Measuring and maintaining the plate parallelism of Fabry-Perot etalons

Many dynamic phenomena on the Sun require observations with high temporal, spatial, and spectral resolution. Fabry-Perot - based imaging spectropolarimetry can serve these needs, especially in combination with adaptive optics and advanced image restoration and processing techniques. Therefore, a detailed understanding of the instrument characteristics is the foundation for exploiting the capabilities of these complex systems. We present a novel, efficient, and robust algorithm for maintaining and quantifying the plate parallelism of Fabry-Perot etalons. The plate characteristics are quantified in terms of Zernike polynomials, finding a direct relationship between the voltages applied to the piezoelectric actuators, which control the tip-tilt motion of the etalon, and the plate parallelism. The plate defects are conveniently described by the Zernike coefficients up to 15th order. The algorithm has been tested with the visible-light imaging magnetograph at the Big Bear Solar Observatory in California.