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The PST is a great staring point; as you say can be easily double stacked with a SM40 to give some very good images. They can also be safely "modded" to apertures up to 90mm.
I read all the other forums re double stacking SM60 filters and concluded, based on almost 50 years of building telescopes and lately spectroscopes, that if each SM60 gave a good FWHM of 0.7A in the Ha (or there about )that the probability of being able to match the "bell curves" ( whether symetrical or not) was just a matter of patience.
I've already had a look at the resonance bands ( in an SM40 etalon) using the spectroscope so I had a fair idea of what might happen. Part of the trick is to adjust the front element FIRST ( to get rid of the multiple reflections) this, by default, just moves the bandwidth off the centre of the Ha emission; the second filter is then "tuned" to give a fainter, but higher contrast image. You can see the effect very easily; when the bandwidths are almost sitting on top of each other the surface brighness is maximum and you know you have to move just that bit "off beam" to get the improved 0.4A bandwidth.
When I can get enough sunlight to test in, I'll take a series of spectroscope shots as I "tune" the etalons and see if I can measure the best ( and actual) double stack bandwidth.......
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