Thanks all for the comments.
Here is an animated gif made with 240s exposures same conditions as the 60s.
Again 1, 4, 9, 16, 25, 36, 49, 64 and 81 frames stacked. 1.5MB
http://d1355990.i49.quadrahosting.co...rtest240s_.gif
It is obvious that after about 25 frames there is not much improvement.
This is an animated gif of a stack of 64 frames at 60s and stack of 16 frames at 240s. 5MB
http://d1355990.i49.quadrahosting.co...9/timecomp.gif
This shows that in this universe there is no free lunch. Nothing beats more photons.
This tells me for this configuration at least 60 minutes of data should be collected whatever the exposure is, to attain a reasonable signal to noise ratio.
Hope this little experiment shows to all of us that the theory is true.
Bert