General Description

FINITO (Fringe-tracking Instrument of NIce and TOrino) is a threebeam fringe tracker that operates in the H-band. Its basic function ist o measure the relative phase difference between the light beams.These measurements allow to identify the piston disturbances due to atmospheric turbulence. An error signal is sent to the OPD Controlle rwhich in return sends a correction signal to the delay lines. Phase measurements of the relative phase difference between the light beamsare performed over short periods of time.

FINITO is a sense, an interferometer based on coaxial (pupil-plane) combination of collimated beams from two or three telescopes at a time. Operating in H-band (λ =1.65μm, Δλ = 300nm) it uses fibre optics for spatial filtering and optical path modulation. An alignment and compensation optical system allows the compensation of transversal and longitudinal atmospheric dispersion.

Currently, FINITO operations are feasible for seeing below 1.2", τ₀above 2.5ms, and airmass < 1.5. The limiting correlated magnitude for FINITO is H = 3 and the minimum visibility in the H band is 15%.These numbers were determined with a seeing < 0.8" and τ₀ above 2.5ms.

The full potential of FINITO comes with the science instrument using higher spectral resolution. Since the fringes are "frozen" in OPDspace, the science instrument can integrate longer and/or one can stack the individual fringes during post-processing.

Operation principle

The components of FINITO are mounted on an optical bench within the interferometric laboratory.

The Alignment and Compensation Unit (ACU) provides the longitudinal and transversal correction required for initial alignment and optical path balancing of the telescope beams on FINITO, and the compensation of the atmospheric effects.

After being superposed with the laser beam of the internal metrology system, the beams from the ACU are sent to the Fibre Modulation Unit (FMU). The FMU acts as a spatial filter, selecting the least aberrated part of the wavefront, and therefore improving the signal coherence. The output beams of the monomode fibres have nearly ideal flat wavefronts. Residual errors from AO (on AT and UT) are translated into beam intensity fluctuations by the spatial filtering process. In addition to its spatial filtering function, the FMU is used for controlled variation of the OPL of the beams, in order to perform the interferogram scan required for fringe tracking and identification of the ZPD fringe.

After the FMU, the metrology and starlight beams are combined separately, using in each case a beam combiner assembly based on semi-reflective beam splitters. The metrology outputs are used to close the internal metrology loop.

The starlight beams are sent to an asymmetric beam combiner, based on beam splitting cubes. The reference beam “0” (not modulated) is split into two beams of equal amplitude for independent pair-wise combination with the two other beams “1” and “2” (both modulated). One of the two polarisation components of the starlight beams is extracted before combination, since the two polarisation have different optical path in the birefringent material of the FMU fibres and are mutually incoherent. The three rejected beams are used as photometric
outputs, for normalization of the four interferometric outputs of thebeam combiner. The seven light spots are injected into monomode optical fibres and sent to an integrating array detector. Residual phase fluctuations observed on the interferometric outputs are considered tobe due to atmospheric turbulence. They are sent to the OPDC within the fringe tracking loop.

FINITO always provides the same basic information derived from its astronomical detector, consisting in phase delay (= optical path difference at mean wavelength), group delay (“coherence signal”) and fringe visibility: the fringes are detected when the visibility, or coherence, reach their maximum value, and the OPD control loop, evaluating the FINITO signals, sets the appropriate DL condition and gives the appropriate information to the scientific instrument, which can start the integration.

More information can be found in the document: "The VLTI Fringe Sensors: FINITO and PRIMA FSU", M. Gai et al., SPIE Proc 5491, Glasgow 2004