First coronographic observations using a 4 Quadrant Phase Mask on the VLT

A team from Paris Observatory has just obtained the first coronographic observations with a 4 Quadrant Phase Mask on the VLT (Very Large Telescope) of ESO. This instrument was built in particular to detect extra-solar planets directly, by avoiding the dazzling due to their central stars.

In spite of their weak luminosity and the proximity of central star, one currently knows a hundred extra-solar planets. Since 1995, their detection was made

possible thanks to clever methods which highlight the disturbance of planet on the central star motion. Direct imagery is the next step in the study of extra-solar

planets. It will be possible to study in more detail the diversity of these objects and their physical characteristics as the mass, the size and the chemical

composition. The indirect methods already highlighted atypical objects (compared to the Solar system) as for example hot Jupiters (planet of the Jupiter mass but

located 100 times closer to their star than is Jupiter to the Sun). The direct imagery will doubtlessly reveal other kinds of planets. Many techniques are thus used to achieve this goal. The difficulty lies in the important contrast between a star and a planet (1 billion in visible and near IR and 1

million in mid IR) and the small angular separation from the central star which is about 0.1" for a planet of the Earth type in orbit around a star at 10pc (or

10 times less than the size of an image after its passage through the atmosphere). The method studied at LESIA is the stellar coronagraphy with high

dynamics and more particularly a type of coronograph called 4 Quadrant Phase Mask (FQPM). A coronograph is an instrument able to attenuate

considerably the light of the central star (several orders of magnitude) without modifying the received flow of an out-axis object like a planet. The first coronographs were

carried out by Bernard Lyot to observe the solar corona in the thirties and were then adapted to the stellar coronography. The 4 quadrant phase mask was invented by Daniel Rouan (cf Rouan et al. 2000, PASP 112, 1479) : it consists in dividing the Airy disk (image of a point

source) in the center of the field into 4 domains, and of applying a phase difference of π to two of them, so that the image is eliminated by destructive interference.

Simulations show that this concept of division in 4 quadrants is more effective and less sensitive to the disturbances of the atmosphere than the previous concepts

of coronograph with modification of phase. We thus continue its development at LESIA. Several phase masks were manufactured like those

presented on Figure 1 and were tested in laboratory to evaluate their quality of realization and to characterize their performance. Figure 1 : 4 quadrant phase mask made by REOSC/SAGEM operating in visible (left) and by Reading University in mid IR (right) With a first assembly at visible wavelengths, it was possible to reach in laboratory an attenuation of the central star by a factor 90000, i.e. making it easy to detect

a companion 10 magnitudes weaker than the star for a separation of 3 times the angular resolution of the telescope. Thanks to these promising performances we could participate in the study of a coronograph for MIRI the infra-red instrument (IR) of the James Webb Space Telescope. For that, we developed an IR bench

characterization at low temperature (MIRI will be maintained at a temperature of -266°C). Preliminary results reach an attenuation of

approximately 400 (in wide strip 10%) in conformity with the expectations for this space telescope. In parallel to this study, we carry out a phase mask for operation in the near IR at 2µm. In agreement with ESO, this coronograph was installed on one of the 4 VLT

telescopes in Chile on the NACO instrument. The adaptive optics of NAOS corrects the effects of atmospheric turbulence and allows the use of such a coronograph

from the ground. The advantage of the phase mask compared to the traditional Lyot coronograph already in place on NACO, is to be able to approach about 4 times closer

to the central star. This represents a privileged region for the search for low mass companions, circumstellar disks, or even of Active Galaxy Nuclei.

Although the "commissioning" (official inauguration) of this component did not take place yet, we could record, in September 2003, some images on a star

allowing us to evaluate promising performances. Figure 2 shows the images obtained with and without coronograph and the radial profile of the images giving

the flux ratio is plotted on figure 3. An animation of the image of the star without coronograph then with coronograph also shows the evolution with time of the

images. Figure 2 : Image of a star of magnitude K=3.16 obtained with the VLT with a narrow-band filter (left) and the same star attenuated by the phase coronograph (right). The attenuation is of 3.5 magnitudes at a separation of only 60 milli-arcseconds. The annular shape of the coronographic image is due to a residual motion of the star caused by the atmospheric turbulence. Figure 3 : Radial profile for the images of Figure 2 giving the attenuation of the star light as a function of the angular separation. The FQPM will allow to detect a hypothetical companion at only one angular resolution of the telescope with a difference of 3.5 magnitudes, while traditional Lyot coronographs are blind until about 6 times the angular resolution. This new coronograph will be offered to the community at the end of 2004. Its principal application will involve the search for substellar companions (low-mass stars

or brown dwarfs) but also the study of circumstellar disks.

This result shows that provided we have an excellent correction of the effects of atmospheric turbulence, this type of device makes a very important

improvement for the dazzling effect, a necessary, but not yet sufficient, step to detect extra-solar planets.

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Reference : Rouan D., Riaud P., Boccaletti A., Clénet Y., Labeyrie A. : 2000 PASP 112, 1479 Contact :

Anthony Boccaletti (Observatoire de Paris, LESIA) Pierre Baudoz (Observatoire de Paris, LESIA) Jacques Baudrand (Observatoire de Paris, LESIA) François Lacombe (Observatoire de Paris, LESIA) Jean-Michel Reess (Observatoire de Paris, LESIA) Pierre Riaud (Observatoire de Paris, LESIA) Daniel Rouan (Observatoire de Paris, LESIA) 

Dernière modification le 4 mars 2013