On-sky Fibre-Target-Alignment of the 4MOST instrument: calibration and performance

Roland Winkler; Weijia Sun; Daniel Sablowski; Thomas Liebner; Ole Streicher; Steffen Frey; Ingo Stilz; Alexander Pramskiy; Scott Smedley; Thomas Szeifert; Diogo Rio Fernandes; Gerard Zins

arXiv · 2606.27882

On-sky Fibre-Target-Alignment of the 4MOST instrument: calibration and performance

Roland Winkler·Weijia Sun·Daniel Sablowski·Thomas Liebner·Ole Streicher·Steffen Frey·Ingo Stilz·Alexander Pramskiy·Scott Smedley·Thomas Szeifert·Diogo Rio Fernandes·Gerard Zins

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Abstract

The 4-metre Multi-Object Spectroscopic Telescope (4MOST) is a new wide-field, fibre-fed spectroscopic survey facility for the VISTA telescope at ESOs Paranal Observatory. The instrument enables the simultaneous acquisition of 2436 spectra across a 4.2 square deg field of view, using a tilting spine fibre positioner feeding three dedicated spectrographs. In this paper, we describe the calibration process, and performance verification of the Fibre-Target-Alignment (FTA) process for 4MOST. We show the complete FTA process, including calibration of the individual hardware- and software components. Namely the Metrology camera system, the Fibre Positioner AESOP, a spine based Secondary Guiding System, the sky to focal surface projection software, and residual minimization via raster scans. In total, the FTA system required one special tool, a large calibration target for the focal surface, and approximately 1 month of accumulated calibration work on the telescope. The FTA process reached approx. 24 um (0.4 arc sec) RMS distance between fibres and targets on sky about 3 weeks after installation of the final hardware components of 4MOST, which is when 4MOST had its first light event. By the time of writing this paper, i.e. 6 months later, we reach approx. 16 um (0.27 arc sec) RMS. Currently, we far exceed our requirements in terms of accuracy, and are doing trade-off studies to maximize scientific returns.

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Roland Winkler, Weijia Sun, Daniel Sablowski, Thomas Liebner, Ole Streicher, Steffen Frey, Ingo Stilz, Alexander Pramskiy, Scott Smedley, Thomas Szeifert, Diogo Rio Fernandes, Gerard Zins. 2026-06-26. On-sky Fibre-Target-Alignment of the 4MOST instrument: calibration and performance. https://arxiv.org/abs/2606.27882

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