Making a measurement of which path a photon took, is forcing the photon to have taken one path. If you don't make the measurement (read, if you don't interact with the photon somewhere along some path it could take), you can get interference between the paths, despite it being a single photon.
Yes I understand that: the single photon interference in the double slit experiment. But what two paths could a photon take in a telescope allowing it to interfere with itself? The mirrors are placed so far apart a photon could never bounce off more than one mirror?
Forget about photons altogether and think of light as a wave and you will understand that yes of course, light bounces on all mirrors and interferes across mirrors. Who cares about the distance between mirrors really, light could (and does) interfere with mirrors miles apart.