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A mirror diagonal or the secondary mirror on a newt need to be more accurate. At a 45 degree angle of incidence and reflection, the wavefront error induced by the surface is doubled. So a 1/10 wave diagonal is 1/10 wave for 90 deg angle of incidence & will propagate the wavefront with a 1/5 wave error.
It is perfectly sensible to invest in a higher quality diagonal for possible future high quality optics, however a diagonal really only needs to be twice the precision of the primary optic it is used with to meet the Rayleigh diffraction limit conditions.
Joe
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