of 16
Optimized actuators for ultrathin deformable
primary mirrors
M
ARIE
L
ASLANDES
,
1
K
EITH
P
ATTERSON
,
2
AND
S
ERGIO
P
ELLEGRINO
1,
*
1
California Institute of Technology, 1200 E. California Blvd, Pasadena, California 91125, USA
2
Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California 91109, USA
*Corresponding author: sergiop@caltech.edu
Received 7 January 2015; revised 10 April 2015; accepted 16 April 2015; posted 17 April 2015 (Doc. ID 231885); published 20 May 2015
A novel design and selection scheme for surface-parallel actuators for ultrathin, lightweight mirrors is presented.
The actuation system consists of electrodes printed on a continuous layer of piezoelectric material bonded to an
optical-quality substrate. The electrodes provide almost full coverage of the piezoelectric layer, in order to maxi-
mize the amount of active material that is available for actuation, and their shape is optimized to maximize the
correctability and stroke of the mirror for a chosen number of independent actuators and for a dominant im-
perfection mode. The starting point for the design of the electrodes is the observation that the correction of a
figure error that has at least two planes of mirror symmetry is optimally done with twin actuators that have the
same optimized shape but are rotated through a suitable angle. Additional sets of optimized twin actuators are
defined by considering the intersection between the twin actuators, and hence an arbitrarily fine actuation pattern
can be generated. It is shown that this approach leads to actuator systems with better performance than simple,
geometrically based actuators. Several actuator patterns to correct third-order astigmatism aberrations are
presented, and an experimental demonstration of a 41-actuator mirror is also presented.
© 2015 Optical
Society of America
OCIS codes:
(110.1080) Active or adaptive optics; (110.1220) Apertures; (110.6770) Telescopes.
http://dx.doi.org/10.1364/AO.54.004937
1. INTRODUCTION
Deformable mirrors are able to correct the wavefront shape in
optical instruments for a variety of applications, including
astronomy [
1
], high-energy lasers [
2
], microscopy [
3
], and oph-
thalmology [
4
]. Each application has different requirements, in
terms of precision of correction and the amplitude, spatial
frequency and temporal frequency of the wavefront error to
be corrected [
5
].
There are three basic technologies for deformable mirrors
[
6
]: surface-normal actuation, surface-parallel actuation, and
boundary actuation.
Surface-normal
actuators apply forces
perpendicular to the optical surface; an array of push/pull
actuators produces local displacements and slopes [
7
]by
reacting against a backing structure. It is an efficient solution
to compensate for relatively high spatial frequencies and low
amplitude errors.
Surface-parallel
actuated systems consist of
an active material laminated to a mirror face-sheet; the in-plane
stretching or contraction of the active material causes the
mirror to bend [
8
11
]. This solution does not require any
backing structure and hence is suited to lighter mirrors and
to the correction of larger amplitude errors. Alternative
implementations have adopted discrete actuators embedded
in a lightweighted structure [
12
]. Systems with
boundary
actuators
use forces and moments along the edge of the
mirror to bend the optical surface [
13
]. This approach mini-
mizes the actuator print-through and is ideal to compensate
for low spatial frequency errors with a relatively small number
of actuators.
Active primary mirrors in earth-based telescopes have al-
ready enabled the emergence of very large apertures [
14
]
and the future development of larger space-based observatories
will require novel active primary mirror technologies [
15
]. This
paper presents the further development of a recently proposed
concept for thin deformable mirrors that promises to drastically
reduce the mass, density, and cost of future telescopes [
10
].
Mirrors based on this approach are lightweight, relatively in-
expensive, and provide a sufficiently large shape correction
capability to allow the use of nominally identical, spherical mir-
ror segments in large segmented primary apertures. Earlier
studies [
9
] have shown that 1 m diameter spherical segments
forming a 10 m diameter segmented aperture with a focal
length of 10 m would require a correction bandwidth of the
order of 250
μ
m to achieve the required shape in all segments.
Accurate shape control would also allow active compensation
for thermal effects and long-term effects such as creep and aging
of the mirror materials.
Research Article
Vol. 54, No. 15 / May 20 2015 /
Applied Optics
4937
1559-128X/15/154937-16$15/0$15.00 © 2015 Optical Society of America