Today's activity...
The last frame is an enlargement to show the solar limb with the SM60/ED80/PST Mod double stack.
One of the issues when using a single stack H alpha filter is leakage of the continuum into the Chromospheric image - due not to the the filter bandwidth but the "foot" at the base of the transmission curve, this can extend beyond the Ha and allow the Photospheric solar edge to be recorded. This shows in images as a double edge - a hard edge (the Photosphere) and an outer soft edge (the Chromosphere)
The use of a double stack system reduces the "foot" of the transmission curve and one measure of a "good" DS system is the ability to image the solar edge without picking up any Photosphere.
The SM60/PST mod DS is getting pretty close.
One of the best solar images I've seen which dramatically shows this effect is a animated GIF from Josef (SolarChat forum). Basically it's built up from images as the tuning of a DS filter is taken from the blue wing (continuum) through the Ha band to centre on Ha where it only shows the Chromosphere - hanging above the Photosphere. Amazing.
https://www.dropbox.com/s/im0ujj8fmz..._edge.gif?dl=0