GAIA1 is a global space astrometry mission, successor to the Hipparcos mission, launched in 1989. The GAIA spacecraft is being built by EADS Astrium France and is scheduled for launch in 2013. At a distance of 1.5 million km from Earth at Lagrangian point L2, slowly spinning around its axis, GAIA will monitor each target star about 100 times over a 5- year period, precisely measuring its distance, movement, and change in brightness. Through spectrophotometric classification, it will provide the detailed physical properties of each star observed: luminosity, temperature, gravity, and elemental composition. This massive stellar census will provide the basic data to tackle an enormous range of important questions related to the origin, structure, and evolutionary history of our Galaxy. The measurements performed with GAIA will be accurate to 24 microarcsec, about 100 times more accurate than Hipparcos. To achieve this extreme accuracy at an operational temperature of 100 K, the entire GAIA Payload is made out of Silicon Carbide (SiC).
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.
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.
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.
TNO has developed a compact BreadBoard (BB) cryogenic Optical Delay Line (ODL) for use in future space
interferometry missions such as ESA's Darwin and NASA's TPF-I. The breadboard delay line is representative of a
flight mechanism. The optical design is a two-mirror cat's-eye. A linear guiding system based on magnetic bearings
provides frictionless and wear free operation with zero hysteresis. The delay line has a voice coil actuator for single stage
Optical Path Difference (OPD) control. The verification program, including functional testing at 40 K, has been
completed succesfully.
A Darwin precursor breadboard, comprising both fine lateral and longitudinal metrology sensors was designed, built and partially tested. The lateral metrology sensor was designed and built by TNO TPD and more than meets the imposed requirements. The longitudinal metrology sensor consists of a dual wavelength interferometer with an integrated delay line for optical path stabilisation. Here TNO TPD supplied the delay line and implemented the optical path difference stabilisation control. Experiments under ambient conditions show that noise reduction up to five orders of magnitude is achievable.
A test-bench simulating the GAIA metrology system has been developed by TNO-TPD. The lines-of-sight of the two telescopes in the GAIA instrument are separated by an angle (called `basic angle') of about 106 degrees. The basic angle stability should be within 10 microarcsecond rms over the satellite revolution period of 3 hours, or should be at least known with this accuracy. The basic angle is monitored by a high-precision interferometric laser metrology system.
A Refocusing Mechanism (REM) has been designed and built for the SEVIRI instrument, the imager in the Meteosat Second Generation (MSG) satellite. The purpose of the mechanisms is in-orbit focal adjustment from its geostationary assembly which forms part of SEVIRI. As the MSG satellite observes the earth and its atmosphere from its geostationary orbit, the REM will be used to compensate for satellite structural distortions due to gravitational-, thermal- and aging effects. It is designed for frequent use. It enables in- orbit refocusing in 1.4 micrometers steps over a range of 2 mm with virtually no spurious rotations and translations. The REM was designed to support a large 3.9 kg mirror module in an ARIANE 5 rocket launch environment without the use of clamping mechanisms. Further major design drivers were the limited volume and the strict mass requirement. The REM function is realized in two modules. A guide module, based on elastic elements provides the required stroke within the strict requirements on lateral deviation and tilt. An actuator module with a motor, a planetary gearbox and a recirculating rollerscrew provides the force for translation of the mirror assembly. Coupling of the two modules has been realized by a dedicated leverarm structure.
A key technique for optical aperture synthesis instruments is the cophasing of the beams from the individual apertures. A cophasing system includes a number of delay lines for the adjustment of the optical path lengths. The design of a high-resolution delay line is based on the mission requirements of a space-borne interferometer. A cat's-eye type of retroreflector is the selected option for the optics. Three delay lines, including actuators and control electronics, have been built and have been tested in a dedicated setup, which includes a star simulator and a cophasing interferometer. The test results support the feasibility of the presented delay line concept.
The Optical Aperture Synthesis Technologies project is aimed at the development of technologies for space interferometry. One of the key technologies is the path length stabilization (or cophasing) of light beams from a guide star. A setup has been developed for testing a system for controlling the cophasing (a delay line) in combination with a system for measuring the cophasing (a cophasing interferometer). The light from a simulated guide star passes a delay line system and it is imaged by the cophasing interferometer on a detector. From the captured (white light) fringe patterns the optical path difference (OPD) is determined in real time. The OPD data are used to drive the delay lines in a control loop system for fast stabilization of the optical paths to a sub-wavelength accuracy. The major design drivers for the testbed development were the optimal mechanical and thermal stability (nanometer level), the control loop bandwidth, the OPD stabilization accuracy and the wavefront quality. The instrumentation (optomechanical breadboard and control system) that has been developed for this project is described, and a first set of test results.
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