The 4-metre Multi-Object Spectroscopic Telescope (4MOST) instrument uses 2436 individually positioned optical fibres to couple the light of targets into its spectrographs. The metrology system determines the position of the back-illuminated fibres on the focal surface of the telescope. It consists of 4 identical cameras that are mounted on the spider vanes of the secondary mirror of the VISTA telescope and look through the entire optical train, including M1, M2 and the WFC/ADC unit. Here, we present an exhaustive study on the expected centroiding errors, including but not limited to lens fabrication errors, seeing, mirror distortions and parallax effects.
The 4-meter Multi-Object Spectroscopic Telescope (4MOST) is a wide-field, high-multiplex spectroscopic survey facility under development for the Visible and Infrared Survey Telescope for Astronomy (VISTA) 4 meter telescope of the European Southern Observatory (ESO) at Cerro Paranal. The objective of 4MOST is to enable the simultaneous spectroscopy of a significant number of targets within a 2.5° diameter field of view, to allow high-efficiency all-sky spectroscopic surveys. A wide field corrector (WFC) is needed to couple targets across the 2.5° field diameter with the exit pupil concentric with the spherical focal surface where ~2400 fibres are configured by a fibre positioner (AESOP). For optimal fibre optic coupling and active optics wavefront sensing the WFC will correct optical aberrations of the primary (M1) and secondary (M2) VISTA optics across the full field of view and provide a well-defined and stable focal surface to which the acquisition/guiding sensors, wavefront sensors, and fibre positioner are interfaced. It will also compensate for the effects of atmospheric dispersion, allowing good chromatic coupling of stellar images with the fibre apertures over a wide range of telescope zenith angles (ZD). The fibres feed three spectrographs; two thirds of the fibres will feed two low resolution spectrographs and the remaining 812 fibres will feed a high-resolution spectrograph. The three spectrographs are fixed-configuration with three channels each. We present the 4MOST optical system together with optical simulation of subsystems.
The 4-meter Multi-Object Spectroscopic Telescope (4MOST) is a wide-field, high-multiplex spectroscopic survey facility under development for the Visible and Infrared Survey Telescope for Astronomy (VISTA) of the European Southern Observatory (ESO). The primary and secondary mirrors (M1 and M2) together with the Wide Field Corrector (WFC) system provide a pupil-centric and aberration corrected focal surface. The WFC is also an integral part of the metrology system. At the focal surface, we meet two wave front sensing (WFS) systems, a deployable camera at commissioning, an acquisition and guiding (A and G) unit and a secondary guiding unit. This paper provides an overview of design details and Manufacture, Assembly, Integration and Verification (MAIV) processes for the 4MOST WFC system.