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Indeed, F/5 is a fast enough light cone. You've also got to think of the other aspects. FSQ operating at F/5 has a critical focus zone of 55 microns. This is quite a small window which is easily displaced as the ambient air drops, hence you need to refocus regularly if you want sharp looking data. Operating at F/3.6 drops the critical focus zone down to 28.5 microns. Here just the very slightest shift will result in softer data. Obviously with automated computer controlled focusing routines, its not a challenge, but expect to be refocusing every 20 to 30mins until the ambient air temperature has stabilised during the course of the night.
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