This may help
http://www.eclipsechaser.com/eclink/...smt.htm#expose,
and also this
http://www.astrosurf.com/luxorion/report-formulae.htm
I'am to much of a newbie to give a right answere, but logic says you should go for a higher f ratio as its for planets and speed isn't needed as much and the mirrors finish will be better. I would email or phone them and just ask, they may even do a custom to suite your need.