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  • 1
    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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  • 2
    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)
    Fulltext
  • 3
    Electronic Resource
    Electronic Resource
    The ALFA Laser: Beam Relay and Control System (2000)
    Springer
    Experimental astronomy 10 (2000), S. 89-101 
    Details
    ISSN: 1572-9508
    Keywords: adaptive optics ; hardware ; laser guide star ; software
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The ALFA laser subsystem uses a 4 watt continuous wave laser beam to produce an artificial guide star in the mesospheric sodium layer as a reference for wavefront sensing. In this article we describe the system design, focusing on the layout of the beam relay system. It consists of seven mirrors, four of which are motor-controlled in closed loop operation accounting for turbulences inside the dome and flexure of the main telescope.The control system features several computers which are located close to analysis and control units. The distribution of the tasks and their interaction is presented, as well as the graphical user interface used to operate the complete system. This is followed by a discussion of the aircraft detection system ALIENS. This system shuts off the laser beam when an object passes close to the outgoing laser.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1008191103168
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    Paper (German National Licenses)
    Fulltext
  • 4
    Electronic Resource
    Electronic Resource
    ALFA: The MPIA/MPE Laser Guide Star AO System (2000)
    Springer
    Experimental astronomy 10 (2000), S. 1-3 
    Details
    ISSN: 1572-9508
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1008119225652
    Library Location Call Number Volume/Issue/Year Availability
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    Paper (German National Licenses)
    Fulltext
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