The purpose of the VLTI is the coherent combination of light beams gathered by 2 or more (up to 8) telescopes. The measurements of the amplitudes and phases of the interference fringes give an information on the image structure at the spatial frequency, corresponding to the physical distance of the telescopes. The optical train of the interferometer must be continually adjusted to equalize the optical path lengths from the observed object, through the atmosphere and the telescopes, to the detector.
The VLTI has been designed to collect the light beams from 8 meter diameter Unit Telescopes (UTs), located at fixed positions, and from 1.8 meter diameter Auxiliary Telescopes (ATs), relocatable among a set of predefined observing stations. Delay Lines (DLs) will cancel the optical path difference (OPD) of the interferometric arms due to the sidereal motion, the geometrical configuration of the telescope array, the internal OPD, and the atmospheric turbulence. A 158 meter long, 8 meter wide, subterranean tunnel will house the Delay Lines. The Transfer Optics (TO) mirrors will convey the beams from the telescopes to the Delay Lines, and from them towards the Interferometer Laboratory. The beams are finally delivered to a VLTI Instrument, which will manage the beam combination to produce an interference pattern.
Figure 1 in this chapter shows a scheme of the VLTI site layout
The general optical VLTI layout, as shown in
Figure 2 in this chapter, includes:The light collected by the primary mirror M1 of each observing telescope is directed through the Nasmyth focus down to the Coudé Station by means of the Coudé Optical Train (M4...M8). The Coudé Relay Optics (M9...M11) injects the light beam into the interferometric tunnel. The Transfer Optics (M12, M16) redirects the collimated light beam towards the assigned Delay Line, where it is retroreflected, and feeds the entrance of the Imaging Beam Combiner or directly a VLTI Instrument.
The next subsections will describe briefly the main VLTI elements.
The Unit Telescope itself contains 8 mirrors (M1...M8) in its f/50 Coudé configuration. The light path between M9 and the entrance to the Delay Line primary mirror M13 lies all in a horizontal plane. M9 is an exchangeable flat beamsplitter, which transmits part of the radiation, in the visible band, to the adaptive optics wavefront sensor and reflects the radiation used by the VLTI. M10 is a field mirror located at the folded Coudé focus. It is curved in order to produce a transferred pupil near the center of the interferometric tunnel. It can be tilted in order to move laterally the pupil image as seen from the beam combining optics (pupil guiding). The Pupil Sensor Unit, located in the Interferometer Laboratory, will provide the measurement of the pupil displacement to steer M10. The off-axis parabolic mirror M11 collimates the beam to the Interferometric Tunnel.
A dual feed capability will support two field angles that may be up to 1' in separation, each of them with a continuous 2'' field diameter. This allows picking up a secondary beam at the Coudé Focus within 1' radius from the center of the primary beam. The dual feed system will be used for reference phase tracking and narrow angle astrometry.
The Auxiliary Telescope, unlike the Unit Telescope, has only one operational focus at the Coudé Station. The optics consists of 11 optical elements: Telescope Mirrors M1...M3, Coudé Mirrors M4...M8, Relay Mirrors M9...M11.
The Coudé Sensor Unit, installed behind M9, which is a dichroïc mirror, provides the necessary data for field acquisition, autoguiding and field stabilization. It comprises:
The field mirror M10 is located in an image plane and can be remotely oriented to control the output pupil position (alike in the Unit Telescope).
Finally the collimator mirror M11 sends the collimated and stabilized light beam horizontally towards the interferometric tunnel.
The dual feed system, as described in the previous section, will be applicable also to the Auxiliary Telescope, when defined.
The Auxiliary Telescope can be relocated by means of a transporter among several observing stations. Observations can be performed only when the Auxiliary Telescope is fixed on a station.
