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 implementation of the 4MOST Facility at the ESO Paranal 4-meter VISTA wide-field telescope requires a substantial modification of the telescope. Since the current acquisition and guiding (A&G) and wavefront sensing optical systems (WFS) are embedded in VIRCAM and will be removed with it, replacements had to be provided. Although the A&G and WFS cameras will serve different purposes, they share common requirements. Among the shared requirements, a few are particularly challenging. For example, the environmental conditions the cameras will be exposed to require them to have an IP54 protection and due to their location, they cannot dissipate heat to the ambient air. To ensure optical alignment, the cameras must have very accurate housing and mechanical interfaces. In addition, both have to be integrated into an existing telescope control environment, with all that this entails in terms of service interfaces and protocols that can be used (e.g. GigE Vision), as well as operational requirements that must be met. After considering the specific performance requirements for the A&G cameras, the WFS detectors and the secondary guider sensor, a decision was made to use the same custom designed CCD camera model for all of them. These cameras are provided by Spectral Instruments. In this work we present the requirements for such cameras, their opto-mechanical design and the first results of their verification campaign, both at Spectral Instrument and AIP premises.
The 4-meter Multi-Object Spectroscopic Telescope (4MOST), wide-field, high-multiplex spectroscopic survey facility will enable the simultaneous spectroscopy of up to 2400 targets within a 2.5° diameter field of view. A secondary guider system optical relay and data production description is described. The role of this guider is for fine rotational and target alignment corrections. The output of the 12 times 7 fibres are arranged such that the CCD detector can be read out in continuous read mode. The position of all fibres are illuminated at their spectrograph end and measured using a camera system at the positioner end. For the secondary guide bundles, only the central fibre is illuminated. A notch filter is used in the image relay in such a way that that the back illumination light is reflected from back illumination fibres to illuminate only the central fibres of each guide probe. This allows on-sky guiding while the fibres are being positioned.
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