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For Ha observing of the Sun, you need an extremely stable, narrowband filter centred on Ha. A "normal" DSO Ha filter may have a bandwidth of say 10nm; the solar filter needs to be down BELOW 0.1nm.
To do this, a Fabry-Perot interference filter (etalon) is usually used in conjunction with a blocking filter.
The F-P etalon produces a series of bands about 0.1nm wide and about 0.8nm apart, these cover the whole spectrum. The particular band we want at Ha is "filtered" out by using a very narrowband Ha filter ( about 0.6nm) called the Blocking Filter ( ie it blocks out everything other than the required wavelength). So the combination of the etalon and the BF give us the resolution at Ha.
Solar filters and scopes are produced by Coronado (PST and Solarmax), Lunt, Daystar and Solarview. They start from around $800 for the introductory 40mm PST/ 60mm Lunt and quickly rise to the $$$$$$ for apertures up to and beyond 90mm.
Hope this helps.
Ken
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