What Limits the Distance of Reflections in Parallel Mirrors?

[Loren Booda]

Two state-of-the-art mirrors are mounted parallel and facing each other. What primarily prevents their reflections from traveling an infinite distance - less than perfect reflectivity, less than perfect flatness, the image of the observer or interference by intervening air?

 

[f95toli]

Impossible to tell.
What you are basically asking it what is limiting the Q value (or if you want, the finesse) of an optical cavity and there is no generic answer to that; it is a combination of many different factors.

Alhough I would expect a high-finesse cavity to be operated in a good vacuum, so I don't think the last reason you mention is actually relevant.