Well, I think I can finally say I have reached the realms of diminishing returns on this target. I have now collected 111 hours of usable data. I have rejected about 28 hours of data in total, making total collected of 138 hours. Slight guiding errors, cloud and the moon being too close have been the main reason for rejection.
In the end I stacked the following:
60 hours in Ha
39 hours in OIII
12 hours in RGB (4 hours in each)
Some things I noticed as time went by.
1. The extra hours in RGB really worked well to show the star colour and smooth the image. I used lighten mode for the addition of the RGB and in several stages. Of interest is the galaxies that have shown up in the background. Whilst there are not hundreds there are a few more than you might expect. I have spent quite some time looking and deciding what is a faint star and what is a distant galaxy. A perfect example of integration have an effect.
2. Dithering becomes really important at this duration and I am going to reconsider my current dithering settings for future images. I noticed some fixed pattern noise appear over time despite dithering of at least 6 pixels. The Ha data I stacked 120 frames and so the dither was deficient for that many frames. Some subs were obviously overlapping.
3. Signal strength climbed slowly the point where some elements of the Helix look as solid as the central core. The outer reaches in particular took a lot of time to get to a point of solid definition and I suspect it would be a quite a lot of hours more to gain further definition and that gain would not be worth the extra hours. I added 29 odd hours to the previous version of this image and it took all that time to make the outer reaches appear more solid and brighter.
Click here for larger resolution image.
I think this image looks better than the previous version. I have cropped this image to remove a very bright flare from a star on the edge of the field, but essentially the full res image is at 100%.