1. Single Pass Front Mount Etalon System:-
50mm without central obstruction
0.75A (single) or 0.55A (double stack)
With blocking filter for up to 600mm focal length
The best thing is the price... just around a PST!!!
2. Single Pass 60mm with Internal Etalon System:-
60mm telescope with internal module (not just a filter)
Just a little bit more expensive than the above.
3. Single Pass 60mm Calcium K line Filter System
Similar to above, but for CaK (internal module)
Again, around the price of a PST!