A Tunable Lyot Filter at Prime Focus: a Method for Tracing Supercluster Scales atz∼ 1

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Abstract
Tunable narrowband emission line surveys have begun to show the ease with which star-forming galaxies can be identified in restricted redshift intervals to z ~ 5 with a 4 m class telescope. These surveys have been carried out with imaging systems at the Cassegrain or Nasmyth focus, and are therefore restricted to fields smaller than 10'. We now show that tunable narrowband imaging is possible over a 30' field with a high-performance Lyot filter placed directly in front of a CCD mosaic at the prime focus. Our design is intended for the f/3.3 prime focus of the Anglo-Australian Telescope (AAT) 3.9 m, although similar devices can be envisaged for the Subaru 8 m (f/2), Palomar 5 m (f/3.4), Visible and Infrared Survey Telescope for Astronomy (VISTA) 4 m (f/6), Mayall 4 m (f/2.6), or the Canada-France-Hawaii Telescope (CFHT) 3.6 m (f/4). A modified Wynne doublet ensures subarcsecond performance over the field. In combination with the new Wide-Field Imager 8k × 8k mosaic (WFI) at the AAT, the overall throughput (35%) of the system to unpolarized light is expected to be comparable to the TAURUS Tunable Filter (TTF). Unlike the TTF, the field is fully monochromatic, and the instrumental profile has much better wing suppression. For targeted surveys of emission line sources at z ~ 1, a low-resolution (R ~ 150 at 550 nm) Lyot filter on a 4 m telescope is expected to be comparable or superior to current instruments on 8-10 m class telescopes. We demonstrate that the 30' field is well matched to superclusters at these redshifts, such that large-scale structure should be directly observable.
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