Multi-Conjugate Adaptive Optics
Theory

Turbulence can not be compensated completely neither by classical AO, nor by MCAO. The AO limits are well understood. What happens in MCAO?

• With only a finite number of Guide Stars the 3D turbulence is not measured completely - hence tomographic error arises.
  • On Extremely Large Telescopes, the de-correlation of wavefronts causes tomographic errors. It means that GS can not be selected in a too wide field. This limit is calculated in paper [7].
  • On 8-m class telescopes, the incomplete overlap of GS beams in the upper atmosphere is the major source of tomographic error.
• Even if the 3D turbulence were known exactly, the number of Deformable Mirrors defines the corrected field of view. The MCAO isoplanatic patch is described in paper [6].
• When Laser Guide Stars are used, the tip-tilt signals remain undefined, hence some unseen modes appear in an MCAO system. Those modes can be measured either by a low-order WFS on a Natural Guide Star, or by tip-tilts from at least 3 NGSs. See paper [2].

The Strehl ratio in the K band for a classical AO system correcting 190, 90, or 55 Zernike modes (full, dashed, dotted lines) in function of off-axis distance. For an MCAO system with 2 DMs at 8 m telescope using 3 NGS as light sources, the Strehl ratio is almost constant in the field. It is higher in 1 arcmin field (upper lines) than in the 2 arcmin field (lower lines) because of tomography errors.

Simulation for MAD

An extensive set of simulations were carried out for the MAD MCAO demonstrator. Here we give two examples of the capabilities of the demonstrator.

The above picture shows, for a given natural guide star configuration (3 stars in a 1' triangular configuration), the K band Strehl ratio obtained within the 2 arcminute corrected field of view of MAD.

As a comparison, at 60'' off-axis, a conventional AO system (with a single guide star and a single deformable mirror) provide a Strehl ratio of less than 5%.

Similar simulations were also carried out on the layer oriented wavefront sensor.

MCAO simulations for OWL

We are currently building a simulation hardware and software architecture. The goal is to be able to predict the performance of an adaptive optics system on OWL.

The harware will be a beowulf cluster of about 20 PCs. We are currently developping software and testing a smaller cluster:

The very preliminary first results are encouraging. We have already been able to produce, with optimized software, the image of Karhunen-Loeve polynomial 40000, and simulate a single conjugate AO system on a 30m telescope !