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

Tunable narrow-band, emission-line surveys have begun to show the ease withwhich star forming galaxies can be identified in restricted redshift intervalsto z ~ 5 with a 4m class telescope. These surveys have been carried out withimaging systems at the Cassegrain or Nasmyth focus and are therefore restrictedto fields smaller than 10 arcmin. We now show that tunable narrowband imagingis possible over a 30 arcmin field with a high-performance Lyot filter placeddirectly in front of a CCD mosaic at the prime focus. Our design is intendedfor the f/3.3 prime focus of the AAT 3.9m, although similar devices can beenvisaged for the Subaru 8m (f/2), Palomar 5m (f/3.4), VISTA 4m (f/6), Mayall4m (f/2.6) or CFHT 3.6m (f/4). A modified Wynne doublet ensures sub-arcsecondperformance over the field. In combination with the new Wide-Field Imaging 8K x8K mosaic (WFI) at the AAT, the overall throughput (35%) of the system tounpolarised light is expected to be comparable to the TAURUS Tunable Filter(TTF). Unlike the TTF, the field is fully monochromatic and the instrumentalprofile has much better wing suppression. For targetted surveys ofemission-line sources at z ~ 1, a low-resolution (R ~ 150 at 550nm) Lyot filteron a 4m telescope is expected to be comparable or superior to currentinstruments on 8-10m class telescopes. We demonstrate that the 30 arcmin fieldis well matched to superclusters at these redshifts such that large-scalestructure should be directly observable.Comment: Astrophysical Journal, accepted. 53 pages, 16 figures, aaste