Due to peculiarities of imaging under an oblique angle of incidence, a minimum working distance is required since the objective should not touch the sample and the beam should not be chopped off by the objective.
Among other issues this is thoroughly addressed in the following publication: "Description of a single modular optical setup for ellipsometry, surface plasmons, waveguide modes, and their imaging techniques including Brewster angle microscopy"
H.Harke, R. Teppner, O.Schulz, H.Orendi, H. Motschmann, Rev.Sci.Instrum. 68(8) August 1997