Novel instrumental effects in surface fluctuation spectroscopy: lens aberrations

The performance of a heterodyne spectrometer used for quasielastic light scattering studies of the fluctuations of liquid surfaces was investigated. Previously unreported instrumental effects were found, which appear to arise from the optical arrangement. It is shown that these effects are due to one particular lens aberration, coma. It is further shown that the spectrometer remains usable even in the presence of such aberrations, provided that care is taken in its calibration.