the new vlt-dsm m2 unit: construction and...
TRANSCRIPT
Adaptive Optics for Extremely Large Telescopes III
THE NEW VLT-DSM M2 UNIT: CONSTRUCTION AND
ELECTROMECHANICAL TESTING
Daniele Gallieni1,a
, Roberto Biasi2
1A.D.S. International Srl, via Roma 87 – 23868 Valmadrera (LC), Italy
2Microgate Srl, via Stradivari 4 – 39100 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 1120mm diameter and 2mm thick Zerodur shell and Thales-SESO made the light-
weighted 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 e-mail : [email protected]
Third AO4ELT Conference - Adaptive Optics for Extremely Large TelescopesFlorence, Italy. May 2013ISBN: 978-88-908876-0-4DOI: 10.12839/AO4ELT3.17883
Adaptive Optics for Extremely Large Telescopes III
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.
Fig.1. Cutaway rendering of the new VLT-DSM M2 unit.
2.1. The DSM
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.
Third AO4ELT Conference - Adaptive Optics for Extremely Large Telescopes
Adaptive Optics for Extremely Large Telescopes III
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; 1170
permanent magnets are glued on the back surface of the shell; the shell is coated by a un-
protected 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 1170 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 1170 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.
Third AO4ELT Conference - Adaptive Optics for Extremely Large Telescopes
Adaptive Optics for Extremely Large Telescopes III
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 2013 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.
Third AO4ELT Conference - Adaptive Optics for Extremely Large Telescopes
Adaptive Optics for Extremely Large Telescopes III
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 ~0.75ms settling time over all 1170 modes, well
exceeding the 1.5ms base requirement and also the 1.0ms 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.5. DSM real time control scheme.
Third AO4ELT Conference - Adaptive Optics for Extremely Large Telescopes
Adaptive Optics for Extremely Large Telescopes III
Fig.6. DSM dynamic response: 0.75msec settling time (within 90% of command) achieved on all modes
(see bottom left plot).
Fig.7. Dynamic performance: following error time history (left) and zoomed view (right).
Median seeing (0.9” λ=0.5µm) 36nm WF; bad seeing (1.5” λ=0.5µm) 49nm WF
0 5 10 15 20 25 30 35 40 45
-1
0
1
2
3
4
5
6x 10
-6 Mode #0 - Settling: 0.721 [ms] - Oversh: 3.5%
Time [ms]
Positio
n [
m]
0 5 10 15 20 25 30 35 40 45
-1
0
1
2
3
4
5
6x 10
-6 Mode #101 - Settling: 0.721 [ms] - Oversh: 5.7%
Time [ms]
Positio
n [
m]
0 200 400 600 800 1000 12000.64
0.66
0.68
0.7
0.72
0.74
0.76Settling time
Mode #
Tim
e [
ms]
0 5 10 15 20 25 30 35 40 45-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
3x 10
-6 Mode #501 - Settling: 0.672 [ms] - Oversh: 0.1%
Time [ms]
Positio
n [
m]
0 500 1000 1500 2000 2500
6.2
6.25
6.3
6.35
6.4
6.45
6.5
6.55
6.6
x 10-5 Act #840 - Ring #1
Time [ms]
Positio
n [
m]
Position
Command
RTC command
RTC command w.compDelay
2465 2470 2475 2480 2485 2490 2495 2500 2505
6.395
6.4
6.405
6.41
6.415
6.42
6.425
6.43x 10
-5 Act #840 - Ring #1
Time [ms]
Positio
n [
m]
Position
Command
RTC command
RTC command w.compDelay
Third AO4ELT Conference - Adaptive Optics for Extremely Large Telescopes
Adaptive Optics for Extremely Large Telescopes III
Another relevant feature of the DSM is its capability of making relatively large mirror strokes up to 100
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 ±20 arcsec steps and restore itself in the Adaptive Optics mode taking
the shape left at the last command step.
Fig.8. Field stabilization: stroke specification of ±12” (mirror) achieved with 70µm actuators stroke,
64 mas error and 10Hz BW
Fig.9. Chopping: large stroke of ±20” (mirror) achieved with 8ms settling of chopping motion plus restore and
104µm actuators stroke.
5. 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.
0 0.5 1 1.5 2 2.5
-10
-5
0
5
10
Field stabilization
Time [s]
Tilt
[arc
sec]
Acquired tilt
Command
RTC command
RTC command w.compDelay
Acquired tip
1.495 1.5 1.505 1.51 1.515 1.52
-7
-6.9
-6.8
-6.7
-6.6
-6.5
-6.4
-6.3
-6.2
-6.1
Field stabilization
Time [s]
Tilt
[arc
sec]
Acquired tilt
Command
RTC command
RTC command w.compDelay
Acquired tip
0 500 1000 1500 2000 2500
3
4
5
6
7
8
9
10
11
12
13x 10
-5 Act #1054 - Ring #18
Time [ms]
Positio
n [
m]
Position
Command
RTC command
RTC command w.compDelay
920 925 930 935 940
6.45
6.46
6.47
6.48
6.49
6.5
x 10-5 Act #1054 - Ring #18
Time [ms]
Positio
n [
m]
Position
Command
RTC command
RTC command w.compDelay
Third AO4ELT Conference - Adaptive Optics for Extremely Large Telescopes
Adaptive Optics for Extremely Large Telescopes III
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 2013. 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.10. 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, 84470J (September 13,
2012); doi:10.1117/12.926074
[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 2012, 8847-88
[4] M.Manetti et al, Control of Massively Actuated Adaptive Mirrors, in IEEE Transactions
on Control System Technology, ISSN 1063-6536
Third AO4ELT Conference - Adaptive Optics for Extremely Large Telescopes