The unique astrometric capability of GRAVITY has already resulted in a serie of transformational results, from the study of the Galactic Center to the characterization of exoplanets. Nonetheless, these breakthroughs have not yet reached the ultimate noise limits of interferometric astrometry, and are currently limited by the systematics of the instrument. As part of the GRAVITY+ project, a major goal is to keep pushing the performances down to the precision of 10-30µas. In this talk, we present the on-going analysis of the precision limits of GRAVITY astrometry, and the potential solutions envisioned to overcome its systematics.
As part of the GRAVITY+ project, the near-infrared beam combiner GRAVITY and the VLTI are currently undergoing a series of significant upgrades to further improve the performance and sky coverage. The instrumental changes will be transformational, and for instance uniquely position GRAVITY to observe the broad line region of hundreds of Active Galactic Nuclei (AGN) at a redshift of two and higher. The increased sky coverage is achieved by enlarging the maximum angular separation between the celestial science object (SC) and the off-axis fringe tracking (FT) star from currently 2 arcseconds (arcsec) up to unprecedented 30 arcsec, limited by the atmospheric conditions. This was successfully demonstrated at the VLTI for the first time.
With the upgrade from GRAVITY to GRAVITY+ the instrument will evolve to an all-sky interferometer that can observe faint targets, such as high redshift AGN. Observing the faintest targets requires reducing the noise sources in GRAVITY as much as possible. The dominant noise source, especially in the blue part of the spectrum, is the backscattering of the metrology laser light onto the detector. To reduce this noise we introduce two new metrology modes. With a combination of small hardware changes and software adaptations, we can dim the metrology laser during the observation without losing the phase referencing. For single beam targets, we can even turn off the metrology laser for the maximum SNR on the detector. These changes lead to a SNR improvement of over a factor of two averaged over the whole spectrum and up to a factor of eight in the part of the spectrum currently dominated by laser noise.
During the past years, the VLTI-instrument GRAVITY has made spectacular discoveries with phase-referenced interferometric imaging with milliarcsecond resolution and ten microarcsecond astrometry. Here, we report on the upgrade of the GRAVITY science spectrometer with two new grisms in October 2019, increasing the instrument throughput by a factor > 2. This improvement was made possible by using a high refractive index Germanium substrate, which reduces the grism and groove angles, and by successfully applying an anti-reflection coating to the ruled surface to overcome Fresnel losses. We present the design, manufacturing, and laboratory testing of the new grisms, as well as the results from the re-commissioning on sky.
Combining adaptive optics and interferometric observations results in a considerable contrast gain compared to single-telescope, extreme AO systems. Taking advantage of this, the ExoGRAVITY project is a survey of known young giant exoplanets located in the range of 0.1” to 2” from their stars. The observations provide astrometric data of unprecedented accuracy, being crucial for refining the orbital parameters of planets and illuminating their dynamical histories. Furthermore, GRAVITY will measure non-Keplerian perturbations due to planet-planet interactions in multi-planet systems and measure dynamical masses. Over time, repetitive observations of the exoplanets at medium resolution (R = 500) will provide a catalogue of K-band spectra of unprecedented quality, for a number of exoplanets. The K-band has the unique properties that it contains many molecular signatures (CO, H2O, CH4, CO2). This allows constraining precisely surface gravity, metallicity, and temperature, if used in conjunction with self-consistent models like Exo-REM. Further, we will use the parameter-retrieval algorithm petitRADTRANS to constrain the C/O ratio of the planets. Ultimately, we plan to produce the first C/O survey of exoplanets, kick-starting the difficult process of linking planetary formation with measured atomic abundances.
The GRAVITY instrument has revolutionized optical/IR interferometry: fringe-tracking and phase-referencing allow for 30 micro-arcsecond astrometry in a dual beam mode, and for spectro-differential astrometry better than 10 micro-arcseconds. The control of systematic effects is essential to fully exploit this technological advancement. Among those systematics are static phase aberrations, introduced along the instrument's optical path, which in particular affect the inferred separation of two unresolved objects within the same FOV. Here, we present how the aberrations can be measured, characterized by low-order Zernike polynomials and, most importantly, how their impact on the astrometry is corrected. The resulting astrometry corrections are verified with calibration observations of a binary before we discuss how they affect GRAVITY's measurement of the galactic center distance.
In VIS/IR interferometry the presence of the turbulent atmosphere induces piston drifts, resulting in the optical path difference (OPD) between each telescope to fluctuate. Typically, this is compensated for by employing a fringe tracker. However, the fundamental need for a fringe tracker effectively limits the sensitivity and sky coverage. Previously, a novel method, the Piston Reconstruction Experiment (P-REx), showed how AO data can be used to estimate the piston drift over individual telescopes in order to stabilise the fringes over short timescales and extend the integration time of the fringe tracker. The principle idea behind this method is that the piston drift is simply the product of the wind velocity and the tip and tilt of the atmosphere. In this paper, new developments in the method for estimating the wind velocity will be presented, as well as comparisons between the P-REx OPD estimates obtained from the VLTI’s CIAO WFS data and GRAVITY fringe tracking data.
We present the successful demonstration of world's first large-separation ~30" off-axis fringe tracking with four telescopes in October 2019. With this technique we increase the sky-coverage for optical interferometry by orders of magnitude compared to current technology. Following the early work at the Palomar Testbed Interferometer, the first demonstration of off-axis fringe tracking at the Keck Interferometer and with PRIMA at the ESO Very Large Telescope Interferometer, and the breakthrough with the GRAVITY Galactic Center observations, we enhanced the VLTI infrastructure for GRAVITY to take advantage of the PRIMA Star separators and Differential Delay Lines for off-axis fringe tracking. In our presentation we give an introduction to the subject, present the enhancements of the VLTI, and present our results from the first on-sky operation in October 2019, with observations of the Orion Trapezium Cluster, a field brown dwarf, and a high redshift quasar.
Instrumental polarization can have large effects on measurements with the VLTI, as it can alter measured polarization and introduce uncertainties. To understand these effects we measured and simulated the instrumental polarization of the VLTI and of GRAVITY. We are able to provide a calibration model for GRAVITY observations and quantify systematic uncertainties due to instrumental polarization. This work has shown to be crucial to measure the polarization of the galactic center black hole Sgr A* where we detect a swing in the polarization angle during flare events. While the analysis was done for GRAVITY, it also gives an important basis for the design of future near-infrared instruments at the VLTI.
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.1 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*2 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.
For sensitive infra-red long-baseline interferometry, it is crucial to control the differential piston between the apertures. Classically this is achieved with a fringe tracker which measures the movement of the interferometric fringes. In this paper, we describe a new method to reconstruct the piston variation introduced by atmospheric turbulence with real-time data from adaptive optics wave-front sensing. Concurrently, the dominant wind speed vector can also be retrieved. The method is analyzed in simulation for atmospheric turbulence of various strength, and wind vectors varying with layer altitude. The results from the simulations show that this method could help to reliably retrieve the piston variation and wind speed from wavefront sensor data. The method is related to concepts of predictive control AO algorithms and reconstruction of the point spread function.
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