MACAO-VLTI first light: adaptive optics at the service of interferometry

The Multiple Application Curvature Adaptive Optics (MACAO) programme was initiated by ESO in 1998 to fulfil the high angular resolution requirements of the VLT Interferometer (Glindemann et al., 2002) and also instruments like SINFONI (Bonnet et al., 2002) and CRIRES (Moorwood et al., 2002). After a learning phase of two years with the laboratory Curvature prototype delivered by Laplacian Optics, the ESO Adaptive Optics Department set up a project team at the beginning of 2000 for the line production of seven MACAO systems (Donaldson et al., 2000): four for VLTI, one for SINFONI, one for CRIRES and one spare. Although the AO key components are similar for these seven systems, the opto-mechanical implementations are different for the VLTI, SINFONI and CRIRES. In the following we will concentrate on the VLTI implementation. The main aim of MACAO-VLTI is to feed the VLTI with a corrected wavefront, to improve light injection efficiency in the monomode fibres. The existing Coude mirror train M8 is replaced by the corrective optics. The MACAO-VLTI Wave Front Sensor (WFS) is located just below M9 on the Coude platform (Figure 1). M9 is a dichroic that allows the wavefront sensing in the visible (transmits 0.45–0.9 μm) and reflects the wavelengths 1 to 13 μm to the recombination laboratory. Simulations done for the design reviews show that MACAO-VLTI will reach a Strehl ratio of 0.58 at 2.2 μm on bright stars (V < 10) with a seeing of 0.65 and this has been confirmed by tests in the laboratory. Limiting magnitude is evaluated at V ~ 18 which would result in a Strehl ratio slightly under ~0.1. MACAO will also operate with worse seeing (1.0 ) but the correction is less spectacular (expected Strehl ratio of 0.39). The first MACAO-VLTI system was delivered to Paranal on UT2 in April 2003, and results from the first light are shown further down in this article. The other three systems will follow at 6 month intervals. By the end of 2003 the first two systems will be available and will allow wavefront corrected beam recombination at the VLTI. MACAO-VLTI First Light: Adaptive Optics at the Service of Interferometry