One should try to wash it from the outside and check with a mirror
in retro-reflection the flux before and after.
According to the "Infrared Materials Handbook" KRS5
can be disolved in alcohol, whereas not so much in distilled water.
However, the window has survived some washings in alcohol so far.
This may be due to the AR-coating, and I do not know if the coating
would be attacked by destilled water. So in that sense I would suggest
to wipe it gain with alcohol, but only shortly and carefully
When you try to clean the entrance window without unmounting, please
take note that there is some vacuum grease on the edges of the window:
avoid to distribute it over the window when cleaning with alcohol