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Tony is right, you only need the band pass filter to double stack the PST. I haven't seen them sold separately, the BP and BF filters are usually sold as a matching set either in BF5, BF10 or BF15.
I have attached a couple of pics of my set up on the Vixen Porta mount. If the seeing is really good I will unscrew the BP filter off the Solarmax40 and double stack it on the PST. The double stacked arrangement does result in a loss of image brightness which is not bad, the contrast is better for surface detail. Prominences lose out with this arrangement though and I have found the PST shows them much clearer. My Solarmax40 has the BF10 filter which is more suited to imaging. Having both scopes side by side is great for viewing with different magnifications. The T-Max filter on the Solarmax40 is better for viewing varying detail than the PST as it has a large range of adjustment.
Something else which is not mentioned is the fact that with double stacking, the T-Max tuner and PST tuner have be "tuned" together. This results in fiddling around with both of them until a sharp and clear image is formed. There appears to be some internal reflection with this arrangement resulting in a red shadow like image of the sun in the field of viewing. You can "tune" this out to a degree by having the shadow off to one side of the eyepiece field.
As a matter of interest, on the Solarmax40, the 40mm lens housing is integral with the BP and T-Max filter and has to be unscrewed each time you want to double stack. You have to be careful in doing this, particularly if you're in a hurry. I always take the two apart over a thick towel just in case and store the 40mm lens assembly in a safe place.
To answer the question, is double stacking worth it? I think it is a pretty expensive way to see just a little bit more surface contrast. The PST to me is really quite adequate and very satisfying particularly for the price. The Solarmax40 is really quite expensive compared to the PST and in actual fact does not perform as well in some respects, particularly viewing prominences.
Cheers, Steve
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