Calibration system for the 4MOST multi object fiber-fed spectrographs

A novel concept for the calibration of multi object fiber-fed spectrographs is described for the 4MOST instrument. The 4MOST facility is foreseen to start science operations in 2022 at the ESO VISTA telescope. The calibration system provides intensity, wavelength and resolution calibrations for the 4MOST spectrographs. The heart of the system is a combination of a bright broad band lamp and a Fabry-Perot etalon. The lamp is able to provide sufficient flux to illuminate the VISTA focal plane and the Fabry-Perot etalon provides a regular comb of spectral lines. The Fabry-Perot etalon can be moved in and out of the optical beam to choose between intensity and spectral calibrations. A fiber bundle of 156 fibers is guided to the VISTA spider arms where each fiber is connected to a small integrating sphere. The integrating spheres are attached to the bottom side of the four VISTA telescope spider struts and provide unvignetted illumination of the telescope. The exit port of the integrating spheres is projected on the VISTA focal plane with a small collimator lens. The integrating spheres assure a uniform illumination of the focal plane and are insensitive to FRD effects of the input fibers due to motion and stress during telescope movements. The calibration system illumination only originates from the telescope spiders and therefore the telescope pupil is not fully filled. The calibration system uses the azimuthal scrambling properties of the fibers that connect the telescope focal plane and the spectrometers to completely fill the spectrograph pupil.