The use of optical fibers in astronomical instrumentation has been becoming more and more common. High transmission, polarization control, compact and easy routing are just a few of the advantages in this respect. But fibers also bring new challenges for the development of systems. During the assembly of the VLTI beam combiner GRAVITY different side effects of the fiber implementation had to be taken into account. In this work we summarize the corresponding phenomena ranging from the external factors influencing the fiber performance, like mechanical and temperature effects, to inelastic scattering within the fiber material.
Since its first light at the Very Large Telescope Interferometer (VLTI), GRAVITY has reached new regimes in optical interferometry, in terms of accuracy as well as sensitivity.<sup>1</sup> GRAVITY is routinely doing phase referenced interferometry of objects fainter than K > 17 mag, which makes for example the galactic center black hole Sagittarius A*<sup>2</sup> detectable 90 % of the times. However from SNR calculations we are confident that even a sensitivity limit of K ~ 19 mag is possible. We therefore try to push the limits of GRAVITY by improving the observations as well as the calibration and the data reduction. This has further improved the sensitivity limit to K > 18 mag in the beginning of this year. Here we present some work we are currently doing in order to reach the best possible sensitivity.
GRAVITY acquisition camera implements four optical functions to track multiple beams of Very Large Telescope Interferometer (VLTI): a) pupil tracker: a 2×2 lenslet images four pupil reference lasers mounted on the spiders of telescope secondary mirror; b) field tracker: images science object; c) pupil imager: reimages telescope pupil; d) aberration tracker: images a Shack-Hartmann. The estimation of beam stabilization parameters from the acquisition camera detector image is carried out, for every 0.7 s, with a dedicated data reduction software. The measured parameters are used in: a) alignment of GRAVITY with the VLTI; b) active pupil and field stabilization; c) defocus correction and engineering purposes. The instrument is now successfully operational on-sky in closed loop. The relevant data reduction and on-sky characterization results are reported.
The VLTI instrument GRAVITY combines the beams from four telescopes and provides phase-referenced imaging as well as precision-astrometry of order 10 μas by observing two celestial objects in dual-field mode. Their angular separation can be determined from their differential OPD (dOPD) when the internal dOPDs in the interferometer are known. Here, we present the general overview of the novel metrology system which performs these measurements. The metrology consists of a three-beam laser system and a homodyne detection scheme for three-beam interference using phase-shifting interferometry in combination with lock-in amplifiers. Via this approach the metrology system measures dOPDs on a nanometer-level.
The GRAVITY Instrument Software (INS) is based on the common VLT Software Environment. In addition to the basic Instrument Control Software (ICS) which handles Motors, Shutters, Lamps, etc., it also includes three detector subsystems, several special devices, field bus devices, and various real time algorithms. The latter are implemented using ESO TAC (Tools for Advanced Control) and run at a frequency of up to 4 kHz. In total, the instrument has more than 100 ICS devices and runs on five workstations and seven vxWorks LCUs.
GRAVITY is the four-beam, near-infrared, AO-assisted, fringe tracking, astrometric and imaging instrument for the Very Large Telescope Interferometer (VLTI). It is requiring the development of one of the most complex instrument software systems ever built for an ESO instrument. Apart from its many interfaces and interdependencies, one of the most challenging aspects is the overall performance and stability of this complex system. The three infrared detectors and the fast reflective memory network (RMN) recorder contribute a total data rate of up to 20 MiB/s accumulating to a maximum of 250 GiB of data per night. The detectors, the two instrument Local Control Units (LCUs) as well as the five LCUs running applications under TAC (Tools for Advanced Control) architecture, are interconnected with fast Ethernet, RMN fibers and dedicated fiber connections as well as signals for the time synchronization. Here we give a simplified overview of all subsystems of GRAVITY and their interfaces and discuss two examples of high-level applications during observations: the acquisition procedure and the gathering and merging of data to the final FITS file.
We present the installed and fully operational beam stabilization and fiber injection subsystem feeding the 2nd generation VLTI instrument GRAVITY. The interferometer GRAVITY requires an unprecedented stability of the VLTI optical train to achieve micro-arcsecond astrometry. For this purpose, GRAVITY contains four fiber coupler units, one per telescope. Each unit is equipped with actuators to stabilize the telescope beam in terms of tilt and lateral pupil displacement, to rotate the field, to adjust the polarization and to compensate atmospheric piston. A special roof-prism offers the possibility of on-axis as well as off-axis fringe tracking without changing the optical train. We describe the assembly, integration and alignment and the resulting optical quality and performance of the individual units. Finally, we present the closed-loop performance of the tip-tilt and pupil tracking achieved with the final systems in the lab.
The acquisition camera for the GRAVITY/VLTI instrument implements four functions: a) field imager: science field imaging, tip-tilt; b) pupil tracker: telescope pupil lateral and longitudinal positions; c) pupil imager: telescope pupil imaging and d) aberration sensor: The VLTI beam higher order aberrations measurement. We present the dedicated algorithms that simulate the GRAVITY acquisition camera detector measurements considering the realistic imaging conditions, complemented by the pipeline used to extract the data. The data reduction procedure was tested with real aberrations at the VLTI lab and reconstructed back accurately. The acquisition camera software undertakes the measurements simultaneously for all four AT/UTs in 1 s. The measured parameters are updated in the instrument online database. The data reduction software uses the ESO Common Library for Image Processing (CLIP), integrated in to the ESO VLT software environment.
The VLTI instrument GRAVITY will provide very powerful astrometry by combining the light from four tele- scopes for two objects simultaneously. It will measure the angular separation between the two astronomical objects to a precision of 10 μas. This corresponds to a differential optical path difference (dOPD) between the targets of few nanometers and the paths within the interferometer have to be maintained stable to that level. For this purpose, the novel metrology system of GRAVITY will monitor the internal dOPDs by means of phase- shifting interferometry. We present the four-step phase-shifting concept of the metrology with emphasis on the method used for calibrating the phase shifts. The latter is based on a phase-step insensitive algorithm which unambiguously extracts phases in contrast to other methods that are strongly limited by non-linearities of the phase-shifting device. The main constraint of this algorithm is to introduce a robust ellipse fitting routine. Via this approach we are able to measure phase shifts in the laboratory with a typical accuracy of λ=2000 or 1 nm of the metrology wavelength.
The laser metrology system in the GRAVITY instrument plays a crucial role in an attempt at high-precision narrow-angle astrometry. With a design goal of achieving 10 microarcseconds precision in astrometry, the system must measure the optical path difference between two beam combiners within GRAVITY to an accuracy of better than 5nm. However in its current design, some parts of the optical paths of the metrology system are not common to the optical paths of starlight (the science path) which it must measure with high accuracy. This state of the design is true for most but not all the baselines which will be used by the GRAVITY instrument. The additional non-common optical paths could produce inaccurate path length measurements and consequently inaccurate measurements of the differential phase between fringe packets of two nearby celestial objects, which is the main astrometric observable of the instrument. With reference to the stability and the sensitivity of the non-common paths, this paper describes the impact of a biased differential phase measurement on the narrowangle astrometry and the image reconstruction performance of the GRAVITY instrument. Several alternative designs are also discussed.
GRAVITY is the second generation VLT Interferometer (VLTI) instrument for high-precision narrow-angle astrometry and phase-referenced interferometric imaging. The laser metrology system of GRAVITY is at the heart of its astrometric mode, which must measure the distance of 2 stars with a precision of 10 micro-arcseconds. This means the metrology has to measure the optical path difference between the two beam combiners of GRAVITY to a level of 5 nm. The metrology design presents some non-common paths that have consequently to be stable at a level of 1 nm. Otherwise they would impact the performance of GRAVITY. The various tests we made in the past on the prototype give us hints on the components responsible for this error, and on their respective contribution to the total error. It is however difficult to assess their exact origin from only OPD measurements, and therefore, to propose a solution to this problem. In this paper, we present the results of a semi-empirical modeling of the fibered metrology system, relying on theoretical basis, as well as on characterisations of key components. The modeling of the metrology system regarding various effects, e.g., temperature, waveguide heating or mechanical stress, will help us to understand how the metrology behave. The goals of this modeling are to <i>1)</i> model the test set-ups and reproduce the measurements (as a validation of the modeling), <i>2)</i> determine the origin of the non-common path errors, and <i>3)</i> propose modifications to the current metrology design to reach the required 1nm stability.
We present in this paper the design and characterisation of a new sub-system of the VLTI 2<sup><i>nd</i></sup> generation instrument GRAVITY: the Calibration Unit. The Calibration Unit provides all functions to test and calibrate the beam combiner instrument: it creates two artificial stars on four beams, and dispose of four delay lines with an internal metrology. It also includes artificial stars for the tip-tilt and pupil guiding systems, as well as four metrology pick-up diodes, for tests and calibration of the corresponding sub-systems. The calibration unit also hosts the reference targets to align GRAVITY to the VLTI, and the safety shutters to avoid the metrology light to propagate in the VLTI-lab. We present the results of the characterisation and validtion of these differrent sub-units.
GRAVITY is a second generation VLTI instrument, combining the light of four telescopes and two objects
simultaneously. The main goal is to obtain astrometrically accurate information. Besides correctly measured stellar
phases this requires the knowledge of the instrumental differential phase, which has to be measured optically during the
astronomical observations. This is the purpose of a dedicated metrology system. The GRAVITY metrology covers the
full optical path, from the beam combiners up to the reference points in the beam of the primary telescope mirror,
minimizing the systematic uncertainties and providing a proper baseline in astrometric terms. Two laser beams with a
fixed phase relation travel backward the whole optical chain, creating a fringe pattern in any plane close to a pupil. By
temporal encoding the phase information can be extracted at any point by means of flux measurements with photo
diodes. The reference points chosen sample the pupil at typical radii, eliminating potential systematics due differential
focus. We present the final design and the performance estimate, which is in accordance with the overall requirements
The GRAVITY acquisition camera measurements are part of the overall beam stabilization by measuring each second
the tip-tilt and the telescope pupil lateral and longitudinal positions, while monitoring at longer intervals the full
telescope pupil, and the VLTI beam higher order aberrations.
The infrared acquisition camera implements a mosaic of field, pupil, and Shack Hartman type images for each telescope.
Star light is used to correct the tip-tilt while laser beacons placed at the telescope spiders are used to measure the pupil
lateral positions. Dedicated optimized algorithms are applied to each image, extracting the beam parameters and storing
them on the instrument database.
The final design is built into the GRAVITY beam combiner, around a structural plane where the 4 telescope folding
optics and field imaging lenses are attached. A fused silica prism assembly, kept around detector temperature, is placed
near to the detector implementing the different image modes.
We present design results of the 2nd generation VLTI instrument GRAVITY beam stabilization and light injection
subsystems. Designed to deliver micro-arcsecond astrometry, GRAVITY requires an unprecedented stability of the
VLTI optical train. To meet the astrometric requirements, we have developed a dedicated 'laser guiding system',
correcting the longitudinal and lateral pupil position as well as the image jitter. The actuators for the correction are
provided by four 'fiber coupler' units located in the GRAVITY cryostat. Each fiber coupler picks the light of one
telescope and stabilizes the beam. Furthermore each unit provides field de-rotation, polarization analysis as well as
atmospheric piston correction. Using a novel roof-prism design offers the possibility of on-axis as well as off-axis fringe
tracking without changing the optical train. Finally the stabilized beam is injected with minimized losses into singlemode
fibers via parabolic mirrors. We present lab results of the first guiding- as well as the first fiber coupler prototype
regarding the closed loop performance and the optical quality. Based on the lab results we discuss the on-sky
performance of the system and the implications concerning the sensitivity of GRAVITY.
KMOS is a multi-object near-infrared integral field spectrometer with 24 deployable pick-off arms. Data processing
is inevitably complex. We discuss specific issues and requirements that must be addressed in the data
reduction pipeline, the calibration, the raw and processed data formats, and the simulated data. We discuss the
pipeline architecture. We focus on its modular style and show how these modules can be used to build a classical
pipeline, as well as a more advanced pipeline that can account for both spectral and spatial flexure as well as
variations in the OH background. A novel aspect of the pipeline is that the raw data can be reconstructed into
a cube in a single step. We discuss the advantages of this and outline the way in which we have implemented it.
We finish by describing how the QFitsView tool can now be used to visualise KMOS data.
Two teams of scientists and engineers at Max Planck Institut fuer Extraterrestrische Physik and at the European Southern Observatory have joined forces to design, build and install the Laser Guide Star Facility for the VLT.
The Laser Guide Star Facility has now been completed and installed on the VLT Yepun telescope at Cerro Paranal. In this paper we report on the first light and first results from the Commissioning of the LGSF.
The PARSEC laser system is designed for the VLT Laser Guide Star Facility to deliver a high power cw laser beam at 589nm, in order to create an artificial guide star in the mesospheric Sodium layer. The laser consists of a resonant, dye based power amplifier which is injection seeded with 589nm, single frequency radiation from a master oscillator. We report on the performance of the system both during the European Acceptance tests, and that which has been achieved in the laboratory. The maximum power we have obtained amounts to 20W cw laser light in a single mode and a single frequency at 589nm. With a beam quality of M<sup>2</sup> of 1.05-1.15 and a long term stability without manual intervention, the laser suits all the demands for operation at the VLT.
Using adaptive optics on the Keck Telescope and the VLT, we are able to probe the dynamics and star formation in Seyfert and QSO nuclei on spatial scales better than 0.1" in the H- and K-bands. Such spectroscopic data are essential for studying the link between AGN and star formation, understanding how gas is driven into the
nucleus, and measuring the black hole mass. In this contribution we present some of our recent results, and consider what an astronomer needs from an adaptive optics system for extragalactic work, as well as what is realistic to expect. We discuss why deconvolution is not appropriate in this context; and examine the scientifically more useful alternative of convolving a model with an estimate of the PSF, describing what level of detail and reliability can actually be achieved in the various methods of measuring the PSF.
For the successful operation of laser referenced adaptive optic systems very powerful lasers for the creation of sodium guide stars are necessary. Here we introduce the design of PARSEC, the cw sodium-line laser for the VLT, and present out first laboratory results on the performance of the system. So far we have achieved a stable output power of 12.8W in a single spatial mode and a single frequency.
We report on the ongoing VLT Laser Guide Star Facility project, which will allow the ESO UT4 telescope to produce an artificial reference star for the Adaptive Optics systems NAOS-CONICA and SINFONI. A custom developed dye laser producing >10W CW at 589nm is installed on-board of the UT4 telescope, then relayed by means of a single mode optical fiber behind the secondary mirror, where a 500mm diameter lightweight, f/1 launch telescope is projecting the laser beam at 90 km altitude.
We described the design tradeoffs and provide some details of the chosen subsystems. This paper is an update including subsystems results, to be read together with our previous paper on LGSF design description.
One of the critical design drivers for the PARSEC laser was that by the time it is integrated into the VLT Laser Guide Star Facility in 2003 it should be able to run remotely without any hands-on tuning for a period of at least one week. In this contribution we describe some of the methods we have employed to achieve this and take a brief look at the proposed operating procedure.
8m class telescopes offer an extremely powerful tool for astronomical research. The light collecting power is enormous and with the combination of adaptive optics and laser guide stars astronomy will make a big step forward in knowledge in almost any field. For the creation of artificial laser-based guide stars that make use of the sodium layer in the earth atmosphere, very powerful lasers at 589nm are necessary. We introduce here the PARSEC laser that will be installed at the UT4 telescope of the VLT, in Chile. This laser will emit cw radiation at more then 10W output power and offers a scalability in power for future multi guide star systems.
We report in this paper on the design and progress of the ESO Laser Guide Star Facility. The project will create a user facility embedded in UT4, to produce in the Earth's Mesosphere Laser Guide Stars, which extend the sky coverage of Adaptive Optics systems on the VLT UT4 telescope. Embedded into the project are provisions for multiple LGS to cope with second generation MCAO instruments.
The Max-Planck institutes for astronomy and for extraterrestrial physics run a high order adaptive optics system with a laser guide star facility at the Calar Alto 3.5- m telescope in southern Spain. This system, called ALFA, saw first light in September 1996. Today, ALFA can compensate for atmospheric turbulences with natural guide stars as faint as 13.5th magnitude in R-band. ALFA recently succeeded in overcoming this limiting magnitude with the deployment of its laser guide star. This paper briefly reviews the ALFA project and its progress over the last 3 years. We further discuss the impact of sodium-layer laser guide stars on wavefront sensing and present results obtained with both kinds of guide stars.
ALFA (Adaptive Optics with Laser for Astronomy) is a joint project between the Max Planck Institut fur extraterrestrische Physik in Garching and the Max Planck Institut fur Astronomie in Heidelberg. To increase the sky coverage of the adaptive optics system, we are using a laser guide star that is created in the sodium layer of the earth atmosphere. The laser consists of a 5W cw dye laser, operated at single frequency and tuned to the sodium D<SUB>2</SUB> resonance transition. Here we report on the present status of the laser system, the technological means to ensure an easy operation of the laser guide star, as well as on developments we are planning for the future.
Observations have shown the presence of sodium layer centroid height variations of a few hundred meters on timescales of tens of seconds. As quality laser guide star (LGS) plus adaptive optics (AO) assisted astronomy, especially on large (8m+) telescopes, will require optimal scheduling of observations and regular laser and wavefront sensor focusing at sites where sporadic sodium layers are frequent, an 'easy to use' sodium layer monitor is required. LIDAR offers a convenient means to achieve this. By pulsing the outgoing sodium laser and performing time-of-flight measurements on the returned photons we can acquire the altitude profile of the sodium layer. Unfortunately, conventional LIDAR requires the laser duty cycle to be very low, therefore large integration times are required. However, by using a cross-correlation technique the duty cycle can be increased to 50%, which gives far better performance. We present the details of this technique which involved amplitude modulation of the MPIA/MPE ALFA cw laser, as well as the following results of such LIDAR measurements performed in October 1999 at the 3.5 m telescope at Calar Alto Observatory in Spain. The altitude of the sodium layer at Calar Alto on 17th and 18th October 1999 was found to be at 90 +/- 3 km and there is evidence for sporadics on one of two nights with sporadic layer FWHM* varying from approximately 240 to 350 m. In addition, a noticeable layer FWHM change (excluding the sporadic layer) from approximately 13 to approximately 5 - 7 km was observed over the two nights. After flux and altitude calibration and correction of the projected altitude range, a very good agreement is found between sodium layer profiles derived from an auxiliary telescope and 3.5 m telescope LIDAR observations. Using an intensity weighted centroid algorithm the centroid height of the sodium layer was observed to have a variation of < 500 m in approximately 10 minutes. Although, shorter timescale variations may be have been present, poor observing conditions and resulting reduced S/N prevents this analysis.
Adaptive optics (AO) coupled to laser guide star systems is crucial to future ground based astronomical observations. It allows correction of image distortion caused by the Earth's turbulent atmosphere, over a hugely larger fraction of the sky than achieved by using only natural stars. Yet there are still very few such systems producing any sort of scientific results. ALFA, now offered on a shared risk basis as a user- instrument at Calar Alto Observatory in Spain, is continuing to improve its performance during closed loop operation on both natural and laser guide stars. The ability to close the loop on the LGS through thin cirrus cloud has the potential to increase the number of nights previously considered suitable for the laser by a factor of about two. In particular, science observations carried out on such a night are described. As part of the TMR network for Laser Guide Stars at Large Telescopes we are studying the distribution of atoms in the mesospheric sodium layer and its evolution over time. Additionally, a new experiment to provide an on- line monitor of the mesospheric sodium layer has been proposed and the results of a simulation are presented. This study will be of importance to large telescopes with laser stars at good astronomical sites where accurate statistics of the sodium layer are required, both for optimal scheduling of observations and for keeping the wavefront sensor focused on the LGS.
The ALFA-Laser of the MPE/MPIA adaptive optic system utilizes a 4W cw-laser for the creation of a sodium layer guide star. The artificial star serves as a reference source for the adaptive optics system installed at the Calar Alto observatory in souther Spain. Several distortion sources are affecting the laser beam and result in a laser guide star spot size too large on which to lock the adaptive optics loop properly. Therefore a number of analysis tools have been installed just before the laser beam expander and measurements of the beam quality have been performed. In this contribution we present parts of the experimental setup and results of these measurements. In addition we report on experimental studies of the guide star brightness when exciting the sodium layer with different polarization states of the laser radiation. A surprisingly large gain in response flux, when using circular polarization, has been measured. The complicated behavior of the polarization state with telescope position, due to phase changes at the beam relay mirrors, makes a control loop necessary to keep the projected beam optimal.
The sodium laser guide star adaptive optics system ALFA has been constructed at the Calar Alto 3.5m telescope. Following the first detection of the laser beacon on the wavefront sensor in 1997 the system is now being optimized for best performance. In this contribution we discuss the current status of the launch beam and the planned improvements and upgrades. We report on the performance level achieved when it is used with the adaptive optics system, and relate various aspects of our experience during operation of the system. We have begun to produce scientific result and mention two of these.