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The First Active Segmented Mirror at ESO

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The First Active Segmented Mirror at ESO Telescopes and Instrumentation The First Active Segmented Mirror at ESO Frédéric Gonté, Christophe Dupuy, The design and integration phases of onal mirrors have to be aligned with a Christoph Frank, Constanza Araujo, APE, which started in 2005, have been precision better than 15 nm rms. It was Roland Brast, Robert Frahm, completed and the test phase will start clearly not feasible to produce a scaled- Robert Karban, Luigi Andolfato, in June 2007 during which time APE down version of the full primary mirror. Regina Esteves, Matty Nylund, will be installed at the focus of one of the However, from a statistical point of view, Babak Sedghi, Gerhard Fischer, Lothar VLT unit telescopes in 2008. a mirror with approximately 50 segments Noethe, Frédéric Derie (all ESO) is already representative of a mirror with Initially the APE team wanted to contract many more segments in terms of the the design and manufacture of the ASM study of issues like alignment algorithms The Active Phasing Experiment (APE) is to a private company. However, when or the effect of misalignments on image part of the Extremely Large Telescope no company could be found which could quality. The main requirements for the Design Study which is supported by the meet the rather stringent requirements, ASM can be summarised as: European Framework Programme 6. it was decided to develop the ASM in- – 61 segments in four rings around the This experiment, which is conducted in house, involving ESO groups in Integra- central segment collaboration with several partners is tion, Optics, Electronics, Software and – Segment size 17 mm to minimise the a demonstrator to test and qualify newly- the ELT Project Office. size of the re-imaged pupil and developed phasing sensors for the align- the relay optics on the optical bench ment of segmented mirrors and test – Three degrees of freedom for rigid body the phasing software within a telescope Design and integration movements, that is piston, tip and tilt control system to be developed for a for each segment future European Extremely Large Tele- The current design of the primary mirror – Precision of displacement better than 2 scope. The segmentation of a primary of the European Extremely Large Tele- nm (similar to an ELT) mirror is simulated by a scaled-down scope consists of 984 hexagonal seg- – Range of displacement more than Active Segmented Mirror of 61 seg- ments (Gilmozzi and Spyromilio 2007). 15 μm (similar to an E-ELT) ments which has been developed in- Each segment has a diameter of around – Size of the gaps between segments house. 1.5 m and the size of the gaps between between 80 and 150 μm (scaled-down the segments is approximately 4 mm. from the gap size in the ELT primary) In order to achieve the performance re- – Surface form quality better than 15 nm Background quired for high-resolution imaging with RMS (similar to the required surface extreme Adaptive Optics (AO), the hexag- quality of a real segment with active In order to gain experience with the phas- shape control) ing of segmented mirrors, in particular – Operational temperature between 0˚C to develop new optical phasing sensors and 25˚C and to test the interface with the rest of – Possibility to exchange a segment in the wavefront control system, ESO de- case of failure. cided to perform an optical-bench type – Optical fibres to be inserted in two of experiment, named the Active Phasing Photo: H. H. Heyer, ESO the hexagonal mirrors (central mir- Experiment (APE). This project is part of ror and one of the perimeter mirrors) to the Extremely Large Telescope Design align APE with high efficiency Study (ELTDS) which is supported by the European Framework Programme 6. The A segment unit, shown in Figure 1, con- project is performed in close collabora- sists of a base, three actuators, three tion with the Istituto Nazionale de Astro- springs and a hexagonal mirror. The 61 fisica di Arcetri, the Instituto de Astrofísica modules were assembled on a base plate de Canarias (IAC) and the Laboratoire (Figure 2). Since the precision of position- d’Astrophysique de Marseille (LAM), and ing the surfaces of the hexagonal mir- the industrial firm Fogale. The core optical rors relative to the base plate is better element of APE, simulating the segment- than 15 µm, it is necessary to be able to ed primary mirror in a large telescope, is actively align all the mirrors. Among the a scaled-down Active Segmented Mirror set of candidates for the commercially (ASM) with 61 segments which can all available actuators, only piezo actuators be controlled in piston, tip and tilt. The could fulfil the ASM requirements. Since pupil of the telescope is re-imaged onto this type of active segmented mirror is the ASM. The optical beam is then re- the first of its kind it was decided to start adapted to behave like the VLT output development in mid-2005 with a proto- beam and distributed to the four phasing type of only seven segments. sensors to be tested in the experiment. Figure 1: Each segment is integrated separately. It is composed of one base, three piezo-actuators, three springs and a hexagonal mirror. The Messenger 128 – June 2007 23 Telescopes and Instrumentation Gonté F. et al., The First Active Segmented Mirror at ESO Figure 3: Integration of the ASM (having a plug and play concept) by Christophe Dupuy. Each segment is added one by one on a support Photo: H. H. Heyer, ESO polished plate which has a surface flatness better than 2 microns. Figure 2: Mechanical design of the ASM made by Christoph Frank. The 61 segments are seen with the three actuators per segment, the module base, the support plate and the connectors behind the support plate. The fibres inserted in the central mirror and one of the mirrors on the right side of the Figure 4: Photo of the ASM can also be seen. fully assembled ASM. Photo: H. H. Heyer, ESO The mechanical design has been made by Christoph Frank. The integration has been conducted by Christophe Dupuy for the opto-mechanical parts (see Figure 3), by Roland Brast for the drive electron- ics and by Robert Frahm for the software. The tests were performed by Constanza Araujo. Information obtained during the tests of the seven-segment prototype was used to improve the final design. obtained by an Internal Metrology sys- Current tests tem which consists of a two-wavelength interferometer developed by Fogale. Several features have been tested on the The seven-segment prototype has been fully integrated ASM (Figure 4), for exam- used to test the closed-loop system ple fatigue of the components, eigen- and achieved a performance better than frequencies, speed and hysteresis of the 5 nm rms for the alignment errors at the positioning, drift of an actuated position, intersegment borders. resolution and accuracy of the position- ing, quality and cosmetics of the surfaces The fully integrated ASM has now fulfilled of the segments. The fully integrated ASM and, in some areas, even surpassed the has fulfilled and, in some cases, even sur- specifications. The success of this realisa- passed the specifications. Figure 5 shows tion now provides the possibility to fully an interferogram of the surface of the test future phasing sensors of the E-ELT ASM while being tested. The tips and tilts within the Active Phasing Experiment. of the segments have been selected such that the letters of the acronym ASM ap- pear in the interferogram. References Figure 5: Interferogram of the surface of the ASM. Some segments have been tip-tilted to create fringes Gilmozzi R. and Spyromilio J. 2007, The Messenger on some selected segments to write on it the During the APE experiment, the ASM will 127, 11 acronym ASM. be driven in closed loop based on signals 24 The Messenger 128 – June 2007.
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