Imaging performance of multi-etalon bidimensional spectrometers

Aims: In recent years, several new solar and nighttime panoramic spectrometers based on Fabry-Perot interferometers have been successfully developed. In this paper we evaluate the imaging performance of the two types of mountings that have been adopted, telecentric and classic, in particular trying to understand which one might be more suitable for future large-aperture solar telescopes. Methods: Numerical code was written to simulate the behavior of such spectrometers, on the basis of the theory of Fourier optics. This code was used to simulate different instrument configurations and was tested on previous results obtained either analytically or numerically by other authors. Results: Calculations of the system MTF and Strehl ratios show that both mountings may perform very close to theoretical expectations. However, gap irregularities in the interferometers may alter the optical quality of the monochromatic images. In the case of the classical mounting in a collimated beam, it is possible to partially compensate for the resulting errors in the wavefront emerging from the interferometers with a suitable phase plate. We also performed an observational test of the optical quality delivered by the IBIS interferometer installed at the Dunn Solar Telescope of the National Solar Observatory, with the results substantially confirming the calculations. Conclusions: It follows from our results that both mountings may be efficiently used for solar bidimensional spectroscopy. The final choice depends on the tradeoff between factors such as image quality, field of view, and acceptable wavelength shift.