Missions

Our instrument concept will be studied in order to be adapted among our 3 science goals:

Our objectives are to elaborate the MOEMS components requirements compatible with Universe Observation, Earth Observation and planetology, to identify the possible future space applications and to assess the instrument environment specifications. After identifying the design drivers coming from Earth observation, Astronomy and planetology, we will be able to set completely the DM specific requirements and to analyze MIRA constraints to be used in space (volume, power …). Developing innovative optical concepts covering all space applications and including both MOEMS devices will be the next step. Instrument architectures for the selected optical concepts will then be derived, including mechanical design and thermal environment assessment at MOEMS component levels. A possible pathfinder will be to propose a demonstrator concept relevant to future missions, in order to validate our models.

Control System evaluation and preliminary architecture

A first control system architectural design will be done. The software technologies and architectural paradigms utilised in the preliminary instrument study are the result of the Instrumentation and Control Group expertise gained in the last years of activity in the ground-based astronomical projects, at Trieste Observatory. The software technologies that will be used for the final instrument operating in space will be investigated in a following phase. The first development will be done with commercial, off-the-shelf components that provide tools for rapid implementation allowing a quick proof of the concept. Examples of such technologies are Programmable Logic Controllers or more specialized Programmable Automation Controllers. Some of the more advanced components available on the market include also building blocks based on FPGA technology. Then, the platform should be shifted to a completely FPGA based solution with System on Chip concept. The same structure as in the first stage is implemented on chip, but taking into account the specific requirements of airborne applications. The components used will be still of the commercially available off the shelf to maintain the costs at a reasonable level, but choosing components that can be migrated to space compliant type.

MOEMS-DM

Major requirements for a space-deployed AO system depend heavily on the target mission; yet there are certain mission-independent qualities that arise from the nature of space-deployment. First and foremost, the developed AO system should be lightweight, robust and power efficient. The DM that sits at the heart of the AO system should be versatile, scalable in size and actuator count and space-qualified. Current DM technologies fail to meet such strict requirements in one way or another. In BATMAN flies, we would like to address this gap and ring the benefits of AO into various types of space missions, including Universe Observation and the search for exoplanets.

The DM is usually the key limiting component in an AO system, and this is also true for BATMAN flies. In order to render the DM for variety of missions, the target specifications are set to provide low-frequency/large stroke and high-frequency/small stroke wavefront correction. The mechanical structure and fabrication process should be compatible with large actuator counts combined with actuator pitches in the order of 1 mm to provide large field of view, particularly for large telescope applications. The summary of the specs we aim to reach for the DM are summarized in table 2.

Table 2:

Target specifications for the deformable mirror system set to maximize.

The BATMAN flies DM represents a major leap forward in DM technology. Batch microfabrication techniques can attain unprecedented precision in DM fabrication leading to higher device uniformity and reproducibility. This in turn improves wavefront correction fidelity. The highly integrated nature of the DM ensures smaller device inertia, and thus, improved response time. This aspect will be critical to push the limits of the capabilities of adaptive optics not only for astrophysics, but also life-science microscopy and ophthalmology. Also, vertical comb drive actuator not only provides displacement-independent force, it also provides considerably higher force density compared to regular parallel-plate actuators and render electrostatic actuators a viable choice for the application proposed here.

This task is conducted by IMTEK with support from LAM and CSEM, and expertise of TAS and Trieste Observatory. The realization of the device is scheduled at CSEM and the complete characterization at LAM.

MOEMS-MIRA

In Europe, an effort is currently under way to develop single-crystalline silicon micromirror arrays for future generation infrared multi-object spectroscopy. A collaboration within LAM, Ecole Polytechnique Federale de Lausanne (EPFL) and CSEM has for purpose to develop a European programmable MMA that can be used as reflective slit mask for MOS; the project is called MIRA. The requirements for this MMA were determined from previous simulation results and measurements. It has to achieve a high optical contrast of 1000:1 (goal: 3000:1), a fill factor of more than 90 % and a mechanical tilt angle greater than 20°. Furthermore, the performance must be uniform over the whole device; the mirror surface must remain flat in operation throughout a large temperature range and it has to work at cryogenic temperature.

Our MMA concept is based on the electrostatic double plate actuator. A micromirror is suspended by two flexion hinges, which were attached to a sustaining frame. To generate an electrostatic force, an electrode is placed underneath the micromirror and pillars are placed to set a precise electrostatic gap. A stopper beam is placed under the frame to set precisely the tilt angle of the micromirror after actuation and electrosatically lock it in this position.

The 100 x 200 μm² micromirrors were made of single-crystal silicon, assuring optical flat surfaces. Silicon being transparent in the infrared range, a gold thin-film coating was deposited on the topside of the mirrors. For MMA realization, a combination of bulk and surface silicon micromachining was used. They were made of two wafers: one for the mirrors and one for the electrodes, which were processed separately and assembled by wafer level bonding. Prototypes of MMA with 2048 individually addressable micromirrors (64 x 32 mirrors) have been successfully realized (Fig. 3) [5,6].

Fig. 3.

64 x 32 micromirror array (a) Close view of 100 x 200 μm² micromirrors (b) Mounted MMA in a Pin Grid Array (PGA)

MMA have been tested at LAM on bench set-ups dedicated to the characterization of MOEMS devices. The surface quality of the micromirror is measured by phase-shifting interferometry, and a total deformation of 10nm peak-to-valley is measured, with 1nm roughness. These mirrors can be electrostatically tilted by 24° at an actuation voltage of 130V. In many MOS observations, astronomers need to have the spectrum of the background nearby the studied object (“long slit” mode, see Fig. 3a). Our locking mechanism is designed in order to ensure this goal and a performance of a few arc-minutes angle difference has been obtained. The fill factor was 83% for the mirror surface and 98% in the direction along the micromirror lines. Individual addressing of the mirrors is based on a line-column scheme. As a proof of concept, a 2 x 2 sub-part of a MMA of 32 x 64 micromirrors was actuated successfully. The contrast of a micromirror was characterized on a dedicated optical bench at LAM: a contrast ratio of 1000:1 was obtained.

In order, to avoid spoiling of the astronomical objects spectra by the thermal emission of the instrument, the micromirror array has to work in a cryogenic environment. For characterising the surface quality and the performance of our MMA’s at low temperature, we have developed a cryo chamber optically coupled to a high-resolution Twyman-Green interferometer. The interferometer provides a sub-nanometer accuracy, and the cryo-chamber allows pressure down to 10^-6 mbar and cryogenic temperatures. Our MMA is conceived such that all structural elements have a matched coefficient of thermal expansion (CTE) in order to avoid deformation or even flaking within the device when cooling down to the operating temperature. This is especially important on the mirrors themselves: they are covered by a thin gold layer, gold having a different CTE than silicon.

The micromirrors could be successfully actuated before, during and after cryogenic cooling at 162K. We could measure the surface quality of the gold coated micromirrors at room temperature and at 162 K without large deformation difference. A 9.8nm PTV surface deformation was measured at 293K, increasing up to 27.2nm PTV at 162K. When coming back at room temperature, we measured again the mirror surface deformation and obtained a value of 9.9nm PTV, identical to the value measured before cooling of the array. The deformation is due to the CTE mismatch between the thick silicon micromirror and the thin gold coating layer on top. However, the surface deformation stays within the limit of 50 nm.

Mission pathfinder: ROBIN, a BATMAN demonstrator

Before developing BATMAN, we have built a demonstrator named ROBIN, for characterizing the actual performance of this new family of instruments, as well as investigating the operational procedures on astronomical objects. The design of the demonstrator is identical to the instrument design for being fully representative, with a global reduced size, on mirrors as well as on the grating. The general mechanical design of ROBIN consists of a main optical bench supporting 2 arms: a spectrograph arm and an imaging arm. The detectors are located on both sides of the bench. Opto-mechanical design is shown in Fig. 4.

Fig. 4.

(a) 3D general design view of ROBIN (in red, beam propagation in the demonstrator); (b) integrated ROBIN picture

ROBIN has been integrated and aligned (Fig. 4b). The optical beam is entering from the top center; the DMD is located at the bottom center and both arms are fed, on the right hand side is the imaging arm and on the left hand side is the spectroscopic arm. Both arms are fully identical except the convex mirror being replaced by the convex grating in the spectroscopic arm. Images and spectra are recorded by two CCD cameras located on both sides (left and right).

First images and spectra have been obtained and measured. In the imaging arm, typical slit mask patterns are recorded (Fig. 5a); the optical quality is good enough for imaging each individual micromirror. In the spectroscopic arm, typical spot diameters are within 1.5 detector pixels, and spectra generated by one micro-mirror slits are displayed with this optical quality over the whole visible wavelength range (Fig. 5b).

Fig. 5.

(a) Image of a typical slit mask in the imaging channel; (b) spectra in the spectral channel, including a single micromirror slit (close view of the spectrum generated by the one-micromirror slit)

We have tested successfully the instrument abilities in terms of variable spatial bin and variable spectral resolution, and any combination of the above modes over the whole FOV; in particular, MOS and IFU-like (scanning slit) modes have been studied, with any slit mask configurations (any shape, including long slit) as well as real time reconfiguration.

BATMAN: on-sky demonstration

BATMAN is a 2048x1080 Digital-Micromirror-Device-based (DMD) MOS instrument to be mounted on the 3.6m Galileo telescope (Fig. 6a). A two-arm instrument has been designed for providing in parallel imaging and spectroscopic capabilities. The field of view (FOV) is 6.8 arcmin x 3.6 arcmin with a plate scale of 0.2 arcsec per micromirror. The wavelength range is in the visible and the spectral resolution is R=560 for 1 arcsec object (typical slit size). The two arms will have 2k x 4k CCD detectors. This general design is identical to the concept shown in the first paragraph (Fig. 1). The mechanical design of BATMAN consists of a main optical bench supporting all optical elements except the detectors mounted on a second bench over the first one and attached to the main bench thanks to two hexapods for an individual alignment of the detector dewars (Fig. 6b).

Fig. 6.

3D general design view of BATMAN (a) mounted on the TNG telescope; (b) the instrument

Building BATMAN is under way, with all optics delivered and tested and some other major instrument parts delivered and tested (hexapods, fore-optics, detectors …). Remaining opto-mechanical parts are ordered and integration and alignment of the instrument is scheduled during spring 2017 [8]. BATMAN on the sky is of prime importance for characterizing the actual performance of this new family of MOS instruments, as well as investigating the operational procedures on astronomical objects. Thanks to a French-Italian collaboration, this instrument will be placed on the Telescopio Nazionale Galileo 3.6-m telescope, at the Nasmyth focus, by 2017.