Double spot formation due to misalignments in the orthogonal angle between the two mirror surfaces. (a) and (c) were simulated at χ = 90.011° and χ = 90.115°, respectively, via pathway S1S2 [inset in (a)]. Same simulations repeated for (b) and (d) via pathway S2S1 [inset in (b)]. Real images of the double spot are shown (f) before and (e) after improving the orthogonality between the mirror surfaces. The entrance aperture was 0.1 mm × 0.1 mm in (e) while it was 0.3 mm × 0.3 mm in (f).