SPICA (SPace Infrared telescope for Cosmology and Astrophysics, see Figure 1) is a proposed next generation space infrared observatory. The mission will study formation of planets, solar system processes, and the origin of the universe. SPICA is an international project, led by the Japanese space agency JAXA, with contributions from Europe. SPICA has been selected as a candidate ESA M-class Cosmic Vision mission. SPICA will have a single 3.5 m mirror operating at 4.5 Kelvin. The wavelength range will cover 5 to 210 =m. The SAFARI (SpicA FAR infrared Instrument) imaging FTS (Fourier Transform Spectrometer) is one of five anticipated focal plane instruments. It will operate in the wavelength range of 35 to 210 =m. The proposed European contributions to SPICA will consist of the telescope subsystem, the ground segment and SAFARI. SAFARI will be financed through national contributions. The Rutherford Appleton Laboratory (RAL) in the UK has been leading the Phase A study for this European SAFARI spectrometer. Currently SRON in the Netherlands has taken over this leading role from RAL. SAFARI contains an Optical Delay Line (ODL) scan mechanism: FTSM. TNO has been participating in a SPICA SAFARI study, led by SRON, to support the RAL SPICA SAFARI Phase A investigations. The objective of this study was to increase the overall SAFARI TRL and improve the chances for SPICA in the ESA Cosmic Vision down selection. TNO participates in the optical design trade off between the SAFARI instrument and the FTS scan mechanism and leads the preliminary design of the FTS scan mechanism. Micromega- Dynamics contributed to the magnetic bearing design. SRON is developing control electronics for the FTSM. TNO has previously been developing a cryogenic optical delay line mechanism breadboard with magnetic bearings together with Micromega-Dynamics and SRON for the Darwin mission. That heritage was the basis for the development of the SAFARI FTSM.
The mirror segments for the E-ELT and TLT are nearly equal in size and shape (hexagonal, 1.2 m over flat sides). They
are very thin (about 50 mm) compared to their size. Supporting these mirrors and obtaining high optical performance is a
challenge from design and manufacturing point of view. TNO has designed and build (together with VDL-ETG) three
identical prototypes for supporting the mirror segments of the E-ELT. These mirror segments vary in size. Hence the
gravity induced deformation of the mirror segments will vary from mirror to mirror segment when no measures are
taken. The paper will concentrate on the design and analysis of the design features within the support structure to
minimize the mirror deformation due to gravity. These features concern passive and active means to influence the mirror
segment shape and to compensate for deformation differences.
H. van Weers, J. den Herder, B. Jackson, P. P. Kooijman, C. Bruineman, K. Ravensberg, M. Bruijn, B. Rangarajan, A. van der Linden, M. Ridder, M. Leeman, B. van Leeuwen, A. Gotink, S. Kwast, T. van der Velde, J. Diesveld, C. Werner, R. Hamelinck
SRON, Netherlands Institute for Space Research, is developing a Focal Plane Assembly for future missions requiring large-format arrays of Transition Edge Sensors. The up-scaling of the amount of pixels together with the mass and volume limitations for a space instrument requires technology developments in several areas. A dedicated program has been initiated to develop the required magnetic shielding, high density electrical interconnects and a thermal insulating suspension. The purpose of the program is to demonstrate Technology Readiness Level 4-5 for these key-technologies before the end of 2015. In this talk we will present the status of the program, as carried out under ESA GSTP.
TNO, together with its partners, have designed a cryogenic scanning mechanism for use in the SAFARI1 Fourier
Transform Spectrometer (FTS) on board of the SPICA mission. SPICA is one of the M-class missions competing to be
launched in ESA's Cosmic Vision Programme2 in 2022. JAXA3 leads the development of the SPICA satellite and SRON
is the prime investigator of the Safari instrument.
The FTS scanning mechanism (FTSM) has to meet a 35 mm stroke requirement with an Optical Path Difference
resolution of less then 15 nm and must fit in a small volume. It consists of two back-to-back roof-top mirrors mounted on
a small carriage, which is moved using a magnetic bearing linear guiding system in combination with a magnetic linear
motor serving as the OPD actuator. The FTSM will be used at cryogenic temperatures of 4 Kelvin inducing challenging
requirements on the thermal power dissipation and heat leak.
The magnetic bearing enables movements over a scanning stroke of 35.5 mm in a small volume. It supports the optics in
a free-floating way with no friction, or other non-linearities, with sub-nanometer accuracy. This solution is based on the
design of the breadboard ODL (Optical Delay Line) developed for the ESA Darwin mission4 and the MABE mechanism
developed by Micromega Dynamics.
During the last couple of years the initial design of the SAFARI instrument, as described in an earlier SPIE 2010 paper5,
was adapted by the SAFARI team in an evolutionary way to meet the changing requirements of the SPICA payload
module. This presentation will focus on the evolution of the FTSM to meet these changing requirements. This work is
supported by the Netherlands Space Office (NSO).
The mirror segments for the E-ELT and TLT are nearly equal in size and shape (hexagonal, 1.2 m over flat sides). They
are very thin (about 50 mm) compared to their size. Supporting these mirrors and obtaining high optical performance is a
challenge from design and manufacturing point of view. TNO has designed and build (together with VDL-ETG) three
identical prototypes for supporting the mirror segments of the E-ELT. These mirror segments vary in size. Hence the
gravity induced deformation of the mirror segments will vary from mirror to mirror segment when no measures are
taken. The paper will concentrate on the design and analysis of the design features within the support structure to
minimize the mirror deformation due to gravity. These features concern passive and active means to influence the mirror
segment shape and to compensate for deformation differences.
ESO is preparing to build the E-ELT which will be the largest optical telescope in the world. Its primary mirror will be
(see manuscript)39.3 m and will consist of nearly 1000 hexagonal segments of 1.2 meter width. Each segment is to be supported by the
same structure although the segments vary in size and shape, hence this will cause variable surface deflection. Its support
structure must guarantee a maximum surface form error of 30 nm rms independent of the segment orientation and
environmental temperature. Measures, such as mass balancing and active surface form compensation are needed to
compensate for these effects.
To meet the main optical and mechanical performance requirements, numerous FEM analysis runs have been made.
Dedicated software was used to calculate mirror deformations under mechanical loads and to express these in Zernike
modes. Especially this tool proved to be very powerful in proving that the optical surface form requirements could be
met for all different sized mirror segments.
The largest optical telescope in the world will be the E-ELT. Its primary mirror will be 42m in diameter. This mirror will
consist of 984 hexagonal segments that are all individually supported. Each mirror will be controlled in six DOF while
local shaping of the segments is provided by so called warping harnesses. These will correct for focus, astigmatism and
trefoil. Hence a mirror with an extreme diameter to thickness ratio of almost 30 is obtained. Its support structure must
guarantee a maximum surface form error of 30 nm rms independent of the segment attitude. Furthermore its stiffness to
mass ratio must allow natural frequencies of 50Hz or higher to obtain sufficient bandwidth for the actuators that control
the piston and tip/tilt of the segment.
Designing such structure is a challenge that has been successfully completed. Three prototypes have been built and are
about to be delivered to ESO. This paper discusses the main performance requirements and how they could be
transferred into an elegant structure design. Furthermore an overview will be given on the main performance parameters
in order to see whether the present design can be further optimized.
TNO, together with its partners Micromega and SRON, have designed a cryogenic scanning mechanism for use in the
SAFARI Fourier Transform Spectrometer (FTS) on board of the SPICA mission.
The optics of the FTS scanning mechanism (FTSM) consists of two back-to-back cat's-eyes. The optics are mounted on a
central "back-bone" tube which houses all the important mechatronic parts: the magnetic bearing linear guiding system, a
magnetic linear motor serving as the OPD actuator, internal metrology with nanometer resolution, and a launch lock.
A magnetic bearing is employed to enable a large scanning stroke in a small volume. It supports the optics in a free-floating
way with no friction, or other non-linearities, enabling sub-nanometer accuracy within a single stage with a
stroke of -4 mm to +31.5 mm.
Because the FTSM will be used at cryogenic temperatures of 4 Kelvin, the main structure and optics are all constructed
from 6061 Aluminum. The overall outside dimensions of the FTSM are: 393 x 130 x 125 mm, and the mass is 2.2 kg.
Adaptive Optics is established as essential technology in current and future ground based (extremely) large
telescopes to compensate for atmospheric turbulence. Deformable mirrors for astronomic purposes have a high
number of actuators (> 10k), a relatively large stroke (> 10μm) on a small spacing (< 10mm) and a high control
bandwidth (> 100Hz).
The availability of piezoelectric ceramics as an actuator principle has driven the development of many adaptive
deformable mirrors towards inappropriately stiff displacement actuation. This, while the use of force actuation
supersedes piezos in performance and longevity while being less costly per channel by a factor of 10-20.
This paper presents a model which is independent of the actuator type used for actuation of continuous
facesheet deformable mirrors, to study the design parameters such as: actuator spacing & coupling, influence
function, peak-valley stroke, dynamical behavior: global & local, etc. The model is validated using finite element
simulations and its parameters are used to derive design fundamentals for optimization.
A new prototype adaptive deformable mirror for future AO-systems is presented that consists of a thin continuous
membrane on which push-pull actuators impose out-of-plane displacements. Each actuator has ±10μm stroke,
nanometer resolution and only mW's heat dissipation. The mirror's modular design makes the mechanics,
electronics and control system extendable towards large numbers of actuators. Models of the mirror are derived
that are validated using influence and transfer function measurements. First results of a prototype with 427
actuators are also presented.
In the design of a large adaptive deformable membrane mirror, variable reluctance actuators are used. These consist of a
closed magnetic circuit in which a strong permanent magnet provides a static magnetic force on a ferromagnetic core
which is suspended in a membrane. By applying a current through the coil which is situated around the magnet, this force
is influenced, providing movement of the ferromagnetic core. This movement is transferred via a rod imposing the out-of-plane
displacements in the reflective deformable membrane. In the actuator design a match is made between the negative
stiffness of the magnet and the positive stiffness of the membrane suspension. If the locality of the influence functions,
mirror modes as well as force and power dissipation are taken into account, a resonance frequency of 1500 Hz and an
overall stiffness of 1000 N/m for the actuators is needed. The actuators are fabricated and the dynamic response tested in a
dedicated setup. The Bode diagram shows a first eigenfrequency of 950 Hz. This is due to a lower magnetic force than
expected. A Helmholtz coil setup was designed to measure the differences in a large set of permanent magnets. With the
same setup the 2nd quadrant of the B-H curve is reconstructed by stacking of the magnets and using the demagnetization
factor. It is shown that the values for Hc and Br of the magnets are indeed lower than the values used for the initial design.
New actuators, with increased magnet thickness, are designed and currently fabricated.
Future large optical telescopes require adaptive optics (AO) systems whose deformable mirrors (DM) have ever
more degrees of freedom. This paper describes advances that are made in a project aimed to design a new AO
system that is extendible to meet tomorrow's specifications. Advances on the mechanical design are reported in
a companion paper [6272-75], whereas this paper discusses the controller design aspects.
The numerical complexity of controller designs often used for AO scales with the fourth power in the diameter
of the telescope's primary mirror. For future large telescopes this will undoubtedly become a critical aspect.
This paper demonstrates the feasibility of solving this issue with a distributed controller design. A distributed
framework will be introduced in which each actuator has a separate processor that can communicate with a few
direct neighbors. First, the DM will be modeled and shown to be compatible with the framework. Then, adaptive
turbulence models that fit the framework will be shown to adequately capture the spatio-temporal behavior of
the atmospheric disturbance, constituting a first step towards a distributed optimal control. Finally, the wavefront
reconstruction step is fitted into the distributed framework such that the computational complexity for
each processor increases only linearly with the telescope diameter.
A large adaptive deformable mirror with high actuator density is presented. The DM consists of a thin continuous membrane which acts as the correcting element. A grid of low voltage electro-magnetical push-pull actuators, - located in an actuator plate -, impose out-of-plane displacements in the mirror's membrane. To provide a stable and stiff reference plane for the actuators, a mechanically stable and thermally decoupled honeycomb support structure is added. The design is suited for mirrors up to several hundred mm with an actuator pitch of a few mm.
One of the key elements in the design is the actuator grid. Each actuator consists of a closed magnetic circuit in which a strong permanent magnet (PM) attracts a ferromagnetic core. Movement of this core is provided by a low stiffness elastic guiding. A coil surrounds the PM. Both the coil and the PM are connected to the fixed world. By applying a current through the coil, the magnetic force acting on the core can be influenced. This force variation will lead to translation of the ferromagnetic core. This movement is transferred to the reflective mirror surface in a piston-free manner. The design allows for a long total stroke and a large inter actuator stroke. The actuators are produced in arrays which make the design modular and easily extendable.
The first actuators and an actuator grid are produced and tested in a dedicated test set-up. This paper describes how relevant actuator properties, such as stiffness and efficiency, can be influenced by the design. The power dissipation in the actuator grid is optimized to a few milliwatts per actuator, thereby avoiding active cooling.
The first prototype of an actuator for a new adaptive deformable mirror (DM) is presented together with the development of a 61-actuator grid element. The DM design consists of a thin membrane which acts as the correcting element. A grid of low voltage electro-magnetical push-pull actuators, impose out-of-plane displacements in the mirror's membrane. To provide a stable and stiff reference plane for the actuators, a mechanically stable and thermally decoupled honeycomb support structure is added.
Several variants of variable reluctance actuators are considered. Each actuator consists of a closed magnetic circuit in which a strong permanent magnet (PM) provides a static magnetic force attracting ferromagnetic material. By applying a current through a coil which is situated around this magnet, this force can be influenced. Depending on the mechanical stiffness of the actuator, this leads to a certain displacement. Both the PM and the coil are connected to the fixed world and only the ferromagnetic material will move. The actuators are produced in arrays which make the design easy and extendable. The power dissipation in the actuator grid is in the order of milliwatts per actuator. Because of this low power dissipation active cooling is not required.
The paper describes how the actuator stiffness and efficiency can be controlled. A test-setup is developed in which the actuator characteristics are measured.
With the future growing size of telescopes, new, high-resolution, affordable wavefront corrector technology with low power dissipation is needed. A new adaptive deformable mirror concept is presented, to meet such requirements. The adaptive mirror consists of a thin (30-50 μm), highly reflective, deformable membrane. An actuator grid with thousands of actuators is designed which push and pull at the membrane’s surface, free from pinning and piston effects. The membrane and the actuator grid are supported by an optimized light and stiff honeycomb sandwich structure. This mechanically stable and thermally insensitive support structure provides a stiff reference plane for the actuators. The design is extendable up to several hundreds of mm's. Low-voltage electro-magnetic actuators have been designed. These highly linear actuators can provide a stroke of 15 micrometers. The design allows for a stroke difference between adjacent actuators larger than 1 micron. The actuator grid has a layer-based design; these layers extend over a large numbers of actuators. The current actuator design allows for actuator pitches of 3 mm or more. Actuation is free from play, friction and mechanical hysteresis and therefore has a high positioning resolution and is highly repeatable. The lowest mechanical resonance frequency is in the range of kHz so a high control bandwidth can be achieved. The power dissipation in the actuator grid is in the order of milliwatts per actuator. Because of this low power dissipation active cooling is not required. A first prototype is currently being developed. Prototypes will be developed with increasing number of actuators.
For high accuracy alignment of optical components in optical instruments TNO TPD has developed dedicated, monolithic, flexure-based alignment mechanisms, which provide accuracies below 0.1 μm or 0.1 μrad as well as stabilities down to tens of picometers per few minutes after locking.
High resolution, high stability alignment mechanisms consist of an adjustment mechanism and a locking device.
Complex monolithic flexure-based mechanisms are designed to align specific degrees of freedom. They are realized by means of spark erosion. The benefits of these mechanisms are no play, no hysteresis, high stiffness, a simplified thermal design, easy to assemble. Using alignment mechanisms a passive system can be maintained.
Locking after alignment is mandatory to guarantee sub-nanometer stability. However, a high accuracy alignment will be disturbed again due to drift during locking if the locking device is not properly designed. Several low-drift locking devices have been designed and developed.
The dedicated alignment mechanisms presented here are based on: (a) the results of an internal ongoing research program on alignment and locking and (b) experience with mechanisms developed at TNO TPD for high precision optical instruments, which are used in e.g. a white light interferometer breadboard (Nulling) and an interferometer with picometer resolution for ESA’s future cornerstone missions “DARWIN” and “GAIA”.
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