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.
The LUCI instruments are a pair of NIR imagers and multi-object spectrographs located at the front bent Gregorian foci of the Large Binocular Telescope (LBT). One of their special features is their diffraction-limited imaging and long-slit spectroscopic capability in combination with the LBT adaptive secondary mirrors. This allows to achieve a spatial resolution down to 60mas and a spectral resolution of up to 25000. Switching from seeing-limited to diffraction-limited observations changes several operational aspects due to features such as the non-common path aberration or the flexure of the instruments. They all require novel techniques to optimize the image quality and to maximize the scientific return. Non-common path aberration can be corrected via look-up tables. For active flexure compensation the night-sky emission is used. The commissioning of the instruments in diffraction-limited mode on sky is largely finished and the instruments have been handed over to the LBT in April 2018.
LUCI1 and LUCI2 are a pair multi-mode, fully cryogenic near-infrared instruments installed at the Large Binoc- ular Telescope (LBT). The instruments provide imaging, long-slit and multi-object spectroscopy over a 4/ FoV in seeing-limited mode. Ground-layer AO (GLAO) correction for imaging and spectroscopy over the 4/ FoV is available using the ARGOS laser system, as well as diffraction-limited AO over a 30// FoV using the LBT first light AO (FLAO) system with natural guide stars. Internal flexure of the instrument is taken care of by passive and active flexure compensation. Image shifts in seeing-limited modes are compensated by a passive flexure con- trol algorithm using pre-defined look-up tables. For AO observations, passive compensation is replaced by active control. In the following, we present the details of the newly developed active flexure compensation algorithm for the LUCI instruments. We also describe some hardware modifications to the instruments and the results obtained with active flexure compensation.
The 4-m Multi-Object Spectrographic Telescope (4MOST) is one high-resolution (R ~ 18000) and two lowresolution (R fi 5000) spectrographs covering the wavelength range between 390 and 950 nm. The spectrographs will be installed on ESO VISTA telescope and will be fed by approximately 2400 fibres. The instrument is capable to simultaneously obtain spectra of about 2400 objects distributed over an hexagonal field-of-view of four square degrees. This paper aims at giving an overview of the control software design, which is based on the standard ESO VLT software architecture and customised to fit the needs of the 4MOST instrument. In particular, the facility control software is intended to arrange the precise positioning of the fibres, to schedule and observe many surveys in parallel, and to combine the output from the three spectrographs. Moreover, 4MOST's software will include user-friendly graphical user interfaces that enable users to interact with the facility control system and to monitor all data-taking and calibration tasks of the instrument. A secondary guiding system will be implemented to correct for any fibre exure and thus to improve 4MOST's guiding performance. The large amount of fibres requires the custom design of data exchange to avoid performance issues. The observation sequences are designed to use spectrographs in parallel with synchronous points for data exchange between subsystems. In order to control hardware devices, Programmable Logic Controller (PLC) components will be used, the new standard for future instruments at ESO.
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