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That's a nice result.
Why only 2 minute subs though? Also ideal ISO for a 60Da is probably more like ISO800. ISO1600 is probably only adding noise compared to ISO800 and losing dynamic range and only makes the same signal easier to see on the LCD screen. There is an ISO sweet spot for each camera. Going above that is simply amplifying noise as well as signal. The engineers tend to make optimizations for read noise at different ISO levels. Higher ISO levels damages Dynamic Range. Above about ISO800 Dynamic Range would be dropping significantly, not that this is a too high a dynamic range image (although the centre is quite bright). But you probably have lost 1 stop of dynamic range if not more by ISO1600 on a 60Da which has small pixels.
ISO is a bit of a funny thing because of course the signal is the signal received by the sensor and higher ISO is simply turning up the volume so to speak, noise along with it. It doesn't add signal, simply amplifies what was received. The only way for more signal is a faster lens or longer exposure time.
Would 5-10 minutes at ISO800 and boost in post processing be a better strategy? Do you expose for 1/3rd of a histogram wide for each shot?
3 x 40 x 2mins = 4hours which is a decent amount. Does it take a long time for Deep Sky Stacker to process that many subs?
Sorry for the long post, - its a nice image - really!
Greg.
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