The NAOS visible wave front sensor
This paper describes the Visible Wave Front Sensor (visible WFS) for the VLT Nasmith Adaptive Optics system (NAOS). This Shack-Hartman-based wave front sensor instrument includes within a continuous flow liquid nitrogen cryostat: - a low noise fast readout CCD camera controlled by the ESO new generation CCD controller FIERA. The readout noise of this system is 3 e- at 50 kilopixel/sec/port, and is only limited by the CCD intrinsic noise. FIERA proposes remotely controlled readout modes with optional binning, windowing and flexible integration time. - two remotely exchangeable micro-lens arrays focusing the analyzed wave front directly on the CCD sensitive surface. The wave front sensor includes also its own atmospheric dispersion compensator. Due to the continuous rotation of the NAOS adapter, the mechanical stiffness of the visible wave front sensor must be very high not to disturb the loop operation (no more than 0.1 μm of lenslet array displacement compared to the CCD location over a 30° rotation angle of the instrument). The following simulations and tests are described: - simulation results providing an estimation of the NAOS maximum operating magnitude - camera optimization - mechanical stiffness measurements.
Commissioning of the Adaptive Optics System NAOS-CONICA for the VLT : the way to First Light
In October 2002 NAOS-CONICA, one of the most powerful adaptive optics systems was offered to the astronomical community. The instrument is installed at the Very Large Telescope in Chile and operated by the European Southern Observatory. The adaptive optics system NAOS corrects for atmospheric turbulence and provides the near-infrared multi-mode camera and spectrograph CONICA with diffraction limited images. Development of NAOS was achieved by a French consortium, while CONICA was developed by a German consortium under the leadership of the Max-Planck-Institut fur Astronomie, Heidelberg. In the context of this PhD thesis several critical contributions to the successful commissioning were made. The major test and calibration results obtained on the way to First Light, i.e., during the laboratory, integration and commissioning periods are presented. They cover cryogenics, mechanics, optics and detector characteristics of CONICA. A major achievement was the development and implementation of a technique for the calibration of static optical aberrations. Since the instrument is designed to achieve absolute Strehl ratios higher than 70%, even the accumulation of residual static wavefront perturbations arising from optical components in the imaging path, critically affect the overall performance. The technique has been completely implemented for the instrument, and the adaptive optics system automatically corrects the static aberrations according to the different instrument configurations. The presented technique will be of great importance for future adaptive optics systems. Particular attention is payed to the calibration of CONICA's high resolution imaging spectroscopy mode, realized by a cold tunable Fabry-Perot interferometer. A detailed guideline is given of how to process a phase-shift map and the capabilities for 3-dimensional structure analysis are demonstrated at Eta Carinae. A first, high-spatial resolution velocity map is created.
Intelligibility of modified speech for young listeners with normal and impaired hearing.
Exposure to modified speech has been shown to benefit children with language-learning impairments with respect to their language skills (M. M. Merzenich et al., 1998; P. Tallal et al., 1996). In the study by Tallal and colleagues, the speech modification consisted of both slowing down and amplifying fast, transitional elements of speech. In this study, we examined whether the benefits of modified speech could be extended to provide intelligibility improvements for children with severe-to-profound hearing impairment who wear sensory aids. In addition, the separate effects on intelligibility of slowing down and amplifying speech were evaluated. Two groups of listeners were employed: 8 severe-to-profoundly hearing-impaired children and 5 children with normal hearing. Four speech-processing conditions were tested: (1) natural, unprocessed speech; (2) envelope-amplified speech; (3) slowed speech; and (4) both slowed and envelope-amplified speech. For each condition, three types of speech materials were used: words in sentences, isolated words, and syllable contrasts. To degrade the performance of the normal-hearing children, all testing was completed with a noise background. Results from the hearing-impaired children showed that all varieties of modified speech yielded either equivalent or poorer intelligibility than unprocessed speech. For words in sentences and isolated words, the slowing-down of speech had no effect on intelligibility scores whereas envelope amplification, both alone and combined with slowing-down, yielded significantly lower scores. Intelligibility results from normal-hearing children listening in noise were somewhat similar to those from hearing-impaired children. For isolated words, the slowing-down of speech had no effect on intelligibility whereas envelope amplification degraded intelligibility. For both subject groups, speech processing had no statistically significant effect on syllable discrimination. In summary, without extensive exposure to the speech processing conditions, children with impaired hearing and children with normal hearing listening in noise received no intelligibility advantage from either slowed speech or envelope-amplified speech.