Paul Girard

Proceedings of the SPIE

Wide-field multi-object spectroscopy is a high priority for European astronomy over the next decade. Most 8-10m telescopes have a small field of view, making 4-m class telescopes a particularly attractive option for wide-field instruments. We present a science case and design drivers for a wide-field multi-object spectrograph (MOS) with integral field units for the 4.2-m William Herschel Telescope (WHT) on La Palma. The instrument intends to take advantage of a future prime-focus corrector and atmospheric-dispersion corrector (Agocs et al, this conf.) that will deliver a field of view 2 deg in diameter, with good throughput from 370 to 1,000 nm. The science programs cluster into three groups needing three different resolving powers R: (1) high-precision radial-velocities for Gaia-related Milky Way dynamics, cosmological redshift surveys, and galaxy evolution studies (R = 5,000), (2) galaxy disk velocity dispersions (R = 10,000) and (3) high-precision stellar element abundances for Milky Way archaeology (R = 20,000). The multiplex requirements of the different science cases range from a few hundred to a few thousand, and a range of fibre-positioner technologies are considered. Several options for the spectrograph are discussed, building in part on published design studies for E-ELT spectrographs. Indeed, a WHT MOS will not only efficiently deliver data for exploitation of important imaging surveys planned for the coming decade, but will also serve as a test-bed to optimize the design of MOS instruments for the future E-ELT.

WEAVE: the next generation wide-field spectroscopy facility for the William Herschel Telescope

We present the preliminary design of the WEAVE next generation spectroscopy facility for the William Herschel Telescope (WHT), principally targeting optical ground-based follow up of upcoming ground-based (LOFAR) and spacebased (Gaia) surveys. WEAVE is a multi-object and multi-IFU facility utilizing a new 2 degree prime focus field of view at the WHT, with a buffered pick and place positioner system hosting 1000 multi-object (MOS) fibres or up to 30 integral field units for each observation. The fibres are fed to a single spectrograph, with a pair of 8k(spectral) x 6k (spatial) pixel cameras, located within the WHT GHRIL enclosure on the telescope Nasmyth platform, supporting observations at R~5000 over the full 370-1000nm wavelength range in a single exposure, or a high resolution mode with limited coverage in each arm at R~20000.

Current results of the PERSEE testbench: the cophasing control and the polychromatic null rate

Stabilizing a nulling interferometer at a nanometric level is the key issue to obtain deep null depths. The PERSEE breadboard has been designed to study and optimize the operation of cophased nulling bench in the most realistic disturbing environment of a space mission. This presentation focuses on the current results of the PERSEE bench. In terms of metrology, we cophased at 0.33 nm rms for the piston and 60 mas rms for the tip/tilt. A Linear Quadratic Gaussian (LQG) control coupled with an unsupervised vibration identification allows us to maintain that level of correction, even with characteristic vibrations of nulling interferometry space missions. These performances, with an accurate design and alignment of the bench, currently lead to a polychromatic unpolarised null depth of 8.9 × 10-6 stabilized at 2.7 × 10-7 on the [1.65 - 2.45] μm spectral band (37% bandwidth). With those significant results, we give the first more general lessons we have already learned from this experiment, both at system and component levels for a future space mission.

PERSEE: experimental results on the cophased nulling bench

Nulling interferometry is still a promising method to characterize spectra of exoplanets. One of the main issues is to cophase at a nanometric level each arm despite satellite disturbances. The bench PERSEE aims to prove the feasibility of that technique for spaceborne missions. After a short description of PERSEE, we will first present the results obtained in a simplified configuration: we have cophased down to 0.22 nm rms in optical path difference (OPD) and 60 mas rms in tip/tilt, and have obtained a monochromatic null of 3 · 10-5 stabilized at 3•10-6. The goal of 1 nm with additional typical satellite disturbances requires the use of an optimal control law; that is why we elaborated a dedicated Kalman filter. Simulations and experiments show a good rejection of disturbances. Performance of the bench should be enhanced by using a Kalman control law, and we should be able to reach the desired nanometric stability. Following, we will present the first results of the final polychromatic configuration, which includes an achromatic phase shifter, perturbators and optical delay lines. As a conclusion, we give the first more general lessons we have already learned from this experiment, both at system and component levels for a future space mission.

Perspective of imaging in the mid-infrared at the Very Large Telescope Interferometer

MATISSE is a mid-infrared spectro-interferometer combining the beams of up to four Unit Telescopes or Auxiliary Telescopes of the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory. MATISSE will constitute an evolution of the two-beam interferometric instrument MIDI. New characteristics present in MATISSE will give access to the mapping and the distribution of the material, the gas and essentially the dust, in the circumstellar environments by using the mid-infrared band coverage extended to L, M and N spectral bands. The four beam combination of MATISSE provides an efficient uv-coverage: 6 visibility points are measured in one set and 4 closure phase relations which can provide aperture synthesis images in the mid-infrared spectral regime. We give an overview of the instrument including the expected performances and a view of the Science Case. We present how the instrument would be operated. The project involves the collaborations of several agencies and institutes: the Observatoire de la Côte d’Azur of Nice and the INSU-CNRS in Paris, the Max Planck Institut für Astronomie of Heidelberg; the University of Leiden and the NOVA-ASTRON Institute of Dwingeloo, the Max Planck Institut für Radioastronomie of Bonn, the Institut für Theoretische Physik und Astrophysik of Kiel, the Vienna University and the Konkoly Observatory.

Chromatic phase diversity for cophasing large array of telescopes

This paper reviews the recent laboratory results we have obtained on the demonstration of a cophasing algorithm based on the chromatic phase diversity method. The SIRIUS testbed was initially dedicated to the demonstration of the direct imaging capabilities of arrays of telescope. We have developed and numerically modeled a piston sensor based on the chromatic dependance of the spectral density phase. This method allows a global cophasing of the array over a capture range of many wavelengths aiming at improving the robustness of the method.

Final results of the PERSEE experiment

Although it has been recently postponed due to high cost and risks, nulling interferometry in space remains one of the very few direct detection methods able to characterize extrasolar planets and particularly telluric ones. Within this framework, several projects such as DARWIN [1], [2], TPF-I [3], [4], FKSI [5] or PEGASE [6], [7], have been proposed in the past years. Most of them are based on a free flying concept. It allows firstly to avoid atmosphere turbulence, and secondly to distribute instrumental function over many satellites flying in close formation. In this way, a very high angular resolution can be achieved with an acceptable launch mass. But the price to pay is to very precisely position and stabilize relatively the spacecrafts, in order to achieve a deep and stable extinction of the star. Understanding and mastering all these requirements are great challenges and key issues towards the feasibility of these missions. Thus, we decided to experimentally study this question and focus on some possible simplifications of the concept. Since 2006, PERSEE (PEGASE Experiment for Research and Stabilization of Extreme Extinction) laboratory test bench is under development by a consortium composed of Centre National d’Etudes Spatiales (CNES), Institut d’Astrophysique Spatiale (IAS), Observatoire de Paris-Meudon (LESIA), Observatoire de la Côte d’Azur (OCA), Office National d’Etudes et de Recherches Aérospatiales (ONERA), and Thalès Alénia Space (TAS) [8]. It is mainly funded by CNES R&D. PERSEE couples an infrared wide band nulling interferometer with local OPD and tip/tilt control loops and a free flying Guidance Navigation and Control (GNC) simulator able to introduce realistic disturbances. Although it was designed in the framework of the PEGASE free flying space mission, PERSEE can adapt very easily to other contexts like FKSI (in space, with a 10 m long beam structure) or ALADDIN [9] (on ground, in Antarctica) because the optical designs of all those missions are very similar. After a short description of the experimental setup, we will present first the results obtained in an intermediate configuration with monochromatic light. Then we will present some preliminary results with polychromatic light. Last, we discuss some very first more general lessons we can already learn from this experiment.