We are finally getting a good spell of clear-ish moonless nights here in the UK and I've been imaging for the first time using my sulphur (SII) and oxygen (OIII) narrowband filters as well as hydrogen (Ha) - all 3nm passband. While I now have 200 minutes of Ha, and 450 minutes of SII I only have 175 minutes of OIII so far. Hopefully I'll get more OIII subs tonight but I think it will take a couple of nights to get to a similar depth as the SII. Whether that will be enough remains to be seen but with my chosen target, NGC 7822, being so faint in both sulphur and oxygen I thought I'd run a test to see if there is actually different structure visible as opposed to just differing brightness. Here's the result:
You can easily see how poor the OIII signal is with just those 7 x 25 minute subs as well as how visible the circular banding (RBI flood induced, for the technically minded) is but that will diminish as I capture more subs and as a result don't have to stretch the image so drastically just to show anything at all.
But the bottom line is that, looking past overall brightness differences, there is different detail present and not just a locally homogenous mixture of gas, with the biggest differences between Ha and SII at the top right and between Ha and OIII in the brighter area towards the bottom left. So the time I'm spending on this object will be worthwhile, something it's nice to be able to confirm as this is my first foray into narrowband other than Ha. If I also have a go with my nitrogen (NII) filter -- it'd be rude not to -- then I have another three nights of work to do just capturing the narrowband data and then there'll be yet another night for the red, green and blue subs to capture some star colour. Oh well, nobody ever said it would be easy... Update:
I added another 8 x 25 minute sub-exposures to my OIII tally last night and the result looks much better but the OIII intensity is definitely some way below the SII.