The COME-ON-PLUS instrument (Gendron et al. 1991), developed in
collaboration by ESO and several French institutes and companies
(Observatoire de Paris, ONERA, Laserdot and LEP), routinely achieves
diffraction-limited images at near infrared wavelengths (down to 1.65 
m)
on the ESO 3.6 meter telescope at La Silla (Rousset et al. 1994;
Beuzit 1995). The images are recorded on the 256 
256 NICMOS 3 detector of the SHARP II infrared camera (Hofmann et
al. 1992) using an image scale of 0.05
/pixel, which leads
to a total field of view of 12.8 12.8. The camera
features the standard J, H, K and short K broad-band photometric filters.
For reference star magnitude lower than 10, i.e. at high photon flux, the
best FWHM is around 0.1 and is obtained in the H and J bands. In
the K band, the 0.12 angular resolution and a typical Strehl ratio,
i.e. the flux concentration with respect to the perfect image case, of 30
to 40% are achieved.
The optical sketch of the COME-ON-PLUS coronograph is shown in
Fig. 2 (click here). Unlike most of the existing coronographs, it is
composed exclusively of reflecting surfaces to avoid chromatic effects when
switching from one wavelength to another. The occulting mask, a flat
metallic mirror with a calibrated central hole, is located at the F/45
output focus of the adaptive optics bench (Mc2). The star light goes
through the hole while the image of the surrounding region is reflected to
the camera. The Mc3 spherical mirror collimates the light beam onto the
Lyot stop (Mc4). The second spherical mirror (Mc5) images the focal plane
onto the infrared detector with the same F/45 aperture. Additional flat
mirrors are used to allow a fast switching from the coronographic mode to
the standard one without removing the coronograph itself.
Different
occulting masks are available with sizes ranging from 245 m to 1.6 mm
i.e. projected diameters from 0.3 to 2, the image scale of
the ESO 3.6 meter telescope being 790 m arcsec-1 at the F/45
focus. The masks were made from stainless steel disks in which holes were
first drilled. They were then optically polished at a surface quality of
/4 at 0.55 m and silver coated.
The Lyot stop or apodizing mask, located at a telescope pupil image, is
used to block the diffracted light from the edge of the telescope's primary
and secondary mirrors and from the secondary support (including the spider
arms). In doing so, it needs to partially obstruct the pupil image. Two
masks, obtained by a microphotolithographic process, were produced by the
LARCA (LAboratoire de Réalisation de Composants pour l'Astronomie) at the
Observatoire de Paris. They correspond to relative obstruction ratios of
10% and 20% of the pupil surface without a significant reduction of the
angular resolution.
Figure 2: Optical sketch of the COME-ON-PLUS coronograph. See Sect. 3 for a
comprehensive description