Optimized fringe sensors for the VLTI next generation instruments

Optical and Infrared Interferometry and Imaging VIII

Hierarchical Fringe Tracking (HFT) is a fringe tracking concept optimizing the sensitivity in optical long baseline by reducing to an absolute minimum the number of measurements used to correct the OPD fluctuations. By nature, the performances of an HFT do not decreases with the number of apertures of the interferometer and are set only by the flux delivered by the individual telescopes. This a critical feature for future interferometers with large number of apertures both for homodyne and heterodyne operation. Here we report the design and first optical bench tests of integrated optics HFT chips for a 4 telescopes interferometer such as the VLTI. These tests validate the HFT concept and confirm previous estimates that we could track accurately fringes on the VLTI up to nearly K~15.9 with the UTs and K~12.2 with the ATs with a J+H+K fringe tracker with one HFT chip per band. This is typically 2.5 magnitudes fainter than the best potential performance of the current ABCD fringe tracker in the K band. An active longitudinal and transverse chromatic dispersion correction allows the optimization of broad band fiber injections and instrumental contrast. We also present a preliminary evaluation of the potential of such a gain of sensitivity for the observations of AGNs with the VLTI.

SPIE Proceedings, 2008

FINITO (the VLTI three beam fringe-tracker) has been offered in September 2007 to the astronomical community for observations with the scientific instruments AMBER and MIDI. In this paper, we describe the last improvements of the fringe-tracking loop and its actual performance when operating with the 1.8m Auxiliary Telescopes. We demonstrate the gain provided to the scientific observations. Finally, we discuss how FINITO real-time data could be used in post-processing to enhance the scientific return of the facility.

New Frontiers in Stellar Interferometry, 2004

FINITO is the first generation VLTI fringe sensor, optimised for three beam observations, recently installed at Paranal and currently used for VLTI optimisation. The PRIMA FSU is the second generation, optimised for astrometry in dualfeed mode, currently in construction. We discuss the constraints of fringe tracking at VLTI, the basic functions required for stabilised interferometric observations, and their different implementation in the two instruments, with remarks on the most critical technical aspects. We provide an estimate of the expected performance and describe some of their possible observing and calibration modes, with reference to the current scientific combiners.

2006

2010

POPS (Planar Optical Phase Sensor) is a second-generation fringe tracker for the Very Large Telescope Interferometer (VLTI), intended to simultaneously measure the cophasing and coherencing errors of up to six Unit Telescopes (UT) or Auxiliary Telescopes (AT) in real time. The most promising concepts are probably based on the utilization of Integrated Optics (IO) components, and were the scope of a Phase A study led by Observatoire de Grenoble (LAOG). Herein is described a tentative design built around a multi-axial IO chip whose fringes are dispersed downstream on a detector array, and a Chromatic Phase Diversity algorithm presented in another paper of this conference 1 . We depict the foreseen opto-mechanical, detection and software implementations, and provide numerical results from a realistic simulation model in terms of group and phase delay measurement accuracy and limiting magnitudes in the K band. The ultimate performance of the method is discussed and compared with the original 2 nd generation VLTI fringe tracker requirements.

Eso Astrophysics Symposia, 2008

The performance of interferometric instruments is strongly linked to the quality of piston stabilization. Next generation VLTI instruments plan to use 4 to 8 beams simultaneously. In the current VLTI implementation, the maximum number of beams that can be phased using FINITO and PRIMA/FSU simultaneously is 5. Therefore, a new fringe sensor is required for the VLTI.

Proceedings of SPIE - The International Society for Optical Engineering

The NOVA Fringe Tracker (NFT) is a proposed solution to the call by ESO for a second generation fringe tracking facility. This instrument at the VLTI will enable the cophasing of up to 6 telescopes simultaneously. Using broad band optics with detection from 1.2 to 2.4 microns, a unique configuration is employed that eliminates so-called “photometric crosstalk.” This refers to imbalance in the beam combiner which results in fluctuations of the incoming wavefronts and the proportion of power accepted by a spatial filter masquerading as a visibility, a common problem afflicting previous interferometric instruments and fringe trackers. Also proposed for use in “science instruments” (for the measurement of visibility), the “Polarization-Based Collimated Beam Combiner,” with its achievement of photometric symmetry in hardware, is particularly suited for combined use of the smaller AT (1.8 meter) telescopes with the UT (8 meter) telescopes involving a 20:1 intensity ratio of the interferin...

2003

The Osservatorio Astronomico di Torino is developing a Fringe Sensor Unit (FSU) for VLTI, in collaboration with ESO. The requirements for interferometric observation at VLTI are reviewed, describing the function of an FSU and its interaction with the instrument complement. The cases analysed are FINITO and the PRIMA FSU. Their basic operating assumptions are described, deducing performance parameters as a function of the magnitude. Specifications for fringe tracking at LBT are deduced by comparison with the VLTI case and from general scientific requirements.

Optical and Infrared Interferometry II, 2010

In a few years, the second generation instruments of the Very Large Telescope Interferometer (VLTI) will routinely provide observations with 4 to 6 telescopes simultaneously. To reach their ultimate performance, they will need a fringe sensor capable to measure in real time the randomly varying optical paths differences. A collaboration between LAOG (PI institute), IAGL, OCA and GIPSA-Lab has proposed the Planar Optics Phase Sensor concept to ESO for the 2 nd Generation Fringe Tracker. This concept is based on the integrated optics technologies, enabling the conception of extremely compact interferometric instruments naturally providing single-mode spatial filtering. It allows operations with 4 and 6 telescopes by measuring the fringes position thanks to a spectrally dispersed ABCD method. We present here the main analysis which led to the current concept as well as the expected on-sky performance and the proposed design.

Astronomy & Astrophysics, 2008

Context. At the Very Large Telescope Interferometer, the purpose of the fringe-tracker FINITO is to stabilize the optical path differences between the beams, allowing longer integration times on the scientific instruments AMBER and MIDI. Aims. Our goal is to demonstrate the potential of FINITO for providing H-band interferometric visibilities, simultaneously and in addition to its normal fringe-tracking role. Methods. We use data obtained during the commissioning of the Reflective Memory Network Recorder at the Paranal observatory. This device has permitted the first recording of all relevant real-time data needed for a proper data-reduction. Results. We show that post-processing the FINITO data allows valuable scientific visibilities to be measured. Over the several hours of our engineering experiment, the intrinsic transfer function is stable at the level of ±2%. Such stability would lead to robust measurements of science stars even without the observation of a calibration star within a short period of time. We briefly discuss the current limitations and the potential improvements.