For information see the attached PSFs and related Strehl's ratios on axis and 0.3 degree off axis (@f/10) for a 6 lambdas
smooth symmetrical (best case) mirror flatness error set at 45 degrees:
f/10 - on axis - back working distance (BWD) = 110mm
f/10 - 0.3 degree of axis - back working distance = 110mm
f/5 - on axis - back working distance = 50mm (which may be challenging to reach)
If the mirror flatness is not a simple smooth deviation across its clear aperture, or does not exhibit the same error in x/y directions (likely) the induced optical aberrations could be worse (such as strong field dependent astigmatism and curvature).
http://www.iceinspace.com.au/forum/d...AAAElFTkSuQmCC