The SINFONI AO module was designed based on the Multi-Application Curvature Adaptive Optics (MACAO) systems which are used in the VLT Coude focii for the VLTI. Modified for the respective focii of the VLT a system with the same 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) AO reference stars. Due to the use of avalange photo diodes (APDs) in the wave front 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 the deformable mirror has to be flattened by the AO system with a calibration light source (optical fiber). There is some overhead time required after any preset or after long integration sequences to correct the shape of the deformable mirror for the respective position on the sky.
- simulated atmospheric turbulance model together with the measured interaction matrix of SINFONI.
- the error budget associated to the instrument (mainly optical aberrations 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 figure for some seeing cases (0.6 to 1.2" at 500nm) and high altitude wind speeds of 11m/s (long), 22m/s (medium) and 33m/s (short):
The main uncertainty in the convertion between optical seeing and IR Strehl ratios is the undefined coherence time of the atmosphere. An example one would read from the figure that the Strehl ratio at seeing of 0.8" may vary between 0.16 and 0.33 for a 14th magnitude AO guide star for different coherence times!
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 altitude wind speed of 33m/s was used to obtain the graph for the short coherence times.
Please note that only the ESO exposure time calculator (ETC) should be used to evaluate the feasibillity of projects. The ETC output was verified with data taken on the sky.
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).