  Large Fabry-Perot Interferometers are used in a variety of astronomical
instrumentation, including spectro-polarimeters for 4-meter class solar
telescopes. In this work we comprehensively characterize the cavity of a
prototype 150 mm Fabry-Perot interferometer, sporting a novel, fully symmetric
design. Of note, we define a new method to properly assess the gravity effects
on the interferometer's cavity when the system is used in either the vertical
or horizontal configuration, both typical of solar observations.
  We show that the symmetric design very effectively limits the combined
effects of pre-load and gravity forces to only a few nm over a 120 mm diameter
illuminated surface, with gravity contributing about 2 nm peak-to-valley (0.3
nm rms) in either configuration. We confirm a variation of the tilt between the
plates of the interferometer during the spectral scan, which can be mitigated
with appropriate corrections to the spacing commands. Finally, we show that the
dynamical response of the new system fully satisfies typical operational
scenarios. We conclude that large, fully symmetric Fabry-Perot interferometers
can be safely used within solar instrumentation in both, horizontal and
vertical position, with the latter better suited to limiting the overall volume
occupied by such an instrument.
