The Apollo spacecraft used a sextant for navigation between the Earth and Moon. Operation of the sextant involved sighting through a telescope; this filter was placed over one of the sextant's two scopes.
This object was flown on Apollo 9, and was transferred by NASA to the Smithsonian in 1975.
This object is on display in Human Spaceflight at the Steven F. Udvar-Hazy Center in Chantilly, VA.