R. Frahm

VLTI technical advances: present and future

The Very Large Telescope Interferometer (VLTI) on Cerro Paranal (2635 m) in Northern Chile reached a major milestone in September 2003 when the mid infrared instrument MIDI was offered for scientific observations to the community. This was only nine months after MIDI had recorded first fringes. In the meantime, the near infrared instrument AMBER saw first fringes in March 2004, and it is planned to offer AMBER in September 2004. The large number of subsystems that have been installed in the last two years - amongst them adaptive optics for the 8-m Unit Telescopes (UT), the first 1.8-m Auxiliary Telescope (AT), the fringe tracker FINITO and three more Delay Lines for a total of six, only to name the major ones - will be described in this article. We will also discuss the next steps of the VLTI mainly concerned with the dual feed system PRIMA and we will give an outlook to possible future extensions.

PRIMA for the VLTI: a status report

PRIMA, the Phase-Referenced Imaging and Micro-arcsecond Astrometry facility for the Very Large Telescope Interferometer, is now nearing the end of its manufacturing phase. An intensive test period of the various sub-systems (star separators, fringe sensor units and incremental metrology) and of their interactions in the global system will start in Garching as soon as they are delivered. The status and performances of the individual sub-systems are presented in this paper as well as the proposed observation and calibration strategy to reach the challenging goal of high-accuracy differential astrometry at 10 μas level.

First results from fringe tracking with the PRIMA fringe sensor unit

The fringe sensor unit (FSU) is the central element of the phase referenced imaging and micro-arcsecond astrometry (PRIMA) dual-feed facility for the Very Large Telescope interferometer (VLTI). It has been installed at the Paranal observatory in August 2008 and is undergoing commissioning and preparation for science operation. Commissioning observations began shortly after installation and first results include the demonstration of spatially encoded fringe sensing and the increase in VLTI limiting magnitude for fringe tracking. However, difficulties have been encountered because the FSU does not incorporate real-time photometric correction and its fringe encoding depends on polarisation. These factors affect the control signals, especially their linearity, and can disturb the tracking control loop. To account for this, additional calibration and characterisation efforts are required. We outline the instrument concept and give an overview of the commissioning results obtained so far. We describe the effects of photometric variations and beam-train polarisation on the instrument operation and propose possible solutions. Finally, we update on the current status in view of the start of astrometric science operation with PRIMA.

Status of PRIMA for the VLTI: heading to astrometry

The Phase Referenced Imaging and Micro Arcsecond Astrometry (PRIMA) facility for the Very Large Telescope Interferometer (VLTI), is being installed and tested in the observatory of Paranal. Since January 2011 the integration and individual testing of the different subsystem has come to a necessary minimum. At the same time the astrometric commissioning phase has begun. In this contribution we give an update on the status of the facility and present some highlights and difficulties on our way from first dual-feed fringe detection to first astrometric measurements. We focus on technical and operational aspects. In particular, within the context of the latter we are going to present a modified mode of operation that scans across the fringes. We will show that this mode, originally only intended for calibration purposes, facilitates the detection of dual-fringes.

Status of PRIMA for the VLTI or the quest for user-friendly fringe tracking

The Phase Referenced Imaging and Micro Arcsecond Astrometry (PRIMA) facility for the Very Large Telescope Interferometer (VLTI), is being installed and tested in the observatory of Paranal. Most of the tests have been concentrated on the characterization of the Fringe Sensor Unit (FSU) and on the automation of the fringe tracking in preparation of dual-field observations. The status of the facility, an analysis of the FSU performance and the first attempts towards dual-field observations will be presented in this paper. In the FSU, the phase information is spatially encoded into four independent combined beams (ABCD) and the group delay comes from their spectral dispersion over 5 spectral channels covering the K-band. During fringe tracking the state machine of the optical path difference controller is driven by the Signal to Noise Ratio (SNR) derived from the 4 ABCD measurements. We will describe the strategy used to define SNR thresholds depending on the star magnitude for automatically detecting and locking the fringes. Further, the SNR as well as the phase delay measurements are affected by differential effects occurring between the four beams. We will shortly discuss the contributions of these effects on the measured phase and SNR noises. We will also assess the sensitivity of the group delay linearity to various instrumental parameters and discuss the corresponding calibration procedures. Finally we will describe how these calibrations and detection thresholds are being automated to make PRIMA as much as possible a user-friendly and efficient facility.

VLTI - PRIMA fringe tracking testbed

One of the key components of the planned VLTI dual feed facility PRIMA is the Fringe Sensor Unit (FSU). Its basic function is the instantaneous measurement of the Optical Path Difference (OPD) between two beams. The FSU acts as the sensor for a complex control system involving optical delay lines and laser metrology with the aim of removing any OPD introduced by the atmosphere and the beam relay. We have initiated a cooperation between ESO and MPE with the purpose of systematically testing this Fringe Tracking Control System in a laboratory environment. This testbed facility is being built at MPE laboratories with the aim to simulate the VLTI and includes FSUs, OPD controller, metrology and in-house built delay lines. In this article we describe this testbed in detail, including the environmental conditions in the laboratory, and present the results of the testbed subsystem characterisation.

ESO ELT M1 local control system software design and development status (Conference Presentation)

The ELT primary mirror is a 39m diameter concave mirror composed of 798 mirror segments. Each mirror segment is equipped with edge sensors, position actuators and a surface deformation warping harness, independently controlled by their backend electronics. The controllers are mounted in cabinets grouping up to seven segments. Each cabinet contains a network switch and a PLC for power control, telemetry and auxiliary tasks. There are in total 132 cabinets, named segment concentrators, grouped in six sectors. This constitutes a system of more than 2600 networked endpoints, including micro controllers, PLCs and network switches, to be controlled and supervised. The M1 Local Control System (LCS) is the subsystem of the ELT responsible for the monitoring and control of M1 segments. Its main goal is to enable the phasing of the M1 mirror to compensate for the presence of disturbances such as changing gravity vector, thermal expansion and wind forces. M1 LCS will provide a reliable and deterministic infrastructure to collect edge sensor and position actuators measurements and to distribute new position references at a frequency of 500 Hz. In addition, the software is responsible for devices synchronization, monitoring, configuration management as well as failure detection, isolation and notification. The M1 LCS passed its final design review and the development commenced. The present paper summarizes the M1 LCS software design, including adopted patterns and technologies, and the current development status.

Rejuvenation of a ten-year old AO curvature sensor: combining obsolescence correction and performance upgrade of MACAO

The MACAO curvature wavefront sensors have been designed as a generic adaptive optics sensor for the Very Large Telescope. Six systems have been manufactured and implemented on sky: four installed in the UTs Coudé train as an AO facility for the VLTI, and two in UT’s instruments, SINFONI and CRIRES. The MACAO-VLTI have now been in use for scientific operation for more than a decade and are planned to be operated for at least ten more years. As second generation instruments for the VLTI were planned to start implementation in end of 2015, accompanied with a major upgrade of the VLTI infrastructure, we saw it as a good time for a rejuvenation project of these systems, correcting the obsolete components. This obsolescence correction also gave us the opportunity to implement improved capabilities: the correction frequency was pushed from 420 Hz to 1050 Hz, and an automatic vibrations compensation algorithm was added. The implementation on the first MACAO was done in October 2014 and the first phase of obsolescence correction was completed in all four MACAO-VLTI systems in October 2015 with the systems delivered back to operation. The resuming of the scientific operation of the VLTI on the UTs in November 2015 allowed to gather statistics in order to evaluate the improvement of the performances through this upgrade. A second phase of obsolescence correction has now been started, together with a global reflection on possible further improvements to secure observations with the VLTI.

MATISSE: alignment, integration, and test phase first results

MATISSE (Multi AperTure mid-Infrared SpectroScopic Experiment) is the spectro-interferometer for the VLTI of the European Southern Observatory, operating in near and mid-infrared, and combining up to four beams from the unit or the auxiliary telescopes. MATISSE will offer new breakthroughs in the study of circumstellar environments by allowing the multispectral mapping of the material distribution, the gas and essentially the dust. The instrument consists in a warm optical system (WOP) accepting four optical beams and relaying them after a dichroic splitting (for the L and M- and N- spectral bands) to cold optical benches (COB) located in two separate cryostats. The Observatoire de la Côte d’Azur is in charge of the WOP providing the spectral band separation, optical path equalization and modulation, pupil positioning, beam anamorphosis, beam commutation, and calibration. NOVA-ASTRON is in charge of the COB providing the functions of beam selection, reduction of thermal background emission, spatial filtering, pupil transfer, photometry and interferometry splitting, additional beam anamorphosis, spectral filtering, polarization selection, image dispersion, and image combination. The Max Planck Institut für Radio Astronomie is in charge of the operation and performance validation of the two detectors, a HAWAII-2RG from Teledyne for the L- and M- bands and a Raytheon AQUARIUS for the N-band. Both detectors are provided by ESO. The Max Planck Institut für Astronomie is in charge of the electronics and the cryostats for which the requirements on space limitations and vibration stability resulted on very specific and stringent decisions on the design. The integration and test of the COB: the two cryogenic systems, including the cold benches and the detectors, have been conducted at MPIA in parallel with the integration of the WOP at OCA. At the end of 2014, the complete instrument was integrated at OCA. Following this integration, a period of interface and alignment between the COB and the WOP took place resulting in the first interference fringes in the L-band during summer 2015 and the first interference fringes in the N-ban in March 2016. After a period of optimization of both the instrument reliability and the environmental working conditions, the test plan is presently being conducted in order to evaluate the complete performance of the instrument and its compliance with the high-level requirements. The present paper gives the first results of the alignment, integration and test phase of the MATISSE instrument.

Generic control software connecting astronomical instruments to the reflective memory data recording system of VLTI - bossvlti

The quality of data obtained by VLTI instruments may be refined by analyzing the continuous data supplied by the Reflective Memory Network (RMN). Based on 5 years experience providing VLTI instruments (PACMAN, AMBER, MIDI) with RMN data, the procedure has been generalized to make the synchronization with observation trouble-free. The present software interface saves not only months of efforts for each instrument but also provides the benefits of software frameworks. Recent applications (GRAVITY, MATISSE) supply feedback for the software to evolve. The paper highlights the way common features been identified to be able to offer reusable code in due course.