If you get bored with H-a...

[riklaunim]

I've seen lately few images made by Richard Crisp - like M57 or M27. What's interesting is that they are using He-II emission band (H-a,O-III,He-II).

There are many emission lines that can be found in planetary nebulae like (from what I found):

He-II is at 468 nm

He-I is at 587 nm

Ne-III is at 386,9 nm

Ar-III is at 775,1 nm

Cl-III at 551,7 nm, Cl-IV at 752,9 and 804,5 nm

So narrowband imagers could need few extra filters but prices for 25 mm dia of scientific/industry 10 nm bandpass filters aren't low.