I've just tried a different processing sequence which has produced tighter stars and sharper detail - drizzle. So instead of a normal stack I've used drizzle stack (within IRIS of course!) to create a drizzled image of twice the size and then used 2x2 binning to reduce it back. Note that I deliberately used dithering when shooting the subs in order to allow this later drizzle processing. See what you think:
Original stack:
(http://www.markshelley.co.uk/webdisk/rosettenodrizzlecrop.jpg)
Drizzled stack:
(http://www.markshelley.co.uk/webdisk/rosettedrizzlecrop.jpg)
And here are the full-size versions:
Original: http://www.markshelley.co.uk/webdisk/tak_rosette14032010.jpg (http://www.markshelley.co.uk/webdisk/tak_rosette14032010.jpg)
Drizzled: http://www.markshelley.co.uk/webdisk/rosettedrizzle.jpg (http://www.markshelley.co.uk/webdisk/rosettedrizzle.jpg)
Clearly I need much more data to reduce the noise. But don't forget this has been taken with a DSLR - so only one quarter of the pixels are responsive to the H-alpha.
I think drizzling may become my processing sequence of choice. In fact, I now want to revist some of my earlier images!
Mark
I prefer the first one, seems better contrast
Have to say mark my vote is with first as well. The effect of doing the drizzle looks almost the same as doing an unsharp mask on stars only - it has made them sharper & much brighter (which has bought out a lot more stars in the background) but perhaps too bright they almost looked clipped. The first for me looks more natural...
John
Hmmm... The second one does seem to show more detail in the nebulosity, but the stars in it have a slightly bloated look when compared with the first image...
I'll give this another go and make sure that I apply exactly the same steps to each image (apart from the drizzling of course) - it will then be a fairer test.
I don't think I applied the same arcsine range scaling on the drizzled image - which is why the stars look brighter and clipped.
Mark
QuoteHmmm... The second one does seem to show more detail in the nebulosity, but the stars in it have a slightly bloated look when compared with the first image...
Yes this was my impression. I prefer some elements of the 2nd one, and some from the first!!
Carole
I agree with Rick but even so it's not bad for a DSLR ;)
If you don't mind I'd like to try it with my FLI camera and Ha filter wheel sometime, I have the canon adaptor so it should just be a matter of fitting it to the bayonet on the camera adaptor (and hoping for no flex !).
I prefer the first one too Mark, second one does look like a slightly over sharpened image.