The current generation of astronomical instrumentation requires new levels of precision to achieve their scientific objectives. The method described by Bland-Hawthorn and co-authors is generally applicable for the calibration challenges presented by any wide-field spectrograph. Developed for and used on the HERMES (High Efficiency and Resolution Multi Element Spectrograph) at the Anglo-Australian Telescope (AAT), the method builds upon recent advances in fiber etalon combs and enables the simultaneous characterization of instrument dispersion, field distortion, and the detailed shape of the point spread function (PSF).

In this approach, an etalon, integrated in a single-mode fiber, illuminated by a supercontinuum light source, is used to realize a photonic comb, which is a light field characterized by a periodic spectrum made of narrowband lines (the “teeth” of the “comb”). The output of this calibration reference is subsequently fed into a multi-mode fiber, creating in this way skew rays, which are in turn fed into a beam expander to match the input of the spectrograph.

This provides a specific advantage in the calibration of the chromatic distortions across the field compared to conventional arc lamp-based calibrators. The resulting distortion mapping is both intuitive and allows reconstruction directly from the comb PSF locations via established computational methods. The reconstruction of the spectrum has been demonstrated with better than 0.5% residuals, with potential for further improvement. The use of photonic combs has great potential in advancing the precision of optical system calibration.

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