POSITIONER SPECIFICATIONS 2250 fibres (c.f. ~400 for FMOS-Echidna)

1. The heart of the Ukidna fibre positioner. 25 linear modules each support 90 fibres, their spines and actuators – a total of 2250 fibres. The spine tips place the fibres accurately in the spherical focal surface. The UK Schmidt 1.2-m telescope currently engaged in RAVE survey work, and proposed for conversion to the Ukidna facility. Side and end views of Ukidna module array. 25 modules each carry 90 actuators, supporting the fibre ends within a few tens of microns of the spherical focal surface. Schmidt corrector plate Telescope primary mirror Schmidt corrector Echidna fibre positioner Ukidna fibre positioner FPI imaging lens Digital FPI camera Ukidna fibre positioner mounted within UK Schmidt telescope. Note that the spine tips face towards the primary mirror. Support structures, electronics and fibre runs have been omitted for clarity Cut-away model of Ukidna installation, showing the FPI camera located in the central aperture of the UK Schmidt primary mirror. Support structures are not shown. Ukidna – the RAVE machine A. McGrath*, W. Saunders, F. Watson and S. Miziarski (Anglo-Australian Observatory) *E-mail: ajm@aao.gov.au UK SCHMIDT Our vision for Ukidna would involve committing the telescope entirely to the RAVE survey, and installation of the fibre positioner and spectrograph will involve significant modifications to the UK Schmidt. Such modifications will preclude easy changeover to other instruments. A study has been undertaken to confirm the suitability of the telescope, and to assess the scope of work required for conversion to the Ukidna facility. SUMMARY The Anglo-Australian Observatory is currently designing a new fibre positioner for the UK Schmidt Telescope. The instrument will have 2250 fibres, positioned with sub-arcsecond accuracy across a six degree field of view, and will have a reconfiguration time of one minute. The instrument is to enable the RAVE survey of high precision abundances and velocities for up to 50 million stars. The design is largely adapted from the AAO's FMOS-Echidna fibre positioner for Subaru. New design challenges for Ukidna include the enormous number of fibres, the large focal surface, and the field curvature of the Schmidt telescope. These features are mostly shared with the expected needs of future prime-focus multi-fibre systems on 8-30m class telescopes. We present details and performance of the multi-actuator design. • POSITIONER SPECIFICATIONS • 2250 fibres (c.f. ~400 for FMOS-Echidna) • Field diameter ~320mm (c.f. ~150mm for FMOS-Echidna) • Fibre core diameter 60µm • Spherical focal surface, ~3m radius • No X-Y positioning gantry for FPI • Spectral coverage 850nm to 875nm • Field configuration time in the order of one minute UKIDNA The proposed positioner configuration is very similar to that of FMOS-Echidna. Certain application differences, including the curvature of the focal surface, the increased size of the focal surface, the required increase in spine packing density, the requirement to minimise vignetting, and inability to accommodate an x-y positioning gantry for imaging the focal plane for spine position feedback, have necessitated design changes. Full advantage has been taken of AAO’s experience in the design and construction of FMOS-Echidna in order to refine design details to improve reliability and reduce cost. A modular arrangement similar to FMOS-Echidna is proposed, but larger, so that 25 modules each carry 90 actuators in two rows of 45 (2250 actuators total). Unlike FMOS-Echidna, the actuators are all mounted at different angles within a straight module, to match the field curvature. The spines are all different lengths to accommodate the difference between the straight line of the module and the spherical focal surface. Modules are nominally identical, and mounted cylindrically (equivalent to bending an FMOS-Echidna system around an axis parallel to its modules). Spine and actuator design has also been considerably refined from the FMOS-Echidna design. Note that this conceived arrangement requires significant evolution from the FMOS-Echidna construction and alignment process. Work is already underway to refine this process for the Gemini WFMOS project3, which incorporates an Echidna positioner with many features in common with Ukidna. POSITION FEEDBACK Closed-loop control of the spine positions is enabled by imaging the focal plane. As a natural evolution of the FMOS-style XY positioning gantry4, in Ukidna this position feedback is provided via a fixed camera capturing the whole focal plane in a single image. This focal plane imaging (FPI) arrangement is similar to the STRIP concept developed for the MOMFOS positioner conceived for the GSMT5. The telescope’s primary mirror incorporates a central aperture providing sufficient space to accommodate the proposed FPI camera. A 2K x 2K format implies that a centroiding accuracy of approximately 1/20 pixels should be adequate to yield 10 micron positioning, however there will be issues regarding the distortion mapping of the field. The FPI camera needs to be defocussed to spread tip images over a suitable area for such accurate centroiding (about four pixels FWHM). FIFTY MILLION STARS The RAVE (RAdial Velocity Experiment) survey is an ambitious plan to measure radial velocities and chemical compositions for up to 50 million stars by 2010 using novel instrumentation techniques on the UK Schmidt telescope and a northern counterpart1. The proposed survey is three orders of magnitude larger than stellar surveys attempted to date. The RAVE survey requires spectroscopic velocity and metallicity measurements of tens of millions of stars in our Galaxy. These stars are relatively bright (V<16) and densely distributed. The calcium triplet (849.8 to 866.2 nm) is selected for the measurements. The project lends itself well to a Schmidt telescope. Eventually, the survey will be extended to the northern hemisphere using a suitably-equipped northern instrument. Desired completion of the survey in a realistic time frame of only a few years poses a considerable technical challenge for the design of the survey instrumentation. In this paper we describe Ukidna, a cost-effective solution based on the innovative Echidna technology originally developed for the Subaru FMOS fibre positioner2. The RAVE survey is currently in progress using the 6dF facility on the Anglo-Australian Observatory’s 1.2-metre UK Schmidt telescope at Siding Spring. This instrument offers 150 fibres over a 6-degree field of view and is typically able to record 700 spectra in an average clear night of RAVE observing. Clearly, this facility cannot deliver the ultimate goal of tens of millions of spectra in a reasonable period. In order to achieve such a target, more than 20,000 stars per average clear night need to be observed. Development and implementation of the Echidna fibre positioner for the FMOS facility on the Subaru telescope offers the promise of radically improved fibre positioner capabilities well suited to surveys like RAVE. An enormous increase in the number of fibres is now possible, with reduced maintenance and operating costs. Our vision is of an Echidna fibre positioner on the AAO’s 1.2-metre UK Schmidt telescope at Siding Springs. Over two thousand fibres are positioned across the six degree field of this survey telescope to feed a spectrograph configured to record just the wavelength range of interest for all the fibres. A field reconfiguration time of one minute or even less permits spectra to be obtained of tens of thousands of targets each night. REFERENCES 1. M. Steinmetz, “RAVE: The RAdial Velocity Experiment”, GAIA Spectrocopy: Science and Technology, ASP Conference Proceedings 298, held 9-12 Sept 2002, Gressoney St. Jean, Aosta, Italy. Ed. Ulisse Munari. p.381, 2003 2. P. Gillingham et al., “Echidna – A multi-fiber positioner for the Subaru prime focus”, Optical and IR Telescope Instrumentation and Detectors, ed. M.Iye & A.Moorwood, Proc. SPIE 4008, 1395-1403, 2000 3. S.Barden et al., “KAOS: kilo-aperture optical spectrograph”, Instrumentation for Future Extremely Large Telescopes, Proc. SPIE 5492-143, 2004 4. J.Brzeski et al., “Echidna: the engineering challenges”, Instrumentation for Future Extremely Large Telescopes, Proc. SPIE 5492-65, 2004 5. A. Moore & A. McGrath “The MOMFOS fiber positioner”, Instrumentation for Future Extremely Large Telescopes, Proc. SPIE 5492-206, 2004