Comments on the optimization of high resolution Fabry-Pérot filtergraphs

Institute for Solar Physics of the Royal Swedish Academy of Sciences, AlbaNova University Center, 106 91 Stockholm, Sweden e-mail: scharmer@astro.su.se

Received: 3 March 2005
Accepted: 23 June 2005

Abstract

In this paper we present results of simulations of Fabry-Pérot (FPI) system performance that address two entirely separate aspects. The first concerns the image quality of FPIs used in telecentric optical setups. We show that the image degradation from phase errors found by von der Lühe & Kentischer (2000, A&AS, 146, 499) can largely be compensated by refocusing and that the major cause of image degradation is the pupil apodization discussed by Beckers (1998, A&AS, 120, 191).
We also discuss the optimization of FPI systems, considering the effects of cavity errors on the homogeneity of the instrument function (transmission profile) across the FOV when two FPIs are combined in a telecentric reimaging system. Our conclusions from several numerical experiments are that telecentric FPI systems that use two or more FPIs with high (94–95%) reflectivities are likely to show large variations in the instrument profile across the FOV unless the cavity errors are significantly smaller than 2.0 nm. We show, that such homogeneity can be obtained at relatively low “cost” in terms of increased parasitic light levels by lowering the reflectivity of the FPI with the smallest cavity separation rather then lowering the reflectivity of both FPIs. We also demonstrate that the choice of cavity ratios has a strong impact on the homogeneity of the spectral transmission profile across the FOV and that double FPI systems with cavity ratios around 0.3 perform much better than at around 0.6.