How to select diagonal subplots of fig1 and put them into a new fig subplot

I have a figure with 17*17 subplots. How can I select the diagonal subplots of this figure and put them into a new figure with say 5*4 subplots.

[Walter Roberson]

It would be much easier to do the necessary work at the time the subplots were being created. Figuring out which axes corresponds to which subplot is certainly possible but a bit of a nuisance.