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The visiting adaptive optics system with ESO’s New Technology Telescope has achieved the first light
Update time: 2014-08-06
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In July 2014, the first visiting Extreme Adaptive Optics (Ex-AO) system in the world has been installed on ESO’s 3.58-m New Technology Telescope (NTT) at the La Silla Observatory in Chile and has achieved first light. The Ex-AO has delivered an excellent imaging quality in H band, which has fully demonstrated its ability to be used for future high-contrast imaging research of exoplanets and brown dwarfs.  

The associate observation with the visiting AO system is supported by a Major International (Regional) Joint Research Project awarded from the National Natural Science Foundation of China (NSFC, Grant No of 11220101001). The observation is performed by the Exoplanet Direct Imaging and Ex-AO Research Group, jointed by Nanjing Institute of Astronomical Optics & Technology (NIAOT, China) and California State University Northridge (CSUN, USA). 

The above visiting instrument observation program aims for the scientific research of the direct imaging of exoplanets. It has been allocated (Awards No: 093.C-0326) with 6 observational nights of NTT, in the collaboration between NIAOT and AIUC at PUC (Prof. Leopoldo Infante’s group). The scientific NIR camera in this work has been supported by the CAS South America Center for Astronomy and NAOC. 

We gratefully acknowledge the on-site assistances from Peter Sinclaire and Ivo Saviane. We also thank Karina Celedón and Angelika Beller from ESO for arranging the shipment of the instrument and travelling. 

The Ex-AO system has been installed successfully during the day time on Jul. 14th (first observational day) on Nasmyth B (NB) platform of NTT, after dismantling the previously existed EFOSC instrument. Figure 1 shows the installed Ex-AO on NB. The system is very compact and has a physical size of 900mm (L) x 600mm (W) x 270mm (W).  


Figure 1. The Ex-AO system installed on the NB of NTT.

With current configuration, our Ex-AO, a natural guide star system can lock on a star with an apparent magnitude of 8. Figure 2 shows typical images of Star Fomalhaut with our Ex-AO off and on (seeing of 0.8″, wind speed 5 m/s). The images were taken in H band (center wavelength of 1500nm, with a bandwidth of 40nm). It has reached a diffraction limited imaging, which demonstrates the system has the ability to be used for the direct imaging observation of exoplanets and brown dwarfs in the next step.  

A movie can be found by the bellowing website, in which most light energy has been concentrated and locked steady in the central airy disk (only flatten and dark correction has applied on each frame, without any co-centering process).  





For Ex-AO coronagraphic research of exoplanets, one of the performance limitations is the none-common path error (NCPE) which is not measured by the AO wave-front sensor (WFS), which is physically separated from the WFS and the science camera. In this observation, the NCPE has been effectively corrected with a PSF evaluation technique based on SPGD (see Ren et al. PASP 2012). The observed images have a similar diffraction pattern with the corrected PSF, where the third diffraction ring has been clearly shown. It can be further improved with more iteration corrections, in which the outer scattered light can be pushed to the PSF center.

Figure 2. Ex-AO’s first light image of Fomalhaut: with AO off (left) and AO on (right).

Due to the limitation of development time and budget, in this observation, our Ex-AO as a proto-type system, has provided a 30-order (of Zernike polynomial) wave-front correction and reached a correction frequency of 1 KHz (open loop), which is limited by current 97-actuator DM. In the next step, a faster 277-actuaotr DM will be induced, which can provide a precise wave-front correction over 100 orders with a correction frequency of 2 KHz.

The system will be finally updated to a facility dedicated for a 4-meter class telescope including both NTT and the Astrophysical Research Consortium (ARC) 3.5-m telescope at Apache Point Observatory. A professional WFS will be used, which will allow us to image faint stars with magnitude down to 10. And a broad-band NIR filter will be added to further increase the throughput of the whole system. Finally a step-transmission filter based coronagraph will be integrated in the Ex-AO for high-contrast imaging research.

Because of the limitation of physical space, none AO system has been installed on a 4-meter class telescope, which can deliver a state of the art performance. For these telescope, only Cassegrain or Nasmyth focus are available. Well our Ex-AO system, with a compact physical size, is able to be installed on a Nasmyth platform as a visiting instrument and deliver an excellent imaging quality.

Please notify that most of current Ex-AO coronagraphic projects for high-contrast imaging research of exoplanets are mainly on 8-meter class telescopes, which includes GPI/GEMINI and SPHERE/ VLT. Due to the large aperture of the primary, Kilo actuator deformable mirrors (DM) are required to reach an extreme high strehl ratio. Furthermore, an 8-m class telescope are very busy and may not be allocated too much observational time for the direct imaging research of exoplanets, which is a time-consuming work. For our Ex-AO, it will allow us to reach a compatible performance to GPI or SPHERE, with a moderate actuator number DM (say 277 actuators), on a smaller aperture size (4-meter class) telescope with a more opportunity to get the observational time, comparing with an 8-meter telescope.

With integration of previously developed coronagraph (Ren, Dou & Zhu, PASP 2010), the system should deliver an observational contrast of 10E-6 at angular distance of 0.4″ in NIR wavelengths. It will be used for the direct imaging research of young giant exoplanets and brown dwarfs, which would be a compensation for 8-m class GPI/SPHERE projects.

Nanjing Institute of Astronomical Optics & Technology ,National Astronomical Observatories ,CAS