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WP 4 - Imaging

Resp. Silvano Desidera
INAF - Osservatorio Astronomico di Padova

Direct imaging (DI) allows accessing the outer regions of planetary systems, where information from other techniques such as RV is incomplete due to the long orbital periods of planets that might be present. Since it provides access to photons originating from planet themselves, it opens the perspective of the direct physical characterization, which bears relevant information of the physical properties of planets and on their formation mechanisms.
Up to now DI is contributing relatively little to the budget of over 800 exoplanet discoveries. This is due to the fact that state-of-art instrumentation on 8m-class telescopes allow us to detect only giant planets at very young ages (when planets are brighter) at large separation from their parent stars. Situation is expected to improve dramatically in the next year, with the advent of new-generation, extreme AO instruments optimized for direct detection of planets, such as SPHERE at VLT and GPI at Gemini. These instruments should allow detection of giant planets with separation as close at that corresponding to the position of the snow-line in proto-planetary disks (where giant planets are expected to form more readily).
The planned activities of this WP include:

  • Completion of the SPHERE instrument and its scientific exploitation.
  • Feasibility studies of technological solutions for the building of new, and much larger, deformable mirrors, and of the optimal wavefront sensor for next generation Adaptive Optics (AO) instruments needed for the direct detection of Earth-like planets and for their atmospheric characterization.


SPHERE (resp. M. Turatto)

The 2nd generation instrument for VLT SPHERE is an European collaboration for the building of an instrument optimized for the direct detection of extrasolar planets direct exploiting the latest technological solutions to afford the challenge of High Contrast Imaging toward unprecedented limits. SPHERE includes an extreme adaptive optics module which feeds, through a complex ''common path'', three scientific arms, two in the near-infrared and one in the visible.
The IFS (Integral Field Spectrograph) instrument, will observe in the zYJH bands performing spectroscopic differential imaging. This instrument is expected to provide the best contrast at the closest separations from the central star. IFS building is under responsibility of INAF.
INAF is also responsible for the instrument control software (INS) of SPHERE, which includes the control software of the three science channels (IRDIS, IFS, and ZIMPOL), of the extreme AO system (SAXO) and of the common opto-mechanical infrastructure (CPI).
During 2013 the integration and validation of the instrument in laboratory at IPAG (Grenoble) will be completed. Then the instrument will be sent to Paranal Observatory (Chile) for integration at the telescope and for commissioning (from late 2013 to early 2014) before the start of science operations in mid-2014. During all these phases the presence of INAF people that worked at IFS and INS is mandatory. Therefore, we expect that activities related directly to the instrument (IFS and INS) will continue for the whole 2014, then requiring adequate resources and manpower. After the commissioning phase the Guaranteed Time Observations, (GTO) will start for a total of 260 VLT nights over 5 years. INAF role in the GTO survey includes the coordination of a large near-infrared planet search survey (NIRSUR - S. Desidera, Co-PI), the responsibility of target selection and characterization, and of the analysis of IFS data. With the start of the science observations, next year will be crucial for the science exploitation of the SPHERE instrument and to confirm the leading role of Italian researchers within the project, a proper return to the large investment by INAF in building the instrument. Large efforts are required for the completion of preparatory programs and the final definition of the target list (several hundreds of nearby, young stars) and for the preparation of tools for handling the observations at the telescope and the data analysis pipelines, which will surely need to be optimized from early observations at the telescope.

Implementation of spectro-polarimetry for SPHERE-IFS

We propose to upgrade SPHERE-IFS with the implementation of a spectro-polarimeter in integral field spectroscopy under the responsibility of dr. Turatto . The measurement of polarization is a sensitive diagnostics of cloud properties in planet atmospheres and on the presence of circumstellar disks. This instrument mode can be implemented using a free position in the prism wheel of SPHERE-IFS. In this position an optical system including a Wollaston prism, a prism with dispersion about half of what currently provided for the Y-H set-up of SPHERE-IFS, and a filter can be placed. With an appropriate choice of the beams emerging from the Wollaston prism, it should be  possible to obtain a configuration of the spectra on the detector similar to that for present spectroscopic mode. For each spectrum about 40 pixels long, two 20-pixels spectra with polarization changed by 90 deg can be obtained. A preliminary estimate of the cost for the whole upgrade is 200 K€ plus a similar amount in FTEs. For the first year, we plan to perform a feasibility study and preliminary design, for a total cost of 25 K€.


Solutions and applications of innovative Adaptive Optics technologies

LEECH:

The LBTI Exozodi Exoplanet Common Hunt is > 100 night exoplanet imaging survey which uses the LBT's adaptive optics system and LBTI. The survey will run in parallel with NASA's 5year exozodi key science program HOSTS (Hunt for Observable Signatures of Terrestrial Systems), taking advantage of LBTI's capability to do nulling interferometry at 8-13 micron simultaneously with direct imaging at 3-5 micron. The goals of the LEECH are the discovery of new exoplanets, their physical characterization and the study of the link between planets and disks. INAF is involved at technical level for the optimization of the LBT AO system for LBTI (INAF-Osservatorio di Arcetri) and on science (target characterization, physical and dynamical characterization of the discovered systems). For this activity we ask for travelling between Italy and the LBT site in Arizona.

Deformable mirrors for high contrast AO systems (resp. S. Esposito).

The aim of this activity, under the responsibility of dr. Simone Esposito, is to investigate the possibility of developing voice-coil actuators with a reduced pitch around 2/3mm to allow to built a 100/150 actuators mirror with a diameter less than 300mm.
Direct observations of extrasolar planets using ground based telescopes is an astronomy field enabled recently by using Adaptive Optics systems. Such systems allows to couple the large collecting power of an 8/10m class telescopes with its extreme angular resolutions like 30/40mas in the NIR. On top of that the new AO systems (FLAO at LBT) demonstrated PSF contrast in excess of 10-4 in their natural focal plane. A key element to achieve such performances is the number of degrees of freedom of the deformable mirror. Presently the best PSF contrast has been obtained using voice-coil deformable secondaries having an actuator pitch of 30mm. It is to be noted that such performance has been obtained with 30 disabled actuators over 672. This is because the shape of a voice coil actuated mirror do not strongly depend on a few failed actuators. The thin reflecting surface, in fact, is not in physical contact with the actuators but is controlled magnetically. On the other side the two more ambitious astronomical planet's finders GPI for Gemini South and SPHERE for VLT use piezo electric and membrane mirrors with a pitch of few and half a mm respectively. All the three DM solutions do not scale very well for the next generation of ELTs where 100/150 actuators per diameter are required to achieve diffraction limited high contrast PSFs.
The voice coil solution requires mirrors of 3/5m in diameter while the last two require to build a piezoelectric mirror with 10k/20k actuators without a few failed actuators. A few uncontrolled actuators in fact seriously reduce the PSF contrast. The voice-coil actuators we want to study coupling a relatively small diameter and a mirror surface shape being not critically dependent on a few failed actuators is a good solutions for planet finder systems of the future and in particular for the ELTs. The INAF - Arcetri AO group has a considerable experience in design and development of voice coil deformable mirror and has already started to study such solution. This element of WP requires design and prototyping activity as well as some laboratory characterization.

Testing Roofs vs. Pyramid WaveFront Sensor for eXtreme Adaptive Optics (resp. R. Ragazzoni).

The objective of the ''eXtreme Roofs'' activity is to analyze theoretically and to double check in laboratory the eXtreme Adaptive Optics capabilities of the double roof vs. pyramid WF sensor. The pyramid WFS has been demonstrated on the sky to be by far the best WFS to achieve extremely high Strehl ratio. This success has been obtained, further to the better sampling of the secondary Adaptive Mirror, thanks to the known aliasing features of the pyramid WFS. It has been shown analytically that the double roof WFS could have some advantages with respect to the pyramid one. However, it is also well known that the double roof is less sensitive than the Pyramid WFS one. In purely geometrical regime this difference is clearly pointing toward the ''classical'' pyramid. In real cases, a moderate amplitude modulation (typically 3 wavelenghts) has been adopted. Is this enough to destroy the advantages of the double roof pyramid? Is the different complexity, especially in calibration, between the two WFS enough to prevent to reach the theoretical advantages of the double roof WFS? Our goal is to analyze thoroughly the problem and to work out an ''ad hoc'' experiment in the laboratory to work out the differences under various kinds of realistic conditions, with the aim to select the very better WFS for XAO purposes for the next generation of AO facilities for planet hunting.
We plan to produce an end-to-end physical optical simulation of the pyramid and double roof WFS. The simulation has to be coded with special care to the issues of modulation and to mis-alignment and mis-placing along the optical axis of the optical components, along with their known imperfections (turned edges, quality of the faces, chromatism and so on). Once a realm of simulated behaviors are obtained, the key features will be individuated and a relatively simple prototype will be built up on the optical bench in order to figure out parameters like sensitivity and dynamic range with the aim to assess performances under eXtreme Adaptive Optics (i.e. Strehl better than 90%) under various conditions (non-common path aberrations, modulations, misplacement of the optical components ...).
Theoretical study and a phase-A concept studies for XAO systems on ELT have been published so far. The best XAO correction has now been achieved with Pyramid WFS, on which the proposing team has almost two decade of expertise. We expect to figure out from analytical, numerical and experimental results under which conditions and by how much amount the double roof WFS over-perform the pyramid one.


Expected results at the end of the 1st year

  • completion of SPHERE commissioning at Paranal
  • fine tuning of instrument control and data reduction softwares
  • implementation of maintenance and calibration procedures
  • final target selection for GTO survey and start of scientific observations
  • data analysis of scientific observations, evaluation of performances, fine tuning of data reduction and analysis procedures
  • early science results
  • Design and prototypical activities of voice-coil deformable mirror.
  • Numerical and analytical model of the pyramid vs. double roof WFS and implementation of a bench prototype