Our H-alpha filter was a "quartz birefringent monochrometer" -- it used a stack of calcite & quartz polarizing filters to get 2 or 3 angstrom bandwidth. Had to be temperature controlled and treated with white gloves. But oh what views of prominences!

ps - we used to call the entrance pupil on a spectroscope a "dekker" because you could choose different slit-widths. I remember being praised by a real astronomer (Helmut Abt) for knowing what this was...