You could combine the image stack using the overlay mode, where lights lighten lighter areas and darks darken darker areas. But the problem with stacking severely underexposed images is simply the lack of information recorded/available.Thanks again. Despite the advice I've been getting I'm sure it is possible to do what you originally suggested, only to use some kind of additive method rather than the averaging method, the technique for which is widely available on the internet.
You could combine the image stack using the overlay mode, where lights lighten lighter areas and darks darken darker areas. But the problem with stacking severely underexposed images is simply the lack of information recorded/available.
I would suggest determining the slowest SS usable w/o blur of stars/land, and then use as high of an ISO as necessary to get the image bright. Take a bunch of these noisy images and align/stack. Then blend in "mean" which removes differences (the random ISO noise).
I don't do a lot of image stacks, but IME the automated processes always leave a little something desired. Once the automated processes are done I almost always need to manually mask some details back in/out.
Is this how Sony does it in camera using there smooth water app?You may not agree with what the chap is saying about needing filters but he explains the stacking alternative well.
This is the video jump to 2m40s to see the technique.
View: https://www.youtube.com/watch?v=YcZkCnPs45s