why is it OIII and SII anyway ?

[RikM]

Rather than an Hb, you can just do Ha:red OIII:green 85%OIII+15%Ha:blue. That gives a pretty representative visual colour.

[DaveS]

Thanks Rik, I might give it a go next time I can see anything in the sky.

[hjw]

I've got a question, which may show me up as a noob in astrophotography, wouldn't it make more sense to filter out what you don't want (streetlight spectrum, common bulb light etc. which tends to be yellow orange) and let everything else through (major and minor emission lines from the elements we are after. I think broadband light pollution filters work on this level. Limiting you light capture to one wavelength only seems like a terrible waste of photons to me. However, I guess there is a good reason for it since anybody showing "nice" pictures is doing it.

[RikM]

That's exactly what you want (and more or less how you go about it) with RGB or one shot colour imaging of broad band targets like galaxies or star clusters. Emission nebulae tend to emit light only in descrete bands of wavelength, so by singling out just those emissions, we can filter out not only light pollution from streetlights but airglow and even starlight to a degree. We can use longer exposures without swamping the faint signal from the nebulae so better signal to noise ratio for nicer sharper images with more contrast.

[DaveS]

Also I'll add, that by isolating specific wavelengths we can reveal different aspects of the nebula's structure.

[glowingturnip]

I've been playing round with some Hubble data, and it's quite interesting.

Here's the Catseye nebula in the regular Hubble HSO Palette, SII=Red, Ha=Green, OIII=Blue:

and then here it is again with HNO Palette, ie using NII=Red instead of SII

Surprising how much difference there is, and considering that the Ha and NII are only 2 nm apart, they'd present as plain red in a 'natural' palette.

Now i need to think of a Palette to use all 9 filters of data Hubble has, including UV and IR...

[glowingturnip]

interesting how the central star is much redder with the Sii=Red palette above, than with Nii=Red.  Guess it shows how the star was burning Oxygen into Sulphur in its death throes