa. It is a symbiotic relationship. We are constantly posing questions like “how can we make better observations of this type of target?” and “how can we extract more information from this type of data?”
If you are a researcher and find yourself asking the same kind of question, then we’d like to hear from you. Perhaps we can help you – and perhaps you can help us.
Already, our skills base has led to much collaborative work in different fields, in particular assisting with and participating in the observational programmes on ultra-cool dwarfs, and pulsars/supernova remnants , which are led by our colleagues Aaron Golden and Andy Shearer in the CfA.
Our team currently consists of:
- Dr. Ray Butler (lecturer in Physics & Astronomy, and CfA secretary)
- Lisa-Marie Browne
- Salam Dulaimi
- plus numerous collaborators within NUI Galway and elsewhere.
Joining the Group
If you are interested in joining the group to carry out an MSc, PhD or as a postdoc, then please contact Ray Butler in the first instance.
Postgrads: We will be supporting a small number of applications for IRCSET and NUI Galway College of Science PhD fellowships.
One last thing: you don’t have to speak IRAF/Pyraf to join our group – but you will be fluent by the time you leave!
We are interested in the stellar populations of star clusters, primarily the globular clusters of the Milky Way galaxy. The rich ecology of populations that we have studied in such clusters includes several classes of variable stars, close binary stars, pulsars, and even an extra-solar planet. At present, we are observing clusters with our custom-designed filters, to measure the variation in absorption of starlight due to varying abundances of carbon and/or nitrogen. The presence of these variations from star to star within some clusters is at odds with the standard model of all stars having been formed at the same time, from the same mix of chemical elements (i.e. the same "metallicity"). Our results to date indicate that these variations are indeed primordial in origin. Kieran Forde is currently on an extended placement at the University of California at Santa Cruz/Lick Observatory, investigating the extension of this field to the regime of extra-galactic globular clusters.
New work underway in this area includes investigation of the use of deconvolution of early-epoch HST data, for more accurate astrometry in cluster cores. This will be used to extend back the baseline in the search for high-proper motion stars, which could be evidence of an intermediate-mass black hole.
Our interest in variable stars began in the 1990s, with the development at Galway of the TRIFFID high angular resolution camera. TRIFFID used imaging photon-counting detectors (MAMAs) for very high-speed imaging, and PEIS post-processing, to achieve what were then some of the sharpest long-exposure ground-based optical images ever taken. This instrument was put to use to target low-mass X-ray binaries, buried in the highly crowded, scarcely resolvable cores of globular clusters. A bonus of this work was the detection or confirmation in the cluster centres of a large number of hitherto unknown variable stars, of various types (RR Lyrae, Type 2 Cepheid, possible eclipsing). Upper limits were also set on the occurrence of dwarf novae.
Subsequently, the need for instruments like TRIFFID to obtain the highest angular resolution for this kind of variability work was superseded by the development of effective image-subtraction algorithms, like ISIS. Although we used ISIS on our later TRIFFID data and the enhanced angular resolution did help, it was clear that very high photometric signal-to-noise (the “depth” of the dataset) made up most or all of the ground lost when resolution was poorer. This handed the initiative back to regular CCD cameras, with their much higher quantum efficiency (q.e.), and much simpler calibration, than TRIFFID-type imaging photon-counting detectors.
But now there’s a new game in town. The development of a new CCD camera technology, dubbed the Low Light Level (L3) CCD, can give us the best of both worlds: the high q.e. & ease of calibration of a CCD, the high speed & lack of readout dead-time of an imaging photon-counting detector, and negligible detector noise. With our colleagues in the Applied Imaging group and Applied Optics Centre, we have built a new camera system based on L3-CCD technology, which we have called GUFI - the Galway Ultra Fast Imager. This camera is suitable for PEIS & Lucky Imaging work, as well as optimised longer-exposure time-series photometry with a 100% duty cycle. The system received "first light" in 2006 on the 1.5m telescope at Loiano, Italy. Brendan Sheehan has been working on building and calibrating the L3-CCD setup - including simulations of its performance, and determining how to optimize its use for a given observational scenario. Brendan has also written a very efficient and highly functional automated pipeline for L3-CCD data reduction and differential photometry. John Chambers designed a two-beam "outrigger" variant for the GUFI optics; the purpose of the steerable second beam would be to help to locate suitably bright comparison stars for differential photometry in sparse fields, even when such stars lay well outside the small field of view of the main beam. Leon Harding is currently converting the old (HP-UX/Digital Unix) PEIS code for the MAMA photon-stream data format, to a new version that will work with FITS image datacubes from the L3-CCD camera.
GUFI's first target was another type of (potentially) variable object that interests us – brown dwarfs/ultra-cool dwarfs. Aaron Golden’s group have been highly successful in radio studies of these odd little objects, and we meld with this group to provide optical/IR timeseries photometry. This has been Caoilfhionn Lane’s main PhD activity.
Still in the domain of variability, we are also interested in detecting extra-solar planets, aka exo-planets, and measuring their properties. This interest originally arose out of the anticipated large numbers of such planets in globular clusters, which however subsequently failed to materialize despite lengthy observational campaigns by a number of groups. It is now (since 2005) known that in fact the metallicity of these clusters is too low for "normal" planets to have formed, so we have adapted our efforts to conduct targeted observations of known, suspected, or possible transiting exoplanets around more nearby stars. If a planet orbits its parent star in the same plane as our line of sight to the star, it passes (transits) across the face of the star and thereby blocks a tiny proportion (~2% or generally much less) of the star's light. This is detectable as a dip in the brightness of the star, measured over the course of a series of repeated images. This observing procedure - transit photometry - is limited by both atmospheric scintillation and inefficiencies or noise imposed by detector readout. Our approach is to minimise the latter limitation, by the development of a new camera system based on L3-CCD technology. The system received "first light" in 2006 on the 1.5m telescope at Loiano, Italy. We attempted to make other observations with a regular CCD on the Faulkes 2m telescope at Maui, Hawaii, in conjunction with transition year students at local schools, but were stymied by tropical typhoons or scheduling overrides every time.
- Gerry Doyle and Tony Antonova ( Armagh Observatory, N. Ireland)
- Walter Brisken ( National Radio Astronomy Observatory, New Mexico)
- Bob Zavala and Fred Vrba ( United States Naval Observatory, Flagstaff, Arizona)
- Richard Boyle ( Vatican Observatory Research Group, Steward Observatory, University of Arizona)