The spectroscopy of DA white dwarfs at high resolution

This thesis is concerned with the spectroscopy of hot, hydrogen-rich white dwarfs, and the development of instrumentation with which to further their study. It begins with a review of white dwarf research, and a summary of results from some notable Extreme-Ultraviolet (EUV) missions. The absence of helium in hot hydrogen-rich white dwarfs is introduced, and the need to precisely determine their composition is explained. These issues provide motivation for the Joint Astrophysical Plasmadynamic Experiment (J-PEX). Using normal incidence, multilayer coated optics and a high resolution focal plane detector, this spectrometer offers a substantial improvement in effective area and spectral resolution over current instrumentation. The design and development of a microchannel plate (MCP) detector for J-PEX is discussed, and a study of MCP sensitivity enhancement processes presented. A CsI photocathode is found to offer superior performance in the 225 - 245 Å band covered by J-PEX. The low quantum efficiencies recently measured for MCPs are also discussed. A preliminary analysis of data from the first successful flight of J-PEX, on board a sounding rocket, is described. Techniques are devised to overcome uncertainties in wavelength calibration, leading to production of the highest resolution EUV spectrum currently available for an astronomical object. These data reveal the presence of helium along the line of sight to the white dwarf G191-B2B, and prove the value of the J-PEX design. Data from the Hubble Space Telescope and the International Ultraviolet Explorer are used to search for white dwarfs with highly ionised, non-photospheric absorption features. Three new identifications are made, in the white dwarfs REJ 1738+665, REJ 0558+165 and WD 2218+706. Possible explanations, including absorption by planetary nebulæ, are suggested. The spectra are of longer wavelength, but higher resolution than currently obtainable by J-PEX, and complement the latter instrument by resolving multiple velocity components along the line-of-sight. The importance of these measurements in the context of J-PEX results, is demonstrated

Autofluorescence Signatures of Seven Pathogens: Preliminary in Vitro Investigations of a Potential Diagnostic for Acanthamoeba Keratitis

PURPOSE: Acanthamoeba keratitis can cause devastating damage to the human cornea and is often difficult to diagnose by routine clinical methods. In this preliminary study, we investigated whether Acanthamoeba may be distinguished from other common corneal pathogens through its autofluorescence response. Although only a small number of pathogens were studied, the identification of a unique Acanthamoeba signature would indicate that autofluorescence spectroscopy as a diagnostic method merits further investigation. METHODS: Samples of 7 common pathogens (Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, Elizabethkingia miricola, Achromobacter ruhlandii, Candida albicans, and Acanthamoeba castellanii) in solution were excited with ultraviolet light at a number of successive, narrow wavebands between 260 and 400 nm, and their fluorescence response recorded. Principal Component Analysis was used to allow better visualization of the differences in response to UV light for different species. RESULTS: Acanthamoeba was found to possess a characteristic autofluorescence response and was easily distinguished from E. coli, S. aureus, P. aeruginosa, E. miricola, A. ruhlandii, and C. albicans over a wide range of excitation wavelengths. We also found a clear discrimination between E. coli, C. albicans, and P. aeruginosa at an excitation wavelength of 274 nm, whereas E. miricola, S. aureus, and A. ruhlandii could be separated using an excitation wavelength of 308 nm. CONCLUSIONS: Our results, although preliminary, indicate that autofluorescence spectroscopy shows promise as a diagnostic technique for keratitis. We intend to expand the set of pathogens studied before assessing the feasibility of the technique in vivo by introducing cultures onto pig corneas

Isolating auroral FUV emission lines using compact, broadband instrumentation

Images of auroral emissions at far ultraviolet (FUV, 122–200 nm) wavelengths are useful tools with which to study magnetospheric–ionospheric coupling, as the scattered sunlight background in this region is low, allowing both dayside and nightside auroras to be imaged simultaneously. The ratio of intensities between certain FUV emission lines or regions can be used to characterise the precipitating particles responsible for auroral emissions, and hence is a useful diagnostic of magnetospheric dynamics. Here, we describe how the addition of simple transmission filters to a compact broadband imager design allows far ultraviolet emission ratios to be deduced while also providing large-scale instantaneous images of the aurora. The low mass and volume of such an instrument would make it well-suited for both small satellite Earth-orbiting missions and larger outer planet missions from which it could be used to characterise the tenuous atmospheres observed at several moons, as well as studying the auroral emissions of the gas giants. We present a study to investigate the accuracy of a technique to allow emission line ratio retrieval, as applied to the OI 130.4 nm and 135.6 nm emissions at Ganymede. The ratio of these emissions provides information about the atmospheric composition, specifically the relative abundances of O and O2. Using modelled FUV spectra representative of Ganymede׳s atmosphere, based on observations by the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS), we find that the accuracy of the retrieved ratios is a function of the magnitude of the ratio, with the best measurements corresponding to a ratio of ~1.

The Joint astrophysical plasmadynamic EXperiment (J-PEX): A high-resolution rocket spectrometer

We report on the successful sounding rocket flight of the high resolution (R=3000-4000) J-PEX EUV spectrometer. J-PEX is a novel normal incidence instrument, which combines the focusing and dispersive elements of the spectrometer into a single optical element, a multilayer-coated grating. The high spectral resolution achieved has had to be matched by unprecedented high spatial resolution in the imaging microchannel plate detector used to record the data. We illustrate the performance of the complete instrument through an analysis of the 220-245Å spectrum of the white dwarf G191-B2B obtained with a 300 second exposure. The high resolution allows us to detect a low-density ionized helium component along the line of sight to the star and individual absorption lines from heavier elements in the photosphere