This is the first time that a variable curvature mirror is achieved with such a large range. Using large deformations theory, the maximal flexion achieved from the initial plane figure, is close to 1200 He-Ne fringes.
The achieved optical quality is in accordance with the ESO requirements for
a 3.5 arcsec FOV (WFE on a 6 mm diam.) and leads to the
realization of a VCM system fulfilling its goals in the forthcoming optical
interferometric array of the VLT. The manufacturing process now represents
the limit of the technology. If a wider FOV of 8'' is planned, we will
have to consider a larger active mirror in order to achieve the required
optical quality on a 14 mm diameter, this can be obtained by using a 28 or
30 mm mirror. Today this VCM system is being included in the delay-line system
in order to setup and calibrate the interfaces with the other devices of the
interferometric mode. A gold coating, easy to apply on a stainless steel
substrate, has been done on the first sample delivered to
ESO in order to
increase the reflectivity of the mirror.
The integration will be done during
this year, and the installation on the Paranal observatory of the first complete
system (delay-line with VCM) will occur around year 2000.
The VLTI will not be the only interferometer to benefit of this type of device,
as recently such a VCM system has been choosen to be used in another optical
interferometric array. Variable curvature mirrors will be part of the upgraded
Grand Interferometer à 2 Telescopes (GI2T) of the Observatoire de la Côte
d'Azur, located in Plateau de Calern.