This may be a bit over the top for this list, but there are a lot of science-leaning types on the list so maybe this will be appreciated.
I have been interested in exploring the [NII] emission lines and recently purchased a filter from Custom Scientific for that purpose.
There are two singly-ionized Nitrogen forbidden lines, 658.3nm and 654.8nm. Located very nearby is our friend, Hydrogen Alpha at 656.3nm.
Most astronomical imaging Ha filters I have seen are too wide to separate the Ha from the two [NII] lines. The Schuler and Astronomik are examples. The Custom Scientific 4.5nm FWHM filters are another example. These filters should really be called Ha + [NII] filters since they really pass the Nitrogen lines along with the Hydrogen.
My Cust Sci filters are the narrower ones, the 3nm FWHM. As a result I can separate the Ha line from the two [NII] lines.
So now the question is why bother; is there anything that shows a difference when imaged through the two filters?
Here is a shot of the Crab Nebula taken with both Ha and the [NII] filter. The Ha data is assigned Red while [NII] is assigned to Green and Blue.
http://www.rdcrisp.darkhorizons.org/m1_crab_nebula_ha_n2_page.htm
As you can see there are really different shapes revealed in the light from the two filters despite the fact that the emission lines are centered only 2nm apart (20angstroms).