Hydrogen alpha (H-alpha) is the name given to a particular emission line of hydrogen centred on the wavelength of 656.3nm.
Since much of our Universe is composed of hydrogen, tuning into this light allows us to concentrate on what is going on in the sky without distraction from other wavelengths.
The Sun is a bright emitter of H-alpha radiation and the Moon reflects a fair amount of that back to Earth.
There are different types of H-alpha filter.
Those used to view the drama that unfolds within the solar chromosphere have a very small passband (the range of wavelengths allowed to pass through), typically one angstrom (10-10m) or less.
Atik Narrowband Filter Set
The filters that can be used to image deep-sky objects and the Moon are called narrowband H-alpha filters and allow a wider set of wavelengths to pass through than their dedicated solar cousins.
Under no circumstances should you attempt to view the Sun with these filters.
Using a narrowband H-alpha filter for imaging the Moon will tend to increase contrast.
In addition, as the image is significantly dimmed, this can be an effective way to bring an overly bright Moon within the range of a sensitive CCD camera.
An interesting effect of narrowband lunar imaging is that certain features are more intense than when viewed under white light.
Specifically, there is a marked difference in Maria Crisium, Foecunditatis and Serenitatis with a narrowband H-alpha filter.
