Roger
You ask a good question. I also wonder why we don't see huge focal lengths being used for planetary imaging. For example I was surprised to learn Bird was going for a 16" f/4.
http://www.iceinspace.com.au/forum/s...ad.php?t=45124
I guess the physical dimensions required for long focal lengths start to be the limiting factor. Are there any 16" f/10 newts out there?
James