A hydrogen alpha filter is the part of a solar telescope that does the difficult work, and it is worth understanding separately because the same phrase is used for two very different products: a solar filter for looking at the sun, and a deep sky imaging filter for photographing nebulae at night.
The solar kind, and why it is expensive
A solar hydrogen alpha filter is an etalon: two precisely spaced reflective surfaces that let one narrow band through by interference. The band must be under a tenth of a nanometre wide to show prominences well, and it must be tuned, because the sun's rotation shifts the line slightly. That precision is the reason a 40mm solar telescope costs $995.00 while a 114mm night telescope costs $91.00 on sale.
The deep sky kind, which is a different product
An imaging hydrogen alpha filter passes a band several nanometres wide around the same wavelength and is used at night, in front of a camera, to photograph emission nebulae through light pollution. It is far cheaper than the solar kind and it is not safe for looking at the sun with. The two products share a name and nothing else, and buying the wrong one is a real risk for a beginner.
What you actually see through the solar kind
The disc turns deep red and the surface breaks up into a mottled texture. Dark filaments snake across it, bright plage surrounds sunspot groups, and at the limb prominences stand out as loops and arches that change shape over hours. It is the one solar view that people describe as looking alive.
Questions people ask about hydrogen alpha filter
What does a hydrogen alpha filter do?
It passes only a narrow band of red light at 656 nanometres. On the sun that reveals the chromosphere; at night it isolates emission nebulae for photography.
Can I add a hydrogen alpha filter to my telescope?
Solar hydrogen alpha systems that fit an existing telescope exist and cost more than most complete instruments. They are not a filter you screw into an eyepiece, and nothing that screws into an eyepiece is safe for solar use.
Is white light or hydrogen alpha better?
They show different layers. White light is cheap and shows sunspots; hydrogen alpha is expensive and shows prominences and chromospheric detail. Most people who own both use white light more often.