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1. Introduction

Narrow band filters used for astronomical imaging have relatively small acceptance angles. As the optical rays deviate from normal incidence (incidence angle tex2html_wrap_inline716), the wavelength tex2html_wrap_inline718 of the filter shifts. For Fabry-Perot filters and etalons the relative shift tex2html_wrap_inline720 equals tex2html_wrap_inline722 for small angles tex2html_wrap_inline724 and for a refractive index in the filter cavity of tex2html_wrap_inline726. To avoid broadening of the bandpath, filters are therefore mostly used in an optical configuration where the sky image is collimated through the filter. The drawback of that is that the wavelength varies across the field of view, an effect that has to be taken into account when analyzing the observations. This is, for example, the case for the TAURUS narrow band imaging interferometer in use at a number of observatories (see Taylor & Atherton 1980 and Atherton et al. 1982).

In order to avoid this variation of wavelength across the field-of-view, Bonaccini et al. (1989) and Cavallini (1997) use a Fabry-Perot etalon in the so-called telecentric mode in which the telescope objective is collimated through the filter. This has two side effects: (i) it broadens the filter bandpath (in the Cavallini instrument from tex2html_wrap_inline728 pm to tex2html_wrap_inline730 pm; 1 pm being 10 mÅ), and (ii) it affects the imaging quality of the telescope in case of diffraction limited imaging because of uneven illumination of the pupil when viewed from behind the etalon, an effect which varies with wavelength. The former effect was recognized by the authors, who in fact have used it to their advantage to broaden the filter bandpath to the desired 2 pm value. The latter effect has not been evaluated anywhere to my knowledge. It is of special interest in an era where the implementation of adaptive optics at many telescopes is sought in order to achieve diffraction limited images. Its assessment is the subject of this paper.


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