THE ING NEWSLETTER No. 5, October 2001
    GENERAL
    SCIENCE
    TELESCOPES AND INSTRUMENTATION
    OTHER NEWS FROM ING
    TELESCOPE TIME

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    First Results from the Rayleigh Laser Guide Star Project

    Paul Clark (University of Durham)
    The Restricted Conjugate Adaptive Optics project is part of the University of Durham Astronomical Instrumentation Group's rolling grant programme, funded by PPARC. The project aims to demonstrate the feasibility of using commercially available laser technology to provide a Rayleigh back-scatter laser guide star system for use with Adaptive Optics. In particular the project investigates low order AO correction over a wide field of view for use with multi-object spectroscopy on the WHT.

    The project is ongoing with the first (Phase A) run having been completed at the Isaac Newton Group of Telescopes on La Palma during May 2001. Launch tests were conducted using both the 4.2-m William Herschel Telescope and a custom-made 30-cm f/6 launch system housed in an outbuilding adjacent to WHT.

    Figures 1 and 2 show the high power laser launch through the 30-cm f/6 launch system and the WHT. Safety considerations form a major part of the project. The laser is only operated by trained staff using protective goggles to prevent eye damage. A plane spotter is used to ensure that no harm comes to aircraft that may fly through the laser beam (the mountain top itself is actually a 'no fly' zone).
     

    Laser launch Laser launch and the WHT
    Figure 1 (left). High power laser launch through the 30-cm f/6 launch system [ JPEG | TIFF ]. Figure 2. The laser launch through the WHT [ JPEG | TIFF ].

    The 'bow tie' structure seen in Figure 1 is a result of  focussing the laser at a particular altitude. The laser is actually pulsed (although the beam appears continuous to the human eye) and a high speed pockels cell shutter is used to range-gate the return pulse at the focal height. The focal spot then appears as a low altitude star which can be used to correct atmospheric turbulence with an Adaptive Optics system. The novel optical configuration used during the Phase A run is shown in Figure 3.
     

    Optical configuration
    Figure 3. Optical configuration used during the Phase A run [ GIF | TIFF ].

    Fluorescence Measurement

    One of the main objectives of the WHT run was to establish if fluorescence from the telescope optics would prevent return measurements during the next phase of the project. The trace in Figure 4, taken just before sunrise on May 13th using a photomultiplier tube, shows return scatter and fluorescence from the telescope optics decaying within 2.5 µs, establishing that return above 400 m should be observable without difficulty.

    Fluorescence Measurement
    Figure 4. Fluorescence measurement [ GIF | TIFF ]

    Range-Gated Return

    The range-gate pockels cell shutter was tested during the May 13th slot on the WHT. The return from differing altitudes was observed using a photomultiplier tube looking at the telescope via a 'mirror with a hole' (to separate launch and return beams — see Figure 3). The 'bow tie' structure is formed as the return passes through focus where most of the light is lost through the hole (see Figure 5). The bow tie was seen to change position as expected as the focal height was altered.
     

    Range-gated return
    Figure 5. Range-gated return [ GIF | TIFF ]

    Cloud Ceiling Detection

    An interesting by-product of the Rayleigh laser system, when used with photomultiplier return detection, is the ability to accurately resolve cloud layers above the telescope. The trace in Figure 6 was taken on May 15th using the 30-cm f/6 launch system from the LN2 building. Two thin cloud layers at 2.8km and 4.7km are clearly visible.
     

    Cloud ceiling detection
    Figure 6. Cloud ceiling detection [ GIF | TIFF ]


    Email contact: Paul Clark (paul.clark@durham.ac.uk)


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    © Isaac Newton Group of Telescopes, La Palma, 2001.