but it doesn't elude to the quality of correction, where a ronchi can.
Even so, the Ronchi is less sensitive. Take k-e readings at the paraxial focus and the edge and you should have a good idea where you are and how much you have to go. The sagitta is D2/8R, where D is the mirror's clear aperture diameter and R its radius of curvature. The Foucault is based on LSA, which which at COC = D2/4R; the Ronchi is based on TSA, and TSA = sinU(LSA), where sinU is the sine of the light cone angle, numerically equal to 0.5/effective F#. Thus, for an f/3.3 mirror at COC, the effective F# = 6.6, hence TSA = (0.5/6.6)(LSA), or only 7.6% of LSA. At COC, In real numbers, for D = 254 mm and R = 1676.4 mm, LSA = 9.6 mm, and TSA = 0.73, so the Ronchi test is 13 times less sensitive.
In short, when testing a mirror at CO,C
(1) LSA = Dz2/4R
(2) TSA = Dz3/8R2
Where Dz = the effective zonal diameter of the mirror. Clearly, for full aperture Dz ≡ D.