4MOST, the 4m Multi Object Spectroscopic Telescope, is an upcoming optical, fibre-fed, MOS facility for the VISTA telescope at ESO's Paranal Observatory in Chile. Its main science drivers are in the fields of galactic archeology, high-energy physics, galaxy evolution and cosmology. The 4MOST consortium consists of several institutes in Europe and Australia under leadership of the Leibniz-Institut für Astrophysik Potsdam (AIP). 4MOST is currently in its Assembly, Integration and Tests Phase with an expected start of science operations in 2025. The design of 4MOST features 2436 fibres split into two low-resolution spectrographs (1624 fibres, three arms, 370-950 nm, R > 4000) and one high-resolution spectrograph (812 fibres, three arms, ~44-69 nm coverage each, R > 18000). The fibre positioner covers a hexagonal field of view of ~4.1 deg2. The fibers are 85μm core with an output beam at f/3. CRAL has the full responsibility of the two Low Resolution Spectrographs. Each of them is composed of a 200mm beam for an off-axis collimator associated to its Schmidt corrector, three “color” arms hosting f/1.73 cameras with standard 6k x 6k 15μm pixel CCD detectors. The local acceptance reviews at CRAL for the both 4MOST Low Resolution Spectrographs were successfully passed respectively in December 2021 and October 2022. In 2022, two SPIE papers described the process performed at CRAL from the integration and alignment of the sub-assemblies up to the procedures developed to test the spectrograph and demonstrate its compliance with the requirements. Then the two LRS were partially disassembled and sent to Potsdam. They have now been fully integrated, aligned and tested at Potsdam by the CRAL team. The integration of both LRS with the others sub-systems in order to validate the global end-to-end tests is foreseen in 2024 and an installation at VISTA telescope is expected in 2025. This paper describes the assembly, integration and performances achieved at Postdam for the both Low Resolution Spectrographs. Special emphasis is put on the update of procedures and components to improve performances and meet the top-level requirements.
The 4-meter Multi-Object Spectroscopic Telescope (4MOST) instrument uses 2436 individually positioned optical fibres to couple the light of targets into its spectrographs. The AESOP fibre positioner is mounted at the Cassegrain focus of the VISTA telescope, which houses the fibres in a hexagon-like structure with a diameter of 535 mm that covers a 2.5 deg diameter field of view on the sky. Fibres are positioned relative to fixed fiducial fibres. The metrology system determines the position of the fibres on the focal surface of the telescope relative to the fiducial fibres. The location of the fibres needs to be measured to better than 3 micron RMS in the focal surface, approximately 0.05 arc seconds on sky. Four imaging cameras are mounted on the VISTA spider vanes that look through the entire optical train, including primary and secondary mirror as well as the wide field corrector (WFC) / atmospheric dispersion compensator (ADC) unit. We recreated the setup for the metrology system in the lab with similar dynamic behavior but different optical design due to the lack of the VISTA telescope. We demonstrate the metrology system measurement accuracy in lab conditions on the full scale test stand. We also show how we measure distortions induced by optical path and the calibration procedure as a precursor for commissioning on the telescope. In particular, we present a method how to measure the surface shape of any optical surface with approx. 10 nm accuracy over its entire optically active surface.
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