I am trying to figure out Ku-band antenna consisting a prime focus parabolic reflector  sub-reflector-->circular outer corrugated waveguide  omt. It is backfire feed so that feed assembly ( OMT) is located behind of reflector. I am having a hard time to understand what is function of outer corrugation of waveguide. The corrugation looks like rings.

Does anyone has any clue of the function of outer corrugation of waveguide ?

Hi
It is called a choke feed arrangement, and is widely used in sat TV antennas.
The purpose is to equalize the E and H plane beamwidths of the exciting circular waveguide. Without the choke the beamwidths are different for the two vectors, leading to a poor cross-polar discrimmination. The OMT selects horizontal polarization in one port and vertical in the other, but if the discrimmination is poor due to the fundamental properties of the feed aperture, then there will be ghost carriers of H pol sources in the V pol port and vice versa.
The corrugations act as chokes that restrict the pasage of unwanted currents across the aperture, and slightly increase the aperture diameter for the broader lobe (E vs H) so it is narrowed and more closely equals the narrower plane width. I know that is a rather vague explanation, but without a lot of math, what you see is what you get