Richard, in regard to the spectrum response, Ha is down there are well and I have plenty of Ha, Sii is not much further along. However, when I look at the spectrum chart on the ZWO website I can see that it is less responsive in that part.
https://astronomy-imaging-camera.com...si1600mm-cool/
But since Ha is so good, unless the cover glass is a cut filter that is affecting Sii spectrum, then I am sure it is just something stupid I have done along the way. I have just been playing around stretching a single Sii sub and it has more signal than my stack of Sii appeared to have. Even on the SGP display I could see signal on individual Sii frames. I need to go back through what I did in DSS with that stack.
Uodate: looking back at my Trifud SII data it was pretty skinny as well when shot on equivalent integration times to Ha and Oiii. I will test tonight on M16 again running just Sii to build up more signal. Is there a 'standard ratio'?