6dF instrument details

6dF (for 6-degree field) is an AAO-built instrument consisting of an automated fibre positioner and multi-object spectroscopic observing facility for the UK Schmidt Telescope. The positioner, like the FLAIR facility that it  replaced (and unlike 2dF on the AAT) operates remotely from the telescope in a special enclosure in the dome. It is shown here under the watchful eye of UKST observer Kristin Fiegert.
 

6dF system outline

  • Robotic r-theta positioning system with curved r-arm to match telescope focal curvature
  • Three interchangeable field plate units, each with up to 150 target fibres of 6.7 arcsec diameter (100um)
  • Four 7-fibre guide bundles for field acquisition
    • Note: because of  fibre breakages and limited access for repairs, the operational number of available spectroscopic fibres is usually less than 150. Please contact UK Schmidt Telescope staff for latest information.
  • Positioning accuracy of ~10um (0.7 arcsec)
  • Field configuration time of < 1 hour
  • Circular field plate of 5.9 degrees accessible diameter
  • Up to 8 fields observable per night (depending on exposure time)
  • Turn-around time between fields (for arcs, flats and field-plate interchange) of 20-30 mins.

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    6dF positioner and field plate units

    The robot's (r, theta) carriage is servo-controlled and carries a pneumatically-actuated gripper. The field plate has a curved steel surface, matching the telescope focal surface, on which the robot places magnetic buttons. The buttons carry cylindrical 90-degree prisms attached to optical fibres in a 10-metre cable that directly feeds a stationary spectrograph in  the dome. This provides an inherently stable instrumental configuration. The 6dF field plate units also carry the fibre retractors, a grid of illuminated fiducial marks to establish the positioner coordinate system, and a plate rotator to compensate for the rotational offset in the telescope.

    The two interchangeable field plate units, each with 150 spectroscopic fibres (shown parked above), allow one of them to be configured while the other is on the telescope. Each unit is loaded into the telescope in a similar way to the photographic plateholders. The 5-metre-long fiducial bundles are cabled separately from the spectroscopic fibres, and feed an intensified CCD camera for field acquisition and guidance.
     

    6dF VPH spectrograph

    The 150-fibre cable from each field plate unit is relayed to an optical bench in a separate enclosure in the dome, where the 6dF spectrograph is located. The instrument uses high-efficiency volume-phase holographic transmission gratings. Resolutions range from about 1 to 7 A. In practice, resolution is limited by the performance of the spectrograph camera (which has to work over a much wider field than it was originally designed for) and is typically 2.8 pixels at the field centre and up to 4.0 pixels in the corners. Each pixel is 13um square.

    A range of 5 gratings is available, and they are used in standard configurations giving the fixed wavelength ranges detailed below. Note that grating changes can only be carried out during daytime. Gratings 425R and 580V (the 6dF Galaxy Survey gratings) are conventionally mounted together in a remotely-controlled slide mechanism that allows rapid interchange between them. Grating 1700I is the RAVE survey grating. 

    Grating      Reciprocal     Instrumental        CCD             Spectral
             dispersion      resolution      resolution           range
                  (Å/mm)            (Å)          (Å/pixel)             (Å)
    425R            169             6.6             2.20            5300-7600
580V            126             4.9             1.64            3900-5600
1201B           60              2.1             0.78            3600-4400
1516R           46              1.5             0.6             6200-6800
1700I           30              1.0             0.4             8415-8800
    (All in 1st order)


    Page maintained by: Fred Watson, AAO

    e-mail: fgw@aaocbn.aao.gov.au

    Last revision: 19 December 2008