  Long-storage-time Fabry-Perot cavities are a core component of many precision
measurement experiments. Optical loss in such cavities is a critical parameter
in determining their performance; however, it is very difficult to determine a
priori from independent characterisation of the individual cavity mirrors.
Here, we summarise three techniques for directly measuring this loss in situ
and apply them to a high-finesse, near-concentric, 2 m system. Through small
modifications of the cavity's length, we explore optical loss as a function of
beam spot size over the 1-3 mm range. In this regime we find that optical loss
is relatively constant at around 10 ppm per mirror and shows greater dependence
on the positions of the beam spots on the cavity optics than on their size.
These results have immediate consequences for the application of squeezed light
to advanced gravitational-wave interferometers.
