What is a Nitrogen II filter good for?

[MikeBJ]

What can an [NII] filter tell me about the astrophysics of an object that I can't learn with H-alpha, [OIII], and [SII] ? So far, the few objects I've photographed through my 3 NM [NII] filter look exactly the same as they do in H-alpha.

To try to make the pitch of my question more concrete, my very limited understanding of my own images is that [OIII] will be strong where there is sufficient energy to produce the relevant excited state, say near the white dwarf of most planetary nebulae, or near a type O star, or perhaps at a very energetic shock front. Conversely, [SII] might be strong in relatively tenuous gas near a supernova remnant (eg Pencil nebula seems a good example), but very weak around the outer ejecta of the non-SNR Norma bipolar nebula, even though the distant outer shell looks rather similar to a SNR.

Can you suggest any objects (star-forming regions, SNR's, WR nebulae, PN's, etc) where an [NII] filter might be especially informative? (I'm typically using 1 hour subs with a 16803 chip on a 20 inch scope).

Cheers,
Mike

[geckzilla]

The one that immediately comes to mind for me is NGC 6369. I think the difference between [NII] and H-alpha for it is quite dramatic. NGC 2818 isn't bad, either. M1-42 and M1-59 have a similar thing going on but they are way too small. There must be some larger targets that are like this but I am only familiar with the ones I've dug out of the Hubble Legacy Archive.

[Chris Peterson]

To try to make the pitch of my question more concrete, my very limited understanding of my own images is that [OIII] will be strong where there is sufficient energy to produce the relevant excited state, say near the white dwarf of most planetary nebulae, or near a type O star, or perhaps at a very energetic shock front. Conversely, [SII] might be strong in relatively tenuous gas near a supernova remnant (eg Pencil nebula seems a good example), but very weak around the outer ejecta of the non-SNR Norma bipolar nebula, even though the distant outer shell looks rather similar to a SNR.

I think that's too narrow. A particular emission band will be observed under two conditions: the proper level of stimulation, as you point out, but also the presence of the emissive species! Doesn't matter how much energy you have, if you don't have any oxygen in a region, you aren't going to get any signal through your [OIII] filter. Hopefully that's obvious, but is isn't stated. So these narrowband filters can teach us about what elements are present, their distribution, and about surrounding energy sources (which are often hidden from direct view).

Have you tried constructing an image made through Ha and [NII] filters by mapping one to red and one to blue? That should serve to highlight differences that might not otherwise be apparent. It's also worth keeping in mind that [NII] and Ha filters (assuming the most common bandpasses) are only about 2 nm apart, and the narrowest amateur filters are typically 3 nm wide. So you're going to get quite a bit of crosstalk, which is going to muddy the differences.

We frequently see [NII] data collected by the HST with the WFPC3. The filter in that case is 2.8 nm wide, but is used in conjunction with an Ha filter only 1.8 nm wide. Many of the HST Ha images, however, are made with a wider filter that passes both Ha and [NII].

[MikeBJ]

Thanks Chris and geckzilla. I'll have a look at both beasts.
Mike