  We present the design of the Fabry-Perot system of the Robert Stobie
Spectrograph on the 10-meter class Southern African Large Telescope and its
characterization as measured in the laboratory. This system provides
spectroscopic imaging at any desired wavelength spanning a bandpass 430 - 860
nm, at four different spectral resolving powers ranging from 300 to 9000. Our
laboratory tests revealed a wavelength dependence of the etalon gap and
parallelism with a maximum variation between 600 - 720 nm that arises because
of the complex structure of the broadband multi-layer dielectric coatings. We
also report an unanticipated optical effect of this multi-layer coating
structure that produces a significant, and wavelength dependent, change in the
apparent shape of the etalon plates. This change is caused by two effects: the
physical non-uniformities or thickness variations in the coating layers, and
the wavelength dependence of the phase change upon refection that can amplify
these non-uniformities. We discuss the impact of these coating effects on the
resolving power, finesse, and throughput of the system. This Fabry-Perot system
will provide a powerful tool for imaging spectroscopy on one of the world's
largest telescopes.
