(a) Meridional slope errors along the surfaces S1 and S2 of the Montel mirror as indicated by the red arrows in the upper inset in Fig. 2. Measurements were performed on both right (S2R) and left edges (S2L) of S2. (b) Sagittal slope errors measurements along the directions indicated by the yellow arrows (x′ = 0, ±30 mm) in the upper inset in Fig. 2 for both S1 (red curves) and S2 (blue curves) surfaces. Data were recorded with a step size of 0.5 mm.