Narrow band filters used for astronomical imaging have relatively small
acceptance angles. As the optical rays deviate from normal incidence
(incidence angle ),
the wavelength
of the filter
shifts. For Fabry-Perot filters and etalons the relative shift
equals
for small angles
and for a refractive index in the filter cavity of
.
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 pm to
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.