Another excellent resource is
CCDCalc. You do need to add your gear to the database.
Canon 40Da chip - 5.7um pixels and 3888x2592 array size.
Canon 85mm f1.8 USM - 47mm aperture and f1.8.
Those values give FOV 899.4x599.6 arcmin.
You didn't list an f ratio for the refractor. 770mm sounds like a 110mm f7 (eg William Optics Fluoro Star 110).
That gives FOV 98.8x65.9 arcmin without the flattener and 116.3x77.5 arcmin with the 0.85