The design of an apochromatic optical spectrograph for astronomical purposes

An optical spectrograph for use in the OGS (Optical Ground Station), a 1 m telescope in Izaña Observatory (Tenerife, Spain) and the GTC telescope at Islas Canarias observatory has been designed, built and tested. The dispersive part was designed and built at the Centro de Investigaciones en Óptica, A.C. Due to requirements from Instituto Astrofísico de Canarias this Spectrograph has an f/5 collimator and camera with a plate factor of 6.55 nm/mm. Since the resolution of the system has to have a high resolution of the order of 15 μm or less, an apochromatic design was selected. This article describes this design and its main characteristics.

Research highlights► The required design characteristics made almost impossible the use of reflective optics. ► The required high resolution makes an almost perfectly flat spectral image necessary. ► Since the spectrum contains a wide range of wavelengths, the system had to be apochromatic. ► The resolution of the system has to be of the order of 15 μm or less. ► Commercial apochromatic lenses cannot work, since the system is symmetric with the pupil at the diffraction grating plane.