Next Contents Previous


Few telescopes offer more than a wide-field imager at prime focus simply because it is very difficult to exploit the fast beam spectroscopically. Spectral passbands are degraded in converging beams which is unfortunate as the widest fields are achieved at the fastest f/ratios. The wide-field expanded Lyot filter is (almost) the last word in exploiting the widest possible field with a given telescope. Remarkably, beams as fast as f/2 can be compensated with crossed birefringent elements, in concert with half-wave plates, such that even a constant sub-Angstrom bandpass is possible over a degree-sized field. This opens up many new astronomical programs, e.g., Lyot filters can be vastly more efficient for redshift-targetted surveys of galaxies (e.g., high redshift clusters) compared with existing multi-aperture spectrographs. To our knowledge, a wide-angle Solc filter using half-wave plates has not been attempted although it is entirely feasible.