A Narrow Passband Polarization Interference Filter for Hydrogen Alpha

Abstract

This paper discusses the construction and testing of a Lyot‐Öhman type polarization interference filter. This filter has a band width of approximately 1A peaked at H alpha. Previous filters have been fabricated, mainly of quartz. For a 1A passband filter a quartz thickness of approximately 24 cm would be needed for the thickest plate. This filter has been constructed using ammonium dihydrogen phosphate. Because the birefringence of this material is much higher than that of quartz, the tolerances associated with its construction are severe. By the use of mica corrector plates or optical shims, the tolerances can be made less critical. The final filter is equipped with a Senarmont compensator at each end so that the passband can be adjusted over a range of about 3A.