3D Printing of VLT Components
10 Shkurt 2014
ESO has recently utilised the innovative technology of 3D printing  to manufacture moulds for the casting of two new telescope components. These are required for the MUSE instrument on ESO's Very Large Telescope (VLT) in Chile and are part of the Adaptive Optics Facility project. The 3D printing technique offers great promise in the manufacture of complex custom items, which are often needed in astronomical instrumentation, delivering the components more quickly and cheaply, and with greater flexibility.
The first item, which was manufactured by the German firm voxeljet AG, is the structural part of a new sensor arm  that was installed in the telescope to work with the MUSE instrument. This change was also needed to accommodate the GALACSI adaptive optics module to be installed in 2015. The sensor arm is a metallic structure that is used to hold three flat mirrors , which feed light beams into sensors that control the active optics systems of the VLT and are used for guiding the telescope during observations.
The component was created using a technique known as investment casting, which has been traditionally used to manufacture components with very complex shapes, such as blades for gas turbine engines. With this casting method, a model is generated in software and its mechanical properties are analysed using standard industrial approaches. Once a suitable virtual model has been created a pattern is printed, in this specific case using polymethylmethacrylate, a type of thermoplastic.
Next, a mould is created from the wax-infiltrated plastic pattern, providing a negative of the original shape. This investment process then involves coating the pattern with a heat-proof ceramic. During the process the ceramic is treated and hardened, the wax-infiltrated pattern is melted out of the new ceramic shell, leaving a perfect die into which the metal for the final cast is poured.
The previous sensor arm was manufactured using beryllium, a lightweight metal that satisfied the requirements for the sensor arm. However, beryllium is highly toxic, and cannot be machined or modified safely once the component has been installed. For this reason, high grade aluminium was selected to make the final cast for the replacement.
The second component, a spacer for the VLT test camera, was manufactured by ACTech GmbH, another German company specialising in metal casting techniques. This component was manufactured from ductile cast iron and used a similar investment casting process, once its original pattern had been laser sintered.
 3D-printing technology is an additive manufacturing process that promises to reduce production time, minimise waste materials and save money for companies. Traditional manufacturing methods, such as milling and lathing are referred to as subtractive processes, because material is removed from a larger piece until the final form is reached. This can generate a lot of waste material in the form of swarf. Additive processes avoid this wastage by building the form up layer by layer to achieve the final form. 3D printing, and more specifically, 3D printing for casts allows for complex internal geometries to be created which are otherwise impossible to achieve.
 The function of the new sensor arm is to trick the telescope active optics system to focus 250 millimetres from the original focus position. The result is that the focal plane now lies 500 millimetres from the Nasmyth flange. This extra distance will provide the space needed to install GALACSI, the adaptive optics module for MUSE.
 These mirrors are made from HB-CESIC®. This is carbon fibre reinforced silicon carbide, a material characterised by its exceptional hardness, very high specific stiffness and a low thermal expansion coefficient. The mirrors were manufactured by ecm Engineered Ceramic Materials GmbH and Berliner Glas.
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