  This paper describes a concept for the high-accuracy absolute calibration of
the instrumental polarization introduced by the primary mirror of a
large-aperture telescope. This procedure requires a small aperture with
polarization calibration optics (e.g., mounted on the dome) followed by a lens
that opens the beam to illuminate the entire surface of the mirror. The Jones
matrix corresponding to this calibration setup (with a diverging incident beam)
is related to that of the normal observing setup (with a collimated incident
beam) by an approximate correction term. Numerical models of parabolic on-axis
and off-axis mirrors with surface imperfections are used to explore its
accuracy.
