[Merlin66 (Ken)]

I've got to say that the SM40 on my ED80 gives far clearer, crisper images of the proms and surface granulation that I ever get with the PST. The contrast is better and obviously I can use that bit higher magnification.
Use a x4 barlow with the DMK21 for best results. All this tells me that the filter has a very good narrow band responce, got to be in the region of 0.5A.

Edit:
Photos added showing the SM40 fitted to the ED80 scope and the BF5 filter