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Paul,
Double stacking when applied to Ha observing means that two of the etalon filters are "stacked" in the optical path.
The amount of solar surface detail perceived, is a combination of the resolution of the scope (or the seeing conditions) and the narrowness of the bandwidth being used.
Most Ha etalon filters are around 0.7A (0.07nm) bandwidth. To improve on this, when double stacked they can produce a (0.7 x 0.7 =) 0.49A bandwidth. This shows more detail in and around active areas, filaments etc.
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