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The etaon is the primary filter near the front or middle of the scope which lets through a narrow band of light (less than an angstrom), but does so at many points across the spectrum. The blocking filter at the rear of the scope filters out all but the single peak needed for Ha observations.
Most Ha scopes/filters are tuned by mechanically tilting the etalon to bring it 'on band', while Lunt also produces etalons which are tuned by varying air pressure in a chamber surrounding the etalon. Changing the air pressure allows the etalon to be kept in the ideal perpendicular position relative to incoming light.
Double stacking refers to putting a second etalon in front of the first one, reducing the passband from about 0.7 to 0.5 angstroms and increasing the amount of detail seen on the solar disc.
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