the adaptive secondary mirrors for the large binocular...

The adaptive secondary mirrors for the Large BinocularTelescope: a progress report

A. Riccardia, G. Brusab, M. Xomperoa, D. Zanottia, C. Del Vecchioa, P. Salinaria, P.Ranfagnia, D. Gallienic, R. Biasid, M. Andrighettonid, S. Millere, P. Mantegazzaf

aINAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, ItalybSteward Observatory - University of Arizona, 933 North Cherry Avenue,

85721 Tucson (AZ), U.S.A.cADS International srl, Via Roma 87, 23868 Valmadrera (LC), ITALY

dMicrogate srl, Via Stradivari 4, 39100 Bolzano, ITALYfMirror Lab of Steward Observatory,

257 National Champion Drive, 85721 Tucson (AZ), U.S.A.eDipartimento di Ingegneria Aerospaziale - Politecnico di Milano,

Campus Bovisa Sud, Via La Masa 34, 20156 Milano, ITALY

ABSTRACT

The two 911mm-diameter adaptive secondary (AS) mirrors for the Large Binocular telescope (LBT) are currentlyunder manufacturing process. Each unit has 672 electro-magnetic force actuators. They control the figure of theGregorian secondary 1.6mm-thick mirrors with an internal loop using the signal of co-located capacitive sensors.The obtained computational power of the on-board control electronics allows to use it as real-time computer forwavefront reconstruction. We present the progress in manufacturing and assembling of the first telescope unit,the progress in software production, the status of the testing facilities and an update on the latest modificationof the design.

Keywords: adaptive optics, wavefront correctors, adaptive secondary

1. INTRODUCTION

Large Binocular Telescope1 (LBT) is equipped with two gregorian adaptive secondary (AS) units (LBT672a andLBT672b). Each one joins the functionalities of a conventional secondary mirror for seeing limited observations,of an active optics and field stabilization compensator, of a chopper for Mid-IR observations and, last but not theleast, of a 672-actuator deformable mirror for Adaptive Optics correction. Because of the location of the ASs, theyrepresent the common AO deformable mirror for all the 4+4 focal stations of the telescope. In particular theyperform AO correction for the the first light adaptive optics system2 (W@LBT) in the front bent-gregorian focalstations serving the spectro-imager Lucifer,3 for the IR interferometer with nulling capabilities4 (LBTI) in thecentral bent-gregorian focal stations and they act as ground layer correctors for the visible-NIR multi-conjugatedinterferometer5 (NIRVANA) located in the back bent-gregorian focal stations.

The adaptive secondary (AS) mirror technology is an on-sky proven technology since late 2001 with the firstlight of the 336-actuator AS unit for MMT6, 7 (MMT336). CAAO (University of Arizona, AZ, USA) routinelyproduced with MMT336 more than one year of scientific results.8 The two 672-actuator AS units for LBT(LBT672a and LBT672b) represent the new generation of the current AS technology. Their design is basedon the experience gained with the MMT unit, but both optical manufacturing procedure and electro-mechanicsdesign have been revised, improving performances, stability, reliability, maintenance and computational powerof the system.9, 10

The partners involved in the project are almost the same team of companies and institutes of the MMT336project. Microgate (Italy) is in charge of the design and production of the electronics and the related microcode.

Further author information: (Send correspondence to A.R.)A.R.: E-mail: [email protected]

Advancements in Adaptive Optics, edited by Domenico Bonaccini Calia, Brent L. Ellerbroek,Roberto Ragazzoni, Proceedings of SPIE Vol. 5490 (SPIE, Bellingham, WA, 2004)

0277-786X/04/$15 · doi: 10.1117/12.551578

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Figure 1. (a) and (b): exploded and 3-D view of the 672-actuator adaptive secondary unit for LBT. The diameter of thethin mirror is 911mm. See the text for explanation. (c): actuator.

ADS International (Italy) manages the mechanical design, mechanical production and assembly. Mirror Lab(Steward Observatory, AZ, USA) produces all the optical elements of the units (thin shells and reference plates).INAF-Osservatorio di Arcetri (Italy) led the conceptual design and is in charge of the diagnostic/calibrationhigh-level software, the laboratory optical and electro-mechanical characterization, and the commissioning atthe telescope. Finally Politecnico di Milano is working on internal control aspects and related dynamical symu-lations.

2. LBT672 ADAPTIVE SECONDARY UNITS

In Fig. 1 the 3-D views of LBT672 show the main six components of the AS unit. From the top (or back) to thebottom (or front) we have:

1. an intermediate flange bolted to the M2 mobile hexapod of the telescope that provides a mechanicalinterface to the unit;

2. three cooled boxes for the electronics. Each box contains 28 DSP boards with 2 DSPs each for the controland the diagnostics of the 672 electro-magnetic force actuators. The boards inside each cooled box areorganized in two internal crates (14 board per crate). Each crate has an independent bus and containsalso a communication and a reference signal generation board. All the communication boards are in daisy-chain and the reference signal boards refers to the same digital signal to assure synchronization. The totalamount of DSPs is 168 for a total computation power of 160 Gflop/s in 32-bit floating-point arithmetic.Each DSP manages four actuators. The huge computational power allows to use the AS electronics asreal-time wave-front reconstructor in the W@LBT AO system2;

3. an aluminum plate (cold plate) that provides support and cooling for the actuators. This plate is connectedto the intermediate flange via a fixed hexapod. The cooling distribution is provided thru copper pipes insideinternal grooves of the cold plate;

4. the 672 electro-magnetic force actuators. A coil is placed on the aluminum cold finger tips (Fig. 1c) thatare faced to the corresponding magnets bonded on the back of the thin mirror. On each actuator there isa board providing the contacts to pick the capacitive sensor signal and the related pre-amplification andde-modulation electronics. The analog signals are converted to digital on the DSP boards. The conversionrate will be set ≈ 100 kHz with the capability of changing it in the 80-160 kHz range depending on theresult of the optimization after the complete characterization of the final unit;

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(a) (b)

Figure 2. (a) new LBT electronics under test with P45 prototype. (b) P45 prototype for testing LBT672 subsystems.

5. a thick (50 mm) Zerodur glass plate (reference plate) with bored holes, attached to the cold plate througha second fixed hexapod and a set of three astatic lever. This plate is used as a position reference for thethin deformable mirror. The coil cold-fingers, supported by the cold plate, pass through the bored holeson the reference plate to reach the deformable mirror. The mirror position is sensed using the co-locatedcapacitive sensors. The conductive area of the capacitive sensors is obtained with a galvanic deposition ofgold;

6. a deformable Zerodur concave shell (thin mirror) of 911 mm diameter and 1.6 mm thickness with 672magnets glued to its back surface. The front surface is ellipsoidal to match the Gregorian optical designof LBT. The back surface is spherical to match the front surface of the reference plate. This shell has acentral hole to which a membrane is attached to provide lateral and in-plane rotational constraint. Whenthe mirror is not active its axial constraint is provided by a set of stops located at the inner and outer edgeof the mirror.

More details about electronics and mechanics can be found in Refs. 11 and 12. Details on the differencesbetween the MMT and LBT design can be found in Refs. 9 and 10.

3. PROJECT PROGRESS

3.1. Mechanics

Hexapod is currently available in the ADS premises. The production of magnets and actuators for the first unitis almost at the end of the precess. The cold-plates of both units finished their first manufacturing process withthe turning machine and are going to start the production of actuator holes. Electronics crates of the first unithave been produced together with their cooling circuitry. The reader can find more details in Ref. 12.

3.2. Electronics

A subset of the new release of LBT electronics has been recently delivered by Microgate, namely one BCU, onesignal reference board and six DSP boards (see Fig. 2a). Boards have been collected in the first electronics crateproduced by ADS and it is currently at Arcetri for running extensive tests with the 45-actuator prototype (P45,

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(a) (b)

Figure 3. (a) 911mm-diameter reference plate from Mirror Lab. (b) Polishing of the Zerodur 911mm-diameter shellfront surface at Mirror Lab.

see Fig. 2b). P45 has been already equipped with the new release of capacitive sensor boards that were testedusing a subset of the old MMT electronics, see Ref. 10 for the related results (capacitive sensor noise, dynamics,etc.). More details about LBT electronics are reported in Ref. 11.

3.3. Optics

Mirror Lab recently delivered the reference-plate of the first LBT672 unit (see Fig. 3a). It is currently at ADSpremises ready for the first step of pre-integration. Thin shells of the two final units are currently in productionat Mirror Lab. The aspheric (front) surface of the first shell has been already polished (see Fig. 3b) and isalmost ready for starting the thinning process. In parallel Mirror Lab is producing a complete set of toolsfor the delicate process for detaching the thin shell from the blocking-body and for its handling, running thecorresponding experimentation.

3.4. Software

Algorithms developed by Arcetri for internal calibration and control optimization have been extensively tested onMMT336 unit during past years. Code for MMT unit has been reorganized by Arcetri and mostly re-written totake advantage of LBT AS new features and to assure its integration in the AO Supervising software developed inArcetri.13 Most of the procedures for system calibration, characterization and initialization have been alreadyproduced for LBT672. They are currently under debugging and test on P45 using the new LBT electronicsrecently delivered by Microgate together with the new firmware of the BCU and DSP boards. The calibration,characterization and initialization code has been written using IDL (Interactive Data Language) that providesan integrate environment with native matrix-oriented syntax and multiprocessor number crunching capabilities,characteristics heavily used during system calibration and characterization.

3.5. Arcetri optical test facility

Arcetri completed the erection of the 14m vertical optical bench10 for testing the LBT672 adaptive secondary inoptical close loop with the wavefront sensor module of first-light AO system (W@LBT) for LBT and simulatingthe same telescope optical coupling. The optical bench, shown in Fig. 4a/b, is a 1.2m-diameter tube decoupledfrom ground using pneumatic isolators. LBT672 unit, together with its hexapod, will be located inside the tubeat its top. The wavefront sensor module will be rigidly connected to the bottom end of the bench, that is locatedinside the tower laboratory (see Fig. 4c). The W@LBT on-board interferometer illuminates the AS unit from theF/15 telescope focal plane. LBT672 has an ellipsoidal shape (gregorian) and produces a spot on the primary focalplane location (F/1.1). A confocal fast relay optics, located in the shadow of the central obscuration, collimatesthe beam and produces an image of the AS shell on a flat mirror. The beam path is reversed relaying the ASmirror (the entrance pupil in this layout) on the same physical plane of the shell, i.e. locating the exit pupilon the same plane where it is located on the telescope. The beam is split close to the F/15 focal plane to feed



Figure 4. 14m-vertical bench for optical test of first light adaptive system for LBT. (a) and (b): view of the bench insidethe solar tower of Arcetri. (c): Interface to the bottom end of the bench for coupling with the W@LBT wavefront sensorunit.(d): step response of temperature of the cooling/heating fluid. On steady state temperature fluctuations are within±0.1 C. Ambient temperature changed of 14 degree during the measurement.

the interferometer and the wavefront sensor. The alignment between the nominal focal point at the wavefrontsensor level and the fast relay optics is obtained using the fine positioning provided by the hexapod (≈ 10 nmresolution).

The optical path volume is controlled in temperature to avoid convection. Temperature stabilization isobtained controlling in closed loop the temperature of a water-glycol mixture that flows in the tube wall. Thethermal control system is currently under test showing a stability of ±0.1 C during steady state regardless toambient temperature variation (see Fig 4d/e). Top to bottom bench temperature difference is less then 1 C withtop always warmer than bottom to avoid convection.

4. THE THIN SHELL STAND-BY SYSTEM

As part of the status report we present in this section the studies on aspects of system design (namely regardingthe support of the shell) that we have developed after the last public report of the project status.10

When the AS is mounted on the telescope but it is not operative, the safest location of the shell is obtainedpulling it against the reference plate. That avoids concentrated stresses in the shell close to the location ofsupport clips. Those stresses are critical for the LBT shell with respect to the MMT one, because the formeris thinner (1.6 mm vs 2.0 mm) and larger (911 mm vs 642 mm), accumulating larger stresses under gravitydeformations when supported by edge clips. Moreover pushing the shell against the reference plate avoids dustcontamination in the gap between them and finally avoids dangerous wandering of the shell in case of wind gustsand gravity load variation during telescope movement. The project baseline uses the TSS (thin shell stand-by)system to inject a bias current through the coils corresponding to 1.5 times the shell weight and pull the glasseven when the control electronics is switched off. We studied a pure passive alternative of TSS, inserting a biasmagnet in the actuator body close to each coil performing the same static bias force. In this case the shell safetyis guaranteed even in case of total disconnection of power lines (e.g. during lightning storms). The effect ofmisalignment of bias magnets with respect to shell magnets could give, in principle, local torque on the shell and



Figure 5. Left side: relative geometry of bias magnet and shell magnet. Right side: top view of shell magnet

cause a local bending that cannot be fully compensated by the actuators. A first analysis of this problem hasbeen studied simulating the interaction of the magnets in case of misalignment.

The magnetic field produced by a magnetized object is equivalent to the field generated by bound surfaceKb and volume currents Jb as follows:

Kb = M × n̂ (1)

Jb = ∇×M (2)

The shell magnets are produced assembling components with uniform magnetization. Inside each componentthe magnetization M is constant giving Jb = 0 and the magnetic field B can be calculated in any point of thespace as follows:

B(r2) =µ0

4π

∫∫S1

Kb(r1) × (r2 − r1)|r2 − r1|3 dS1 (3)

The forces and moments of the magnetic field exerted on a magnetic surface are given by:

F =∫∫

S2

Kb(r2) ×B(r2)dS2 (4)

�τ =∫∫

S2

r2 × [Kb(r2) ×B(r2)] dS2 (5)

.

In order to optimize the magnetic coupling between the shell magnet and the corresponding coil,14 the magnethas been designed as a cylindrical magnetic core and eight prisms with trapezoidal base with magnetizationvectors as shown in Fig. 5. The bias magnet, located inside the head of the actuator cold finger, is simply acylindrical magnet. The bias magnet size and distance with respect to the shell magnet have been dimensionedin order to apply the target pulling force of 1.5 times the weight per actuator.

The magnetic forces and moments can be computed by numerical integration of Eqs.3, 4 and 5. Datasheetsof the used magnetic materials give the following maximum magnetization values:

• Ms = 1.39µ0

: the nominal magnetization of petals (Vacodym 510) [A/m];

• Mc = 1.39µ0

: the nominal magnetization of the core cylindrical magnet (Vacodym 510) [A/m];

• M = 1.03µ0

: the nominal magnetization of the bias cylindrical magnet [A/m].



(a)(b)

0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3 0.32−0.08

−0.06

−0.04

−0.02

0

0.02

0.04

0.06

0.08

isocontours of u⊥ [nm]: detail of one of the three stretched areas (the ×’s indicate the actuators)

−80

−60

−60

−40

−40

−20

−20

−20

0 0

0

0

0

0

0

0

0

00

0

0

0

0

00

0

0

0

0

0

0

20

20

2040

40

60

6080

Figure 6. (a) Measured and simulated forces in function of the coil-to-magnet gap.(b) residual surface deformationinduced by 4.5 Nmm torque in the mean vain of shell on the location of a median ring actuator

ADS provided the measurements of the interaction forces between two bias magnets and a bias magnet with ashell magnet. Comparing the above forces with the numerical values obtained by the simulation we computed theeffective values of magnetization. The effective magnetization is only 0.3% lower with respect to the maximumvalue for bias magnets, while the shell magnet has a 22% lower effective magnetization. The reduction of theeffective magnetization of shell magnets is probably due to small defects in magnetization of petals and thepresence of the finite thickness of glue between the components that has not been considered in the simulation.In any case the scatter of the actual magnetization value is quite small12 (5%) assuring the requested uniformityof the bias forces. Fig. 6a shows the measured and numerical force (considering the effective magnetization)between the bias and the shell magnets in function of the gap between the bottom end of the coil enclosure andthe shell magnet upper surface (0.2 mm nominal).

The calibration of the effective magnetization allows to compute, using Eq. 5, the torque generated bymisalignment due to decentering between the axes of the two magnets and the tilt of the bias magnet. In theexpected range of decentering d (±0.4 mm) the torque is linear with respect to d with a slope of 0.12 Nmm/mm.The effect of the tilt is negligible. We estimated the effect of such a torque applied to the mean vane of the shellwith finite element analysis using FEMLAB software. In the analysis the actuators are used to compensate forthe deformation. Only the residual error of the compensation is relevant. The result is the butterfly shape inFig. 6b. It has been obtained applying 4.5 Nmm of tangential torque at the location of one of the central ring ofactuators. The scaling of the surface error peak-to-valley to the maximum expected torque (0.048 Nmm) gives asurface residual error of 2.3 nm. The small value of the residual suggests that the use of bias magnets is a validalternative to the active TSS system with all the advantages of a passive solution.



5. CONCLUSIONS

In the present paper we gave an up-to-date description of the two adaptive secondary units for the LargeBinocular Telescope (LBT672a/b). We also reported an overview of the current progress in the various aspectsof the project: mechanics, electronics, optics, software and test facilities. We presented also an alternative passivesolution to the active ”thin shell stand-by” (TSS) system in order to avoid to apply critical stresses close to thesupport clips of the shell in case the active TSS system failure or in case it has to be switched-off (i.e. in case oflighting storms).

REFERENCES

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5. W. Gaessler, “LINC-NIRVANA: how to get a 23-m wavefront nearly flat,” in Advancements in adaptiveoptics. Edited by D. Bonaccini, B. Ellerbroek and R. Ragazzoni, Proc. SPIE 5490, 2004. in press.

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8. M. A. Kenworthy, D. L. Miller, G. Brusa, P. M. Hinz, D. L. Fisher, M. Lloyd-Hart, F. P. Wildi, D. W.McCarthy, D. L. Dylan, C. Kulesa, P. A. Young, B. D. Oppenheimer, W. Liu, M. R. Meyer, and J. Greissl,“Scientific results from the MMT natural guide star adaptive optics system,” in Advancements in adaptiveoptics. Edited by D. Bonaccini, B. Ellerbroek and R. Ragazzoni, Proc. SPIE 5490, 2004. in press.

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10. A. Riccardi, G. Brusa, P. Salinari, S. Busoni, O. Lardiere, P. Ranfagni, D. Gallieni, R. Biasi, M. Andrighet-toni, S. Miller, and P. Mantegazza, “Adaptive secondary mirrors for the Large binocular telescope,” inAstronomical Adaptive Optics Systems and Applications. Edited by Tyson, Robert K.; Lloyd-Hart, Michael,Proc. SPIE 5169, pp. 159–168, Dec. 2003.

11. R. Biasi, M. Andrighettoni, A. Riccardi, V. Biliotti, L. Fini, D. Gallieni, and P. Mantegazza, “Dedicated flex-ible electronics for adaptive secondary control,” in Advancements in adaptive optics. Edited by D. Bonaccini,B. Ellerbroek and R. Ragazzoni, Proc. SPIE 5490, 2004. in press.

12. D. Gallieni, V. Anaclerio, A. Ripamonti, R. Biasi, M. Andrighettoni, D. Veronese, and W. Ponzo, “LBTadaptive secondary inits construction: a progress report,” in Advancements in adaptive optics. Edited by D.Bonaccini, B. Ellerbroek and R. Ragazzoni, Proc. SPIE 5490, 2004. in press.

13. L. Fini, A. Puglisi, and A. Riccardi, “LBT-adopt control software,” in Advanced software, control, andcommunication systems for astronomy. Edited by L. Hilton and G. Raffi, Proc. SPIE 5496, 2004. in press.

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