Help me keep my wife happy

[earth titan]

Strangely my wife won't let me buy anothr scope, cos I already have one (and bins etc. etc.) but she has encouraged me to build one..... Despite the fact it will cost about the same if not more.

I'm about to order mirror blanks, grinding tools etc and I have a sketch in my mind but I wonder if someone can sanity check my thoughts before I start getting materials together.

I want to build a scope for planetery work (theory beeing this will be my first mirror so a longer focal length will be more tolerant). Blanks are little difference in price between 6" and 8" so I think it will be 8".

Question is, what focal ratio? I'm considering f7 and making it a dob. There don't seem to be many of this size available commercially, so is there good reason why? Is it just the focal length and thus length of tube which makes them rare? I estimate it will need to be around 1.5m long.