[glend (Glen)]

There is an element of trial and error in working this out for your location. Your scope f ratio is 7.5 so It's not particulary fast. But I think you have picked the right ISO, I use ISO1600 with my f8 RC08. Without going into all the jargon, pick a bright object like Eta Carinae and run some test subs at various times and check the histogram. I'd suggest between 180" - 240" to start with but if your location over powers that you might have to reduce it and shoot more shorter subs. You could try a CLS filter but you will need to increase your sub lengths to compensate. Have fun.