12" SCT @ F/5 has a CFZ of 55microns.
12" SCT @ F/6.3 has a CFZ of 87microns.
Of course as you change your F ratio, your focal length and in turn your FOV also changes.
This is the DSI gain reference I mentioned - I found it again!
http://www.hamiltonastronomy.com/ART...o_Gain_DSI.pdf