The Dark-frame subtraction will also be of higher quality if done with an (averaged) master dark-frame.
Right you are, Bart. Dark frame subtraction with a single dark frame will reduce the read noise component that is invariant frame to frame, but will
increase the read noise that changes from frame to frame by a factor of 1.414, since sqrt(n^2 + (-n)^2) = sqrt(2*n^2) = 1.414*n.
In some cameras,, like the D810, the frame to frame invariant read noise has almost all of the low-frequency (objectionable) content:
http://blog.kasson.com/?p=7936In other cameras, like the Sony a7S, that is unfortunately not true:
http://blog.kasson.com/?p=7971I don't know which camp the 645Z falls into.
Jim