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6. Comparison with expected performance and other systems

  Section 5 (click here) gives the effective performance of the instrumental configuration used. We discuss them in comparison with theoretical calculations and others systems.

First, the rejection capability of the coronograph appears to be in good agreement with theoretical expectations, even though it is slightly reduced. The following effects may explain the remaining discrepancy. First, the optics are not perfect. In particular, the mirror scattering decreases the efficiency of a coronograph. For instance, the latest observations occurred after the primary mirror had been re-aluminized and the improvement in the image quality was spectacular. Second, the wavefront
after the adaptive optics system is not perfectly flat and the wings of the PSF are correspondingly modified. Also, since the efficiency of the coronograph critically depends on the accurate centering of the star, a slight variation in the position during observation, possibly due to a small mechanical deformation, affects the whole shape of the residual light profile.

The importance of the star position stability underlined below explains why observing with little, or no image correction does not allow the use of very small masks. Consequently, the close vicinity of the star may not be investigated without AO. Quantitatively, no precise measurements have been achieved closer than 3tex2html_wrap_inline1120 from a star without AO correction (Kalas & Jewitt 1995) or closer than 2.5tex2html_wrap_inline1120 with tip-tilt correction (Golimovski et al. 1993). This is to be compared to our present limit of 1.5tex2html_wrap_inline1120, where the uncertainty of the whole observational and reduction procedure discussed in Sect. 5.2 is lower than the disk brightness (Mouillet et al. 1995, here after Paper II). We have to stress that our observational parameters were optimized for observation close to the star. Different observational parameters would be chosen to observe at greater distances. A larger mask would provide a better S/N with longer exposure times. The compromise between the pixel sampling and the field of view depends on the particular application concerned.

Finally, the use of a coronograph may also be discussed. In terms of the requested dynamic range, our detection limit at 2tex2html_wrap_inline1120 and beyond is around tex2html_wrap_inline1264 times fainter than the peak intensity of the non occulted star observed with the adaptive optics system under the same conditions. This limit could even be improved by a total exposure time larger than 10 minutes since the limiting factor is in this case the low flux noise. Yet to achieve the same result without a coronograph, the detector intrinsic dynamic range (in the linear regime) would need to be much larger. Indeed, the subtraction of a reference profile requires a very good signal to noise ratio in the wings and this implies a significant number of electrons in the low flux regions.


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