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Peter,
Each filter works by blocking certain wavelengths and transmitting others.
The Ha filter (DSO imaging) is designed to block all wavelengths other than those centred on the Ha wavelength.
If you look at the transmission curve of each filter the transmission width is usually given as the FWHM (Full Width Half Max) in Angstroms or nm. (1 nm =10A)
The narrower the FWHM the less background light (around the target wavelength) from the surrounding area is allowed through - highlighting the selected emission band (ie Ha).
Similar for the other DSO filters - Sii, OIII, H beta etc.
Hope this helps.
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