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  • 1
    Electronic Resource
    Electronic Resource
    Rayleigh Scattering and Laser Spot Elongation Problems at ALFA (2000)
    Springer
    Experimental astronomy 10 (2000), S. 123-133 
    Details
    ISSN: 1572-9508
    Keywords: adaptive optics ; laser guide star ; Rayleigh scattering ; spot elongation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract This article describes the qualitative effects of LGS spot elongation and Rayleigh scattering on ALFA wavefront sensor images.An analytical model of Rayleigh scattering and a numerical model of laser plume generation at the altitude of the Na-layer were developed. These models, integrated into ageneral AO simulation, provide the sensor sub-apertureimages. It is shown that the centroid measurement accuracyis affected by these phenomena. The simulation was made both for the ALFA system and for the VLT Nasmyth Adaptive Optics System (NAOS).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1008117126814
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    The ALFA Laser Guide Star: Operation and Results (2000)
    Springer
    Experimental astronomy 10 (2000), S. 103-121 
    Details
    ISSN: 1572-9508
    Keywords: Abell Galaxy Clusters ; adaptive optics ; atmospheric turbulence ; Herbig Ae/Be stars ; laser guide star ; sodium profile
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The importance of laser guide stars to the practical usefulness ofadaptive optics cannot be understated, and yet there are veryfew working systems.This contribution discusses the current status of the ALFAlaser guide star, with regard to the particular difficulties encounteredwhile observing as well as both the expected performance and that so farachieved from scientific observations.A description is given of a number of experiments involving ALFA whichaim to determine the atmospheric turbulence and sodium layercharateristics, and demonstrate the possibility for tilt recovery fromlaser guide stars.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1008165517543
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    Paper (German National Licenses)
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  • 3
    Electronic Resource
    Electronic Resource
    The ALFA Laser and Analysis Tools (2000)
    Springer
    Experimental astronomy 10 (2000), S. 75-88 
    Details
    ISSN: 1572-9508
    Keywords: adaptive optics ; laser guide star ; atmospheric turbulence
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The optimal performance of adaptive optics systems can only be maintainedif the wavefront reference is bright and compact.Experience has shown that both of these important criteria are remarkablydifficult to achieve with laser guide stars. This contribution gives an account of the methods by which ALFAattempts to reach them.First, the production of a high quality, high power laser beam isdescribed.However, this quality is unavoidably compromised along the path to thelaunch telescope.In order to rectify this, a new set of diagnostic tools which monitorthe quality of the out-going beam has been installed, and these arealso described.Lastly, we outline a number of possible modifications on which we areworking.If successful, these may allow a substantial improvement in the beamquality.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1008137214239
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    Paper (German National Licenses)
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  • 4
    Electronic Resource
    Electronic Resource
    Absolute Tilt from a Laser Guide Star: A First Experiment (2000)
    Springer
    Experimental astronomy 10 (2000), S. 135-145 
    Details
    ISSN: 1572-9508
    Keywords: adaptive optics ; laser guide star
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Absolute tip–tilt recovery using a tilt signal measured on a Laser Guide Star is a central problem in the framework of the development of Adaptive Optics Systems reaching full sky coverage down to visible wavelengths. In the past few years, various techniques aimed at solving this problemhave been proposed. However only a couple of these has been recentlytested in practice.We report about an experiment aimed at evaluating the performance of one of these techniques called the ‘Elongation Perspective’ technique. Our experiment has been performed using the ALFA system inCalar-Alto (Spain) and involves the simultaneous operation of the 3.6 m and the 2.2 m telescopes at the Observatory.This article describes the telescope configuration used, as well as the datareduction process carried out in order to estimate the scientific objecttilt. The technique performances are discussed in terms of the residualtilt error variance and related correlation coefficient. The analysisshows that, despite the low SNR of our measurements, the atmospheric tiltvariance is reduced to 80% of its initial value corresponding to acorrelation coefficient of about 0.6. To get a betterestimate of the performance achievable using this technique,the tilt error variance due to photon noise in the laser measurementis estimated and removed from the obtained tilt error variance.When this correction is done, thisvariance is reduced to about 50% of its initial value, showingthat the use of this technique can give rise to a significant reduction of the scientific object image motion.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1008115109976
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    Paper (German National Licenses)
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  • 5
    Electronic Resource
    Electronic Resource
    Short timescale variability of the mesospheric sodium layer (2000)
    Springer
    Experimental astronomy 10 (2000), S. 147-156 
    Details
    ISSN: 1572-9508
    Keywords: adaptive optics ; laser guide star ; mesospheric sodium layer
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract In this article we investigate the short-term characteristics of the sodium layer and their implications for laser guide star systems. We report measurements of sodium density andcentroid-height variations on timescales of 100 ms upwards. Significant centroid-height variations on short timescales may necessitate frequent refocussing of the beam and wavefront sensor system.We present results from observations of the mesospheric sodium layer taken at the Max Planck observatory in Calar Alto, Spain in September 1997 and August 1998. We describe our experiment which uses the resonant optical backscatter of 589.2 nm laser light from the upper atmosphere as a measure of sodium abundance.Short-term variations are dominated by the formation of dense sporadic layers in the normal sodium layer. Measurements were made on 3 nights in 1997 and on 2 nights in 1998. Somewhat unexpectedly for a mid-latitude site, sporadic sodium layers were seen on 4 of these 5 nights. One of the sporadic layers was observed for its duration. The 2 km wide layer reached a maximum intensity of approximately two and a half times that of the background layer and could be distinguished from the background for over five hours. Centroid height variations of up to 400 m were observed on timescales of 1–2 min. In 1998 we were sensitive to variations of 5% or more in total sodium abundance on timescales of 100 ms upwards. We found no evidencefor variations of this level on these short timescales.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1008126318786
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    Paper (German National Licenses)
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