THE NEW VLT-DSM M2 UNIT: CONSTRUCTION AND ELECTROMECHANICAL TESTING

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1 Florence, Italy. Adaptive May 213 Optics for Extremely Large Telescopes III ISBN: DOI: /AO4ELT THE NEW VLT-DSM M2 UNIT: CONSTRUCTION AND ELECTROMECHANICAL TESTING Daniele Gallieni 1,a, Roberto Biasi 2 1 A.D.S. International Srl, via Roma Valmadrera (LC), Italy 2 Microgate Srl, via Stradivari Bolzano (BZ), Italy Abstract. We present the design, construction and validation of the new M2 unit of the VLT Deformable Secondary Mirror. In the framework of the Adaptive Optics Facility program, ADS and Microgate designed a new secondary unit which replaces the current Dornier one. The M2 is composed by the mechanical structure, a new hexapod positioner and the Deformable Secondary Mirror unit. The DSM is based on the well proven contactless, voice coil motor technology that has been already successfully implemented in the MMT, LBT and Magellan adaptive secondaries, and is considered a promising technical choice for the E-ELT M4 and the GMT ASM. The VLT adaptive unit has been fully integrated and, before starting the optical calibration, has completed the electromechanical characterization, focused on the dynamic performance. With respect to the previous units we introduced several improvements, both in hardware and control architecture that allowed achieving a significant enhancement of the system dynamics and reduction of power consumption. 1. Introduction The VLT Deformable Secondary Mirror will replace the existing Dornier secondary unit for ESO Adaptive Optics Facility on VLT UT-4 Yepun [1]. The whole M2 hub has been re-designed around the deformable mirror unit and a new hexapod positioner, while keeping the present interfaces to the telescope top ring. The unit is now performing its acceptance and optical calibration tasks on the ASSIST optical test bench at ESO Garching facility, where it will be eventually used to calibrate the two modules for which it has been designed, GRAAL and GALACSI. After that everything will be packed and moved to ESO Paranal for a final testing before being integrated on the telescope. Microgate with A.D.S. International have designed and built the new M2 unit; REOSC (SAFRAN- Sagem) made the 112mm diameter and 2mm thick Zerodur shell and Thales-SESO made the lightweighted Zerodur reference body. 2. System description The new VLT-M2 unit is made of three subsystems, the hub structure, the hexapod positioner and the Deformable Secondary Mirror. a gallieni@ads-int.com

2 The hub will be permanently mounted on the telescope top ring, with the hexapod inside. The latter can be removed for maintenance purposes and put back in place without affecting hub own alignment on the telescope top ring. The DSM is installed on the M2 hub and removed from it in the same way the current Dornier M2 units are managed, i.e. with the telescope laying at the maintenance elevation with the service platform installed The DSM Fig.1. Cutaway rendering of the new VLT-DSM M2 unit. The Deformable Secondary Mirror is made by a number of subsystems: at the highest level we can identify the adaptive mirror and the electronics crates managing real time control, power and communication functions. The crates are removed from the telescope together with the adaptive mirror, which can be run independently from the telescope. Both electronics crates and the adaptive mirror are actively cooled by the same water-glycol system running in the rest of the telescope.

3 Each one of the three crates comprehends two electronics backplanes comprehending 13 control boards, each controlling up to 16 actuators, besides the real time and diagnostic communication electronics and the ancillary supply and housekeeping electronics. Inside the adaptive mirror we can recognize: the 2 mm thick mirror shell made of Zerodur, built by REOSC (SAFRAN-Sagem), which is restrained to the adaptive mirror by the sole flexure put in the central hole of the mirror; 117 permanent magnets are glued on the back surface of the shell; the shell is coated by a unprotected Aluminum layer on both sides, the front being the optical surface and the back the common armature of the capacitive sensors co-located to the actuators; the Zerodur light-weighted Reference Body made by Thales-SESO, with 117 holes to allow the Voice-Coil-Motor actuators reaching the permanent magnets on the back of the shell; around each hole a metallization ring is deposited to make the armature of the capacitive sensor which measures the local distance (gap) between the shell back surface and the Reference Body; the 117 VCM actuators, which are made of an aluminum cold finger with the coil mounted on top and a PCB mounted aboard, where the capacitive sensors signal is amplified and converted from analog to digital and the coil driving current is transferred to the voice coil motors; an aluminum plate which plays the function of actuators support structure and heat sink for the power dissipated by the actuators by means of the coolant pipes mounted on its surface; the top of the cold-plate is covered by a set of PCBs divided in sectors, each one collecting the signals of the actuators and interfacing them through flat cables to the control boards installed in the crates. Fig.2. Installation of the thin shell on the DSM unit at Microgate.

4 3. The new M2-DSM unit MAIT The manufacturing, assembly and opto-mechanical integration of the whole unit was carried out by ADS in parallel to the electronics manufacturing and modular testing made at Microgate. The optical pieces, namely the thin shell and the Reference Body have been processed at ADS, by means of a series of dedicated handling tools that allowed safe execution of both cleaning and coating tasks. In particular for the thin shell a novel sandwich box has been developed to be compatible with all the steps of the shell integration process, so that once the glass piece has been transferred on this tool there is no further need of handling it directly. The magnets bonding process has been implemented by dedicated tools so that the integration of the second thin shell due to be delivered by REOSC (SAFRAN-Sagem) in late 213 can be made independently from the DSM unit. Fig.3. VLT-DSM thin shell during the cleaning and coating process at ADS and ZAOT (Milan, IT). Once the DSM sub-system opto-mechanical integration was completed at ADS, the unit was moved to Microgate premises where the electronics sub-systems have been integrated on it. At Micorgate all the actuators have been individually calibrated and tested on a dedicated bench with automated procedures, that allowed to characterize and qualify them before being integrated into the DSM. Also the real time electronics boards were thoroughly tested individually before the whole crates had been qualified by burn-in procedures and environmental testing. Once the actuators have been installed and aligned in the DSM and the electronics crates mounted in the assembly, Microgate run the DSM first with white-tests first and then controlling the actual shell [3]. Worth to be noticed that DSM actuators capacitive sensors make a quite efficient embedded metrology for this system: after an initial calibration they allow running the full set of dynamics qualification of the adaptive mirror without optical feedback. The latter is then performed at a subsequent stage to achieve the static calibration of the mirror under the interferometer.

5 Fig.4. VLT-DSM during dynamics testing at Microgate. 4. DSM dynamic performances The VLT-DSM is the first among Microgate-ADS adaptive units where we have implemented a novel control concept, the so called Dynamic Feed-Forward which allowed reaching unprecedented response time performances. In particular we achieved ~.7ms settling time over all 117 modes, well exceeding the 1.ms base requirement and also the 1.ms goal requirement. Typical control error (the difference between commanded and achieved mirror shape) is also presented hereafter for two cases of median and bad seeing for which 36 and 49 nm RMS WF have been achieved respectively. Fig.. DSM real time control scheme.

6 Third AO4ELT Conference - Adaptive Optics for Extremely Large Telescopes Mode #11 - Settling:.721 [ms] - Oversh:.7% x 1-6 Mode # - Settling:.721 [ms] - Oversh: 3.% x 1 4 Settling time Mode #1 - Settling:.672 [ms] - Oversh:.1% x Mode # Fig.6. DSM dynamic response:.7msec settling time (within 9% of command) achieved on all modes (see bottom left plot). - - Act #84 - Ring #1 x 1 Act #84 - Ring #1 x Position w.compdelay Position w.compdelay Fig.7. Dynamic performance: following error time history (left) and zoomed view (right). Median seeing (.9 λ=.µm) 36nm WF; bad seeing (1. λ=.µm) 49nm WF

7 Tilt [arcsec] Tilt [arcsec] Third AO4ELT Conference - Adaptive Optics for Extremely Large Telescopes Another relevant feature of the DSM is its capability of making relatively large mirror strokes up to 1 microns at actuator level, which means essentially being able to implement field stabilization and even chopping functions. Worth to be observed in the plots reported hereafter is the response time of the chopping steps: in 8ms the mirror makes the requested ±2 arcsec steps and restore itself in the Adaptive Optics mode taking the shape left at the last command step. Field stabilization Field stabilization 1 Acquired tilt w.compdelay Acquired tip Acquired tilt w.compdelay Acquired tip Time [s] Time [s] Fig.8. Field stabilization: stroke specification of ±12 (mirror) achieved with 7µm actuators stroke, 64 mas error and 1Hz BW x 1 - Act #14 - Ring #18 13 x 1 - Act #14 - Ring # Position w.compdelay Position w.compdelay Fig.9. Chopping: large stroke of ±2 (mirror) achieved with 8ms settling of chopping motion plus restore and 14µm actuators stroke.. Remaining steps to telescope installation After completing the electro-mechanical test phase at Microgate, which included environmental testing of the whole DSM that was run at -18 C, the adaptive mirror subsystem was moved back to ADS premises where it has been integrated into the new M2 unit (hub plus hexapod). In this phase all the handling tools and procedures foreseen to be done at the telescope have been tested and finally the electro-mechanical (dynamic) tests have been repeated on the complete M2 unit mounted on a telescope simulator bench, being this the Factory Acceptance of the system.

8 After that the M2 unit was cleared to be moved to ESO premises in Garching where it has been installed on the ASSIST facility in early 213. This was also the opportunity to test all the packing and transport procedures and tools that will be used to move the new M2-DSM unit to Paranal after the optical testing will be finished in Garching. Fig.1. VLT-DSM installation on ASSIST at ESO Garching. 6. Conclusions The new VLT-DSM M2 unit has been designed, built and tested by Microgate-ADS team and it has been delivered to ESO for optical calibration, that has been performed in collaboration with INAF- Osservatorio Astrofisico di Arcetri [2] still under responsibility of the companies team. This program consolidated the capability of our companies to cover the entire production and qualification process of these systems, with great improvement on the risk reduction and overall efficiency on the delivery of our Deformable Mirror units for 8-m class telescope as well as the next generation of ELTs. 7. References [1] Robin Arsenault et al, Proc. SPIE 8447, Adaptive Optics Systems III, 8447J (September 13, 212); doi:1.1117/ [2] Runa Briguglio et al., Optical calibration and test of the VLT Deformable Secondary Mirror, in this conference [3] R.Biasi et al, VLT Deformable Secondary Mirror: integration and electromechanical tests results, SPIE 212, [4] M.Manetti et al, Control of Massively Actuated Adaptive Mirrors, in IEEE Transactions on Control System Technology, ISSN