Kort,  the secret, imo,  is a sturdy tripod, and when time is on your side...is mirror lock up (most dslr's should have this facilty), and cable release (or use self timer!)..and I used spot metering with manual settings.

ps...the A3 print is ....wonderful :)

I sold the 2x extender as f8 is not good! (would have gave me 1000mm though!)
plus you loose a few more stops over the 1x4 which is what I used on the 500mm @ 700mm (f5.6) on this shot.