Adaptive Optics

AO-Module Overview

The SINFONI AO module was designed based on the Multi-Application Curvature Adaptive Optics (MACAO) systems which are used in the VLT Coude focii forthe VLTI. Modified for the respective focii of the VLT a system with thesame wave front sensor and deformable mirror is used for the SINFONI instrument and the future high resolution infrared spectrograph CRIRES.

The MACAO systems are highly reliable curvature sensing adaptive optics systems with excellent performance for bright (R < 11 magnitude) AOreference stars. Due to the use of avalange photo diodes (APDs) in the wavefront sensor the limiting magnitudes for AO reference stars are fainter then for comparable instruments. The system is extremely user friendly and requires no manual intervention to be started apart from entering the target and guide star coordinates with reasonable precission.

SINFONI can be also used without AO guide stars in seeing limited mode.The deformable mirror is then used as a folding mirror. The shape of thedeformable mirror has to be flattened by the AO system with a calibrationlight source (optical fiber). There is some overhead time required after anypreset or after long integration sequences to correct the shape of thedeformable mirror for the respective position on the sky.

AO-Module System Performance

Natural Guide Star Mode

The Strehl performance achieved in NGS mode has been evaluated for on axis observations by combining

  • simulated atmospheric turbulance model together with the measured interaction matrix of SINFONI.
  • the error budget associated to the instrument (mainly opticalaberrations of SPIFFI)
  • the resulting Strehl ratios were compared with data taken with SINFONI

The predicted Strehl performance in K band is shown on the following figurefor some seeing cases (0.6 to 1.2" at 500nm) and highaltitude wind speeds of 11m/s (long), 22m/s (medium) and 33m/s (short):

The main uncertainty in the convertion between optical seeingand IR Strehl ratios is the undefined coherence time of theatmosphere. An example one would read from the figure thatthe Strehl ratio at seeing of 0.8" may vary between 0.16 and0.33 for a 14th magnitude AO guide star for different coherencetimes!

An estimate of the high altitude wind speed for the coming week can be obtained from the Long term predictions for Paranal. In the figure above a high altitudewind speed of 33m/s was used to obtain the graph for the short coherence times.

Please note that only the ESO exposuretime calculator (ETC) should be used to evaluate the feasibillity of projects.The ETC output was verified with data taken on the sky.

Laser Guide Star Mode

The Strehl performance achieved in LGS mode has been evaluated for on axis observations by combining:

  • the performance estimation obtained for the NGS mode with a guide star brightness corresponding to the expected NGS brightness
  • the estimated degradation factors undegone in LGS operations:
    • cone effect
    • trombone control error
    • LGS jitter
    • effect of the LGS spot size
    • aberrations introduced by the LGS additional optical elements
    • STRAP performance (depending on the pointing star brightness)
    • tip/tilt anisoplanetism

The predicted Strehl performance in K band is shown on the following figure (for a 0.65" seeing at 500nm).