My Vixen DED108SS is a 4 element Petzval Nagler astrograph refractor. It is f/5 natively. I wonder how fast I could go? (120 sec exposures even cause blooming with the CCD currently at f/5)
Now you have me contemplating making it f/3 or so... but then I may sacrifice the field flatness
(perhaps my collimation I did recently is still a micron off)