Research and Training Network – Adaptive Optics for Extremely Large Telescopes
June 2000 – June 2001
Annual Report on Workpackage 4
M. Ferrari1, F. Zamkotsian1, K. Dohlen1, S. Esposito2, C. Del Vecchio2, A. Riccardi2
1
Observatoire de Marseille-Provence
Osservatorio Astrofisico di Arcetri
2
General.
WP 4.1
Purpose – Analyse the deformable mirror (DM) specifications set in WP 1 in terms of
number of actuators, diameter, stroke, bandwidth, position control etc… as well as
the impacts on the technological constraints.
I- Specifications of the Deformable Mirror
A first meeting with ONERA gives preliminary requirements for the adaptive optics
parameters of Extremely Large Telescopes. Discussions have converged towards these
values:
 Number of actuators: > 100 000
 Inter-actuator spacing: 500 µm – 1 mm
 Actuator stroke: 5-10 µm
 Inter-actuator coupling: 20-30 %
 Bandwidth: 1 kHz
Pupil diameter is an overall parameter and for a 100 m primary telescope, the internal
pupil diameter cannot be reduced below 1 m. According to the maximal size of the
wafers (8 inches), a deformable mirror based on MOEMS technology cannot be build
into one piece. New AO architectures are under investigation to avoid this limitation.
The actuator stroke was recently revised as we have to take into consideration the outer
scale. Some discussions were engaged during the Venice conference, but no conclusion
has been yet issued.
II- Micro-Deformable Mirror survey
The required number of actuators (> 100 000) is prohibitive for conventional
technology (stacked piezoelectric actuators, bimorph mirrors), but can be achieved by
the development of new technologies based on micro-opto-electro-mechanical systems
(MOEMS). MOEMS devices are realized by the mature microelectronics process and
they show great potential to reach the specifications required by AO in astronomy. We
have reviewed the three main kinds of micro-deformable mirrors (MDM) which are
under study and illustrate their great potential to reach the required specifications. These
are the bulk micro-machined continuous-membrane MDM, the surface micro-machined
continuous-membrane MDM, and the segmented MDM. All these devices present high
density of actuators and are driven by electrostatic field, but they differ in terms of
surface quality, size of the device, number of actuators, inter-actuator coupling effects,
deformation motion (maximal deflection) and driving voltage. A major advantage of the
MOEMS technology is the possibility to build complete addressing and driving
electronics in an underlying level, under the optical architecture (into the device). This
survey is extensively described in the Survey Report.
RTN – AO-ELT
WP4 Annual Report I
06/02/2016
M. Ferrari
WP 4.2
Purpose: Development of the AO deformable mirror simultaneously by the two
network partners (OM and OAA). 1) MOEMS technology is investigate by OM with a
suitable industrial/laboratory partner. 2) OAA develop an alternative technology
based on electromagnetic actuators and a lower actuator density.
A - MOEMS technology is investigated by OM
Since September 2000, OM has engaged an active collaboration with a French
laboratory expert in micro-technologies, the Laboratoire d’Automatique et d’Analyse
des Systèmes (LAAS) in Toulouse, for the conception and the realization of deformable
mirror prototypes, based on MOEMS technology.
A schematic view of a Micro-Deformable Mirror (MDM) is presented in Figure 1.
The MDM is designed around
three building blocks:



the mirror surface
the actuation mechanism
the driving electronics.
Fig. 1: Schematic view of a Micro-Deformable Mirror (MDM)
Our development plan is split towards two directions for the development of the three
basic building blocks, process (technologies, materials) and architecture of the different
building blocks.
1) Process (technologies, materials)
These investigations concern the structural, the sacrificial and the contact layers
(electrodes)
Process optimization requires precise characterization of the layers in term of thickness
(Scanning Electron Microcope view), layer homogeneity (large scale defaults,
microscopy), surface quality(flatness, roughness) and long term stability. Parallel
studies of these key steps of the MDM process are under way.
We have developed a contact less characterization procedure at LAM in order to
measure the surface quality of the deposited layers at each key step of the process. By
measuring the position of an optical “needle” focused on the surface, we determine the
surface topography and its roughness.
As a test device, we have used a tiltable micro-mirror designed and realized by LAAS,
using the silicon bulk micro-machining process. A SEM view of this tiltable micromirror is shown in Figure 2.
From the characterization, we are able to view a reconstruction of the surface in 3
dimensions (Figure 3).
RTN – AO-ELT
WP4 Annual Report I
06/02/2016
M. Ferrari
Fig. 2: SEM view of LAAS tiltable micro-mirror
Mirror: 3 x 0.8 mm2,
Torsion beam section: 20 x 20 µm2
Fig. 3: 3D surface reconstruction of
the LAAS tiltable micro-mirror
A spherical deformation and an additional astigmatism are visible. The longitudinal sag
is 3.3 µm and the lateral sag 0.8 µm. Deformation is induced by the stress of the
reflective layer. However, the surface roughness is found to be excellent and
measurements show values lower than 3 nm rms.
2) Architecture of the building blocks
Architecture of micro-deformable mirrors is based on three parts, developed hereafter,
and each of them presents technological challenges to be solved.
a) Mirror surface
The goal is to obtain a continuous and smooth surface in order to minimize stray-light
and diffraction effects. We are currently studying Material (without stress) and
Planarization (print-trough effect)
b) Actuation mechanism
Our actuation mechanism is based on electrostatic effects. We are optimizing two key
parameters, the stroke and the driving voltage. The 5-10 µm stroke is a goal and we
hope to obtain our first actuators soon in order to characterize the actuator design and to
up-grade our process. Low driving voltage is not a priority, but our actuators must have
the potential to reach acceptable voltages for a conventional micro-electronics circuit.
c) Driving electronics
As our goal is to control and command a device with several hundred of thousand
actuators, we cannot wire each of them. A dedicated micro-electronics has to be
designed and developped. The success of the Micro-Mirror Array developped by Texas
Instruments indicates that integration of the optical architecture and the driving
electronics is the most reliable approch. However, further research has to be done for:
 Underlying electronics design
 Compatibility with the optical architecture process
We are currently focusing our efforts on the two first parts, the mirror surface and the
actuation mechanism.
RTN – AO-ELT
WP4 Annual Report I
06/02/2016
M. Ferrari
B - Alternative electromagnetic technology developed by OAA
The adaptive secondary mirror for the 6.5 conversion of the Multiple Mirror Telescope
Laboratory tests were performed on the non-conventional secondary mirror that will be
used as corrector for the adaptive optics system of the 6.5m conversion of MMT. The
design of the unit consists of a 2mm-thick 642mm-diameter convex deformable mirror
whose figure is controlled by 336 electro-magnetic force actuators, a thick shell used as
position reference and an aluminium shell as actuator support and cooling. The actuator
forces are controlled using feed-forward and de-centralized closed loop compensation
thanks to the 40kHz feedback signals from the 336 capacitive position sensors. The
digital real-time control and the unit monitoring is obtained using custom-made onboard electronics based on 168 DSPs. Results were presented in the Venice
International Conference ”Beyond conventional Adaptive Optics”. The paper address
the extensive static and dynamical measurements aimed at characterize the mirror
response function, that shows a proper atmospheric turbulence compensation.
The LBT adaptive secondary mirror:
Enhancements in the magnetic, mechanical and thermal design
Several simulations of the deformable mirror of the secondary unit have been run in the
past, in order to determine the parameters able to achieve the best performances in the
various static and adaptive operations of the mirror. The experience gained by those
analyses allowed to freeze the magnetic circuit design, the actuator geometry, and the
thickness profile of the mirror. These choices, now included in the telescope design
baseline, were presented in the Venice International Conference ”Beyond conventional
Adaptive Optics”. The paper describe the improvements of the new magnetic,
mechanical, and thermal design.
Meetings
1. Paris (ONERA), June 2000
The meeting was attended by representatives from the Observatoire de Marseille, and
ONERA. Topics addressed:
i.
Each team made a presentation of relevant research work:
- Adaptive Optic Systems and ELTs requirements (ONERA)
- Developments in MOEMS technologies (OM)
ii.
The various problems encountered with AO for ELTs were discussed:
- number of actuators stroke, density, coupling, bandwidth etc ..
iii.
Possible solutions suitable for ELTs were considered.
2. Florence, January 2001
There was a session on WP4 at the general meeting of the RTN, held in Florence,
January 2001. The following presentations were made:
i.
Introduction and planning (M. Ferrari)
ii.
MOEMS: Test procedures and key technologies steps (F. Zamkotsian)
iii.
Deformable secondary: Results from LBT secondary
iv.
Work plan/discussion (M. Ferrari)
RTN – AO-ELT
WP4 Annual Report I
(S. Esposito)
06/02/2016
M. Ferrari
3. Venice, May 2001
Discussions between participants in WP4 took place during the International
Conference “Beyond conventional Adaptive Optics” held in Venice (7-10 May 2001).
The following papers relative to the WP4 topics were presented:
P. Dierickx (ESO)
Optical Design and adaptive optics properties of the OWL-100m telescope
J. Farinato, E. Fedrigo, E. Marchetti (ESO), R.Ragazzoni (OAPD)
More DMs (and higher Sthrel) in layer-oriented for free
Riccardi, A. et al. (OAA)
The adaptive secondary mirror for the 6.5 conversion of the Multiple Mirror
Telescope
Frederic Zamkotsian, Kjetil Dohlen, Marc Ferrari (OM)
Prospects for MOEMS - based adaptive optics on extremely large telescopes
J. Charton , E. Stadler, J.-L Beuzit, P. Kern, W. Schwartz, J. Margail, P. Puget
Micromachined deformable mirrors for adaptive optics
C. Del Vecchio, D. Gallieni, B. Martin, A. Riccardi, G. Brusa (OAA)
The LBT adaptive secondary mirror: enhancements in the magnetic, mechanical
and thermal design
Post-Docs
During this first year, a young researcher (post-doc) in the micro-electronics domain,
was planned to begin his work on MOEMS technology in close collaboration with OM
and an industrial/laboratory partner. As this year has been dedicated to the technology
survey and the identification of the suitable partner, decision was taken to postpone the
opening of this position to the second year. This will allow to offer a better research
environment, and a strong collaboration between OM and the micro-technology partner,
to the young researcher. Post-doc position will start in 2002, for about 6 months in 2nd
RTN year.
2nd Year Planning
- WP meeting before end 2001
- Opening post-doc position on micro-technologies beginning 2002
- MOEMS preliminary design prototype in 2002
RTN – AO-ELT
WP4 Annual Report I
06/02/2016
M. Ferrari