You know my experience with that comet filter got me to thinking about some of the claims of it's use as a cyanogen filter. As you know, most times the filter does not work on comets but occassionally it seems to work on the odd comet, as it certainly does on this comet. But when I saw improvement using the filter on M27 it got my mind to thinking is this really correct, that it screens for cyanogen?
Supposedly this selects for the cyanogen emission line, but wait. Cyanogen ( N≡C−C≡N) emits in the 350 nm range, below human visual limits which cut off at 380nm. It would make no sense to select for this line of transmission in a filter. This is at the start of the ultraviolet range which is invisible to the eye. I think the idea of a "cyanogen" filter is a misnomer. Now the green or bluish color as noted by both Kent and myself may be more attributable to the "Swan Bands" of the spectrum for carbon up around 550nm, well within range of human vision.
If the comet filter did select close to the UV range (supposed cyanogen) that very marginally could explain some improvement in planetary nebula which is gas excited by UV radiation from the central star of such nebula. I think the effect though should be small to invisible due to the marginal nature of enhancement at the edge of visual range.
Looking at the filter's specs it also passes in the ionized oxygen range. It does not select for less than 500nm. That fits nicely with what I saw and is close to the swan bands of carbon.
The commonly held idea of a cyanogen source of the comets green tint or a filter that selects for this is wrong.
I may get several filters and use my diffraction grating to look at band pass of several types compared to the "comet filter".