In order to see features on the sun in the presence of its extreme glare, Ha scopes and the like select out a very narrow band of frequencies with an (expensive) filter called an etalon.
In order to do this the etalons have to be “tuned” by various methods. To use them visually you need a very good contrast, and sometimes 2 etalons are used – the so called “double stacking” to narrow the bandwidth (and double the expense!) and improve the contrast. This can show breathtaking visual views.
However this only improves the contrast – not the spatial resolution, I believe. Resolution is controlled by the scope aperture.
Now, when imaging, you can readily improve contrast by signal processing with “Curves” and the like, as long as your camera has sufficient dynamic range. Tuning is therefore a lot less critical, and “double stacking” probably unnecessary.
Have I got this right? Any comments?