The chopper is a cryogenic tip-tilt mechanism in the Common Fore Optics (CFO) of METIS, the mid-infrared imager and spectrograph for ESO’s Extremely Large Telescope (ELT). In this paper we delve into the milling process of the mirror of the chopper mechanism, denominated as CM5. This highly accurate mirror is fully integrated into a tip-tilt mechanism and operates to reduce the background noise by shifting between the target object and the background, this happens with a frequency of 5Hz. We will primarily focus on the challenges encountered during the mirror-mechanism fabrication. Notably, the tip/tilt mechanism in the chopper requires precision levels that surpass the accuracy of the 5-axis milling machine that is used. The general accuracy of the used machine on a multi sided part is typically 20 to 30 microns, and is dominated by thermal drift of the machine itself. This significantly increases the manufacturing complexity, therefore a specialized production methodology was devised to ensure the achievement of the micron-level tolerances. In the next paragraphs we will describe how the critical tolerances of down to 5 microns in the mechanism features have been achieved. The use of a so-called Golden Sample provides a known and stable reference during manufacturing, allowing to bypass the drift and static errors of the machine. The usage of CMM measurements proves that the developed production method delivers excellent results overcoming the limitations of the machine accuracy. This monolithic aluminium cryogenic mirror has been produced through in-house CNC programming, milling, and thermal ageing followed by thermal cycling, Single point diamond turning (SPDT) and Wire Electrical Discharging Machining (Wire EDM) by external suppliers. After SPDT of the optical surface, the surface is polished to optical quality and finally a gold coating layer is applied to increase the reflectivity in the desired wavelength.
To achieve superb stability in cryogenic optical systems, NOVA-ASTRON generally designs optical instruments on the basis of a 'no adjustments' philosophy. This means that in principle no corrections are possible after assembly. The alignment precision and consequently the performance of the instrument is guaranteed from the design, the tolerance analysis and the detailed knowledge of the material behavior and manufacturing process. This resulted in a higher degree of integrated optomechanical-cryogenic design with fewer parts, but with a higher part complexity. The 'no adjustments' strategy is successful because in the end the risk on instrument performance and project delays is much reduced. Astronomical instrument specifications have become more challenging over the years. Recent designs of the European Southern Observatory Very Large Telescope Interferometer (ESO VLTI) 4 Telescope combiner MATISSE include hundreds of optical components in a cryogenic environment. Despite the large number of optical components the alignment accuracy and stability requirements are in the order of nanometers. The 'no adjustments' philosophy would be too costly in this case, because all components would need to meet extremely tight manufacturing specifications. These specifications can be relaxed dramatically if cryogenic mechanisms are used for alignment. Several mechanisms have been developed: a tip-tilt mirror mechanism, an optical path distance mechanism, a slider mechanism, a bistable cryogenic shutter and a mirror mounting clip. Key aspects of these mechanisms are that the optical element and mechanism are combined in a compact single component, driven by e.g. self braking piezo actuators in order to hold position without power. The design, realization and test results of several mechanisms are presented in this paper.
This paper discusses the development, realization and initial characterization of a demonstrator for a cryogenic 'set and forget' deformable mirror. Many optical and cryogenic infrared instruments on modern very and extremely large telescopes aim at diffraction-limited performance and require total wave front errors in the order of 50 nanometers or less. At the same time, their complex optical functionality requires either a large number of spherical mirrors or several complex free-form mirrors. Due to manufacturing and alignment tolerances, each mirror contributes static aberrations to the wave front. Many of these aberrations are not known in the design phase and can only be measured once the system has been assembled. A 'set-and-forget' deformable mirror can be used to compensate for these aberrations, making it especially interesting for systems with complex free-form mirrors or cryogenic systems where access to iterative realignment is very difficult or time consuming.
The mirror with an optical diameter of 200 mm is designed to correct wave front aberrations of up to 2 μm root-mean square (rms). The shape of the wave front is approximated by the first 15 Zernike modes. Finite element analysis of the mirror shows a theoretically possible reduction of the wave front error from 2 μm to 53 nm rms. To produce the desired shapes, the mirror surface is controlled by 19 identical actuator modules at the back of the mirror.
The actuator modules use commercially available Piezo-Knob actuators with a high technology readiness level (TRL). These provide nanometer resolution at cryogenic temperatures combined with high positional stability, and allow for the system to be powered off once the desired shape is obtained. The stiff design provides a high resonance frequency (>200 Hz) to suppress external disturbances.
A full-size demonstrator of the deformable mirror containing 6 actuators and 13 dummy actuators is realized and characterized. Measurement results show that the actuators can provide sufficient stroke to correct the 2 μm rms WFE. The resolution of the actuator influence functions is found to be 0.24 nm rms or better depending on the position of the actuator within the grid. Superposition of the actuator influence functions shows that a 2 μm rms WFE can be accurately corrected with a 38 nm fitting error. Due to the manufacturing method of the demonstrator an artificially large print-through error of 182 nm is observed. The main cause of this print-through error has been identified and will be reduced in future design iterations. After these design changes the system is expected to have a total residual error of less than 70 nm and offer diffraction limited performance (λ14) for wavelengths of 1 μm and above.
EMIR1,2 (Espectrógrafo Multiobjeto Infra-Rojo) is a wide field multi-object spectrograph already installed in the Nasmyth focus of GTC (Gran Telescopio Canarias). It operates in the near-infrared (NIR), in the wavelength range from 0.9 μm to 2.5 μm and it will include several mechanism working in cryogenic conditions.
A key component of EMIR is the CSU (Configurable Slit Unit), which is a robotic cryo-mechanism used to generate a multi-slit configuration and a long slit on EMIR focal plane when working in spectroscopic mode. The system has 110 sliding bars which can be configured at cryogenic working temperature to create up to 55 slits with a high position accuracy and repeatability. The movement of the bars is performed by an actuator which allows reaching a relatively high speed for the coarse movement and controllable steps up to 2 microns for the fine positioning. This subsystem has been designed and manufactured by the Dutch company Janssen Precision Engineering (JPE) and the Spanish company NTE-SENER. Afterwards, it was thoroughly verified at the IAC (Instituto de Astrofísica de Canarias) facilities.
In this paper, the CSU will be briefly described. One of the more important parts of the CSU is the actuators, which move the bars by means of a stick-slip effect. A set of tests designed for characterizing and improving the robustness and performance of the actuators will be presented. Finally, an overview of the current CSU performance will be presented.
The Mid-infrared E-ELT Imager and Spectrograph, or METIS, is foreseen as an early instrument for the European
Extremely Large Telescope (E-ELT). A key part of METIS is the Cold Chopper (MCC) which switches the optical beam
between the target and a nearby reference sky during observation for characterization of the fluctuating IR background
signal in post-processing. This paper discusses the development and characterization of the realized MCC demonstrator.
The chopper mirror (Ø64mm) should tip/tilt in 2D with a combined angle of up to 13.6mrad with 1.7μrad stability and
repeatability within 5ms (95% duty cycle at 5Hz) at 80K. As these requirements cannot be met in the presence of friction
or backlash, the mirror is guided by a monolithically integrated flexure mechanism. The angular position is actuated by
three linear actuators and measured by three linear position sensors, resulting in a fast tip, tilt, and focus mirror. Using
the third actuator introduces symmetry, and thus homogeneity in forces and heat flux.
In an earlier paper, Ref. [1], the design of the chopper and the breadboard level testing of the key components were
discussed. Since then, the chopper design has been revised to implement the lessons learned from the breadboard test and
a demonstrator has been realized. This demonstrator has undergone an elaborate test program for characterization and
performance validation in a cryogenic environment, as discussed in this paper.
The Configurable Slit Unit (CSU) is a key module of EMIR (wide field NIR multi-object spectrograph) which will be one of the key next generation instruments of the Gran Telescopio de Canarias (GTC). The CSU enables a multi-slit configuration, a long slit, or an imaging aperture in the 6’x6’ (340mm x 340mm) field of view. This is realized by 110 sliding bars which can be configured at cryogenic working temperature to create 55 slits with a position accuracy of 6 micron. The CSU incorporates a number of enabling technologies which have been developed, validated and matured as a part of the total development of the CSU. Dedicated actuator drive and position measurement technologies have been successfully developed. Also a selective surface treatment technology, to give detailed features on the same part opposite emissivity performances, has been developed. All these technologies are currently implemented in the realization of the unit. Manufacturing of components for the unit has challenged state of the art production equipment and skills to the limit due to the size, number, accuracy and complexity of the parts and features. Integration and verification of the CSU is advancing. Both mechanics as electronics have been tested at sub-module level. Ahead is the challenge of actual integration of the electronics and software in order to get the mechanical hardware to operate within specification. Control strategies are developed and tuned to guarantee robust operation of the unit in cryogenic working environment. As a final integration step all individual axes are calibrated with an external interferometer measurement system.
The Mid-infrared E-ELT Imager and Spectrograph, or METIS, is foreseen as the third instrument for the European
Extremely Large Telescope (E-ELT). A key part of METIS is the Cold Chopper (MCC) which switches the optical beam
between the target and a nearby reference sky during observation for elimination of the fluctuating IR background signal
in post-processing. This paper discusses the development of the MCC demonstrator.
The chopper mirror (Ø64mm) has to tip/tilt in 2D with a combined angle of up to 13.6mrad with 1.7μrad stability and
repeatability within 5ms (95% duty cycle at 5Hz) at 80K. As these requirements cannot be met in the presence of friction
or backlash, the mirror is guided by a monolithically integrated flexure mechanism. The angular position is actuated by
three linear actuators and measured by three linear position sensors, resulting in a fast tip, tilt, and focus mirror. Using
the third actuator to introduce symmetry, homogeneity in forces and heat flux is obtained.
Both the actuators and the sensors are key components. A voice coil actuator had to be custom designed, to achieve the
required acceleration force within the specified 1W heat load. The requirements for the displacement measurement can
be met with a commercially available, fiber interferometry system. For integration of this system, stray light elimination
is a critical design aspect and retro-reflectors have been used to reflect sufficient power into the fiber at large tip/tilt
angles.
Cryogenic mechanisms are needed for the alignment plan of MATISSE, a mid-infrared spectro-interferometer for the
European Southern Observatory Very Large Telescope Interferometer (ESO VLTI) that combines up to four Unit
Telescopes or Auxiliary Telescopes. Telescope beams are split into 16 beams that need to be aligned on the detector and
corrected for OPD (Optical path difference) in order to create an interference pattern.
Alignment accuracy and stability specifications are of the order of nanometers and arcsec's. These specifications cannot
be met by warm alignment nor by manufacturing tolerances, therefore 16 motorized tip-tilt units are needed that operate
in a vacuum cryogenic environment.
Key aspects of the mechanisms are that the optical element and mechanism are combined in a compact single
component, driven by self braking piezo actuators in order to hold position without power. The design, realization and
test results of these mechanisms are presented.
METIS: "Mid-infrared ELT Imager and Spectrograph" is the mid-infrared (3 - 14 microns) instrument for imaging and
spectroscopy for the European Extremely Large Telescope (E-ELT). To ensure high detection sensitivity the internal
radiation of the instrument needs to be eliminated (sufficiently reduced) and thus needs to be operated at cryogenic
temperatures.
The instrument is divided in a cold and warm system. The cold system, the actual heart of the system, is subdivided into
five main opto-mechanical modules located within a common cryostat (part of the warm system). The warm system
provides the crucial environment for the cold system, including the instrument control and maintenance equipment. The
end 2009 finished Phase-A study carried out within the framework of the ESO sponsored E-ELT instrumentation studies
has been performed by an international consortium with institutes from Netherlands (PI: Bernhard Brandl - NOVA),
Germany, France, United Kingdom and Belgium. During this conference various aspects of the METIS instrument
(design) are presented in several papers, including the instrument concept and science case, and the system engineering
and optical design.
This paper describes the design constraints and key issues regarding the packaging of this complex cryogenic instrument.
The design solutions to create a light, small and fully accessible instrument are discussed together with the specific
subdivision of the cold and warm system to ensure concurrent development at various different institutes around Europe.
In addition the paper addresses the design and development studies for the special, challenging units such as the large
optical image de-rotator, the (2D) chopper mechanism and the special cryogenic drives.
The Configurable Slit Unit (CSU) for EMIR shall enable the possibility to generate a multi-slit configuration, a long slit,
or an imaging aperture at the entrance focal plane of the GTC-EMIR instrument. The CSU is therefore a cryogenic
reconfigurable slit mechanism. It contains 110 sliding bars which can be positioned within the 340x340mm wide
aperture of EMIR's instrument field of view.
Based on the results which haven been obtained in the previously performed demonstration programme, the current
developments have focused on the optimization of the performance and up scaling of the construction. Major progress
and improvement have been made on the position measurement of the slits, as a result of conceptual improvements in the
measurement system but also as a result of fundamental hardware changes. Furthermore an improvement of the thermal
household of the instrument will contribute to the position stability performance. Main development advances on several
aspects of the instrument design are presented. This work is performed under contract of the Instituto de Astrofisica de
Canarias as part of the development of the EMIR instrument to be installed at the GTC telescope.
Currently, a number of astrophysical institutes all over the world are working on the design of Extremely Large
Telescopes (ELT). Due to the enormous size of the primary mirror these telescopes make use of segmented mirrors.
These segments have to be positioned with respect to each other with nanometer accuracy in spite of all kind of external
disturbances such as wind loads, thermal loads, deformation of the base frame, varying orientation with respect to the
field of gravity, etc.
Janssen Precision Engineering (JPE) developed a revolutionary position actuator called the HiPAC which is able to
fulfill the demanding requirements for this kind of actuators.
The actuator is based on an integrated system of a pneumatic actuator, an electric voice coil and smart control strategy
and has the following features:
high positioning accuracy performance due to play-free and frictionless actuation;
high reliability and maintenance free operation due to flexure-based frictionless guiding;
system behavior is constant in time, because no parts affected by wear are used in the actuator;
low cost, because no highly accurate machined parts required to reach high end performance;
the position actuator acts as an integrated vibration isolator which isolates the segmented mirrors from
external vibrations induced in the telescope frame;
In this paper the design, simulation and measurements of the HiPAC actuator will be presented.
The Configurable Slit Unit (CSU) for EMIR shall enable the possibility to generate a multi-slit configuration, a long slit, or an imaging aperture at the entrance focal plane of the GTC-EMIR instrument. The CSU is therefore a cryogenic reconfigurable slit mechanism. It contains 100 sliding bars which can be positioned within the 307x307mm wide aperture of EMIR's instrument field of view. The development of the CSU has been a challenging task for several reasons: the high number of elements to control to configure a single observing pattern; the nominal working temperature of 77 K at which all the functionalities have to be accomplished, the stringent requirements in both accuracy and repeatability for most of the functionalities and the rotating nature of the EMIR instrument. The combination of these requirements urged the need to develop new pioneering concepts for actuation and position measurement. An actuation mechanism has been developed based on a piezo drive concept. A dedicated incremental, endless capacitive measurement system has been developed to measure the position of each separate bar. Both technologies are successfully realized in the demonstration programme that has been launched to prove the feasibility of the CSU concept. Besides actuation and position control of the bars, also thermal behavior of the CSU concept have been evaluated within the demonstration programme.
Currently, a number of astrophysical institutes all over the world are working on the design of Extremely Large
Telescopes (ELT). Due to the enormous size of the primary mirror these telescopes make use of segmented mirrors.
These segments have to be positioned with respect to each other with nanometer accuracy in spite of all kind of external
disturbances such as wind loads, thermal loads, deformation of the base frame, varying orientation with respect to the
field of gravity, etc.
Janssen Precision Engineering (JPE) developed a revolutionary position actuator called the HiPAC which is able to
fulfill the demanding requirements for this kind of actuators.
The actuator is based on an integrated system of a pneumatic actuator, an electric voice coil and smart control strategy
and has the following features: high positioning accuracy performance due to play-free and frictionless actuation; high reliability and maintenance free operation due to flexure-based frictionless guiding; system behavior is constant in time, because no parts affected by wear are used in the actuator; low cost, because no highly accurate machined parts required to reach high end performance; the position actuator acts as an integrated vibration isolator which isolates the segmented mirrors from external vibrations induced in the telescope frame; In this paper the design, simulation and measurements of the HiPAC actuator will be presented.
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