Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

First Light for the GRAVITY+ Adaptive Optics: Extreme Adaptive Optics for the Very Large Telescope Interferometer

GRAVITY+ Collaboration, :, F. Allouche, C. Bailet, M. Benisty, A. Berdeu, J. -P. Berger, P. Berio, A. Bigioli, C. Blanchard, O. Boebion, H. Bonnet, G. Bourdarot, P. Bourget, W. Brandner, J. Brulé, P. Burgos, M. Carbillet, C. Correia, B. Courtney Barrer, S. Curaba, R. Davies, D. Defrère, A. Delboulbé, F. Delplancke, R. Dembet, A. Drescher, N. Dubost, A. Eckart, C. Édouard, F. Eisenhauer, L. Esteras Otal, M. Fabricius, H. Feuchtgruber, P. Fédou, G. Finger, N. M. Förster Schreiber, R. Frahm, E. Garcia, P. Garcia, R. Garcia Lopez, R. Genzel, J. P. Gil, S. Gillessen, T. Gomes, F. Gonté, V. Gopinath, C. Gouvret, J. Graf, P. Guajardo, S. Guieu, W. Hackenberg, M. Hartl, X. Haubois, F. Haußmann, T. Henning, P. Hibon, S. Hönig, M. Horrobin, M. Houllé, N. Hubin, I. Ibn Taieb, L. Jochum, L. Jocou, A. Jost, J. Kammerer, L. Karl, A. Kaufer, P. Kern, P. Kervella, J. Kolb, H. Korhonen, L. Kreidberg, P. Krempl, S. Lacour, S. Lagarde, O. Lai, V. Lapeyrère, R. Laugier, V. Leal, J. -B. Le Bouquin, J. Leftley, P. Léna, B. Lopez, D. Lutz, Y. Magnard, F. Mang, A. Marcotto, D. Maurel, A. Mérand, F. Millour, M. Montarges, N. More, N. Morujão, T. Moulin, H. Nowacki, M. Nowak, S. Oberti, T. Ott, L. Pallanca, F. Patru, T. Paumard, K. Perraut, G. Perrin, P. O. Petrucci, R. Petrov, O. Pfuhl, N. Pourré, S. Rabien, C. Rau, M. Riquelme, S. Robbe-Dubois, S. Rochat, M. Salman, J. Sánchez-Bermúdez, J. Schubert, J. Scigliuto, P. Shchekaturov, N. Schuhler, J. Shangguan, T. Shimizu, S. Scheithauer, C. Soenke, F. Soulez, E. Stadler, J. Stadler, C. Straubmeier, E. Sturm, M. Subroweit, C. Sykes, L. J. Tacconi, K. R. W. Tristram, S. Uysal, S. von Fellenberg, F. Widmann, E. Wieprecht, E. Wiezorrek, J. Woillez, S. Yazici, G. Zins

Abstract

GRAVITY+ improves by orders of magnitude the sensitivity, sky-coverage and contrast of the Very Large Telescope Interferometer (VLTI). A central part of this project is the development of Gravity Plus Adaptive Optics (GPAO), a dedicated high-order and laser-guide star Adaptive Optics (AO) system for VLTI. GPAO consists of four state-of-the-art AO systems equipping all 8m-class Unit Telescopes (UTs) for the wavefront correction of the VLTI instruments. It offers both visible and infrared Natural Guide Star (NGS) and Laser Guide Star (LGS) operations. The paper presents the design, operations and performances of GPAO. We illustrate the improvement brought by GPAO with interferometric observations obtained during the commissioning of the NGS mode end-2024. These science results include the first optical interferometry observations of a redshift $z\sim4$ quasar, the spectroscopy of a cool brown-dwarf with magnitude $K\sim 21.0$, the first observations of a Class I young star with GRAVITY, and the first sub-micro arcsecond differential astrometry in the optical. Together with the entire GRAVITY+ project, the implementation of GPAO is a true paradigm shift for observing the optical Universe at very high angular resolution.

First Light for the GRAVITY+ Adaptive Optics: Extreme Adaptive Optics for the Very Large Telescope Interferometer

Abstract

GRAVITY+ improves by orders of magnitude the sensitivity, sky-coverage and contrast of the Very Large Telescope Interferometer (VLTI). A central part of this project is the development of Gravity Plus Adaptive Optics (GPAO), a dedicated high-order and laser-guide star Adaptive Optics (AO) system for VLTI. GPAO consists of four state-of-the-art AO systems equipping all 8m-class Unit Telescopes (UTs) for the wavefront correction of the VLTI instruments. It offers both visible and infrared Natural Guide Star (NGS) and Laser Guide Star (LGS) operations. The paper presents the design, operations and performances of GPAO. We illustrate the improvement brought by GPAO with interferometric observations obtained during the commissioning of the NGS mode end-2024. These science results include the first optical interferometry observations of a redshift quasar, the spectroscopy of a cool brown-dwarf with magnitude , the first observations of a Class I young star with GRAVITY, and the first sub-micro arcsecond differential astrometry in the optical. Together with the entire GRAVITY+ project, the implementation of GPAO is a true paradigm shift for observing the optical Universe at very high angular resolution.

Paper Structure

This paper contains 88 sections, 8 equations, 19 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Context at the VLTI and GRAVITY+
  3. Optical-Infrared interferometry with large telescopes and adaptive optics:
  4. GRAVITY overview
  5. From GRAVITY to GRAVITY+
  6. Instrument design
  7. General design guidelines and trade-offs
  8. ALPAO 43x43 deformable mirror
  9. OCAM2 camera and frame grabber
  10. Shack Hartmann versus pyramid wavefront sensors
  11. Number of measured modes
  12. Piston-free operation
  13. Integration of CIAO
  14. Zonal versus modal control
  15. Software derotations
  16. ...and 73 more sections

Figures (19)

  • Figure 1: Location of the subsystems of GPAO within the structure of the unit telescope. The beam of light is represented in light red, from the vertical incidence on M1 down to the exit toward the VLTI tunnel. Although they are not formally GPAO subsystems, the location of the M2 mirror, the elevation and azimuth axes, the Nasmyth beacon, the mirrors of the Coudé train, the M9 dichroic, and the star separator are also highlighted because of their particular importance for GPAO.
  • Figure 2: Control scheme of GPAO. Disturbances and corrections are represented but sensing is not represented. The derotators LROT, NROT, and DROT are shaded to express that the hardware is physically present but that they are kept at a fixed angle.
  • Figure 3: Images of unresolved stars (point spread function) measured in K-band with the IRIS guiding camera of VLTI, corrected by GPAO in NGS_VIS in the bright regime (left, $G_\mathrm{RP}=5$, Strehl 77%) and faint regime (right, $G_\mathrm{RP}=12$, Strehl 11%). The flux is represented in log scale.
  • Figure 4: Strehl in the K-band versus the Gaia $G_\mathrm{RP}$ magnitude of the guide star. Points indicate the measurements with GPAO in NGS_VIS mode. The seeing conditions are proportional to the diameter of the point. The blue curves are the expectations for GPAO in NGS_VIS mode and two seeing values, the red curve is the expectation for GPAO in LGS_VIS mode with seeing 0.6. The gray curve shows the performance of the former MACAO system of VLTI.
  • Figure 5: Comparison between MACAO (entire year 2023) and GPAO NGS_VIS (from December 2024 to May 2025) of the $P_{5\%}/P_{95\%}$ injection metric histograms for the GRAVITY fringe tracker. The flux dropouts, represented by $P_{5\%}/P_{95\%}$ values close to zero, are significantly reduced by GPAO.
  • ...and 14 more figures