The layer-oriented wavefront sensors

The Layer-Oriented WaveFront Sensors

The star-oriented and layer-oriented concepts

Introduction

In 1999, while classical adaptive optics systems were implemented on all the 10m class telescopes, it seems more and more evident that a correction on a larger field of view is needed especially for the extremely large telescopes under study.
The Multi Conjugate Adaptive Optics (MCAO) concept has already been proposed by Beckers in the early 1990 and several groups have studied theoretical aspects of the atmospheric tomography (Ellerbroek 1994, Tallon 1990).
In the first proposed MCAO concepts and also in some on--going projects, several wavefront sensors conventional in their concept although especially designed for such a task, are coupled to one out of several point (or quasi- point, in case for istance of Laser Guide Stars) sources. In 1999, a different approach has been introduced by Ragazzoni, later defined in its detailed form (Ragazzoni, 2000) including variations on the theme like the Multiple Field of View leading to expectations of even much better performance than what originally claimed. In what has been called layer-oriented wavefront sensing, several stars are sensed simultaneously and optically coupled to a single detector conjugated to a specific altitude, or, in other words, to a specific layer.
On practical point of view, Gemini Observatory is currently building a MCAO system for Gemini South based on Shack-Hartmann wavefront sensors and laser guide stars. The European Southern Observatory has chosen to build a demonstrator of the MCAO concept with two possible wavefront sensors concept: the star-oriented and the layer-oriented.

The star-oriented and layer-oriented concepts

In the star-oriented concept (left figure), a WFS is associated to each reference star and the real-time wavefront computer retrieve the deformable mirror commands from all the measurements.

In the Layer-oriented concept (right figure), the WFS is conjugated to a particular altitude and has therefore an associated deformable mirror. The photons of the stars are then are supposed to act collectively.

The Breadboard

In order to give a practical demonstration of the concepts embedded in the layer--oriented idea we decided to build a prototype instrument of the LO WFS for MAD. The goal was also to acquire a much deeper knowledge of the technicalities linked with this novel concept of wavefront sensor in order to have a much detailed background for the design of on--sky experiments and instrument facilities, like NIRVANA aboard LBT and one of the channels of MAD aboard VLT.
In this framework we designed, built and tested a prototype of layer--oriented wavefront sensor that, although limited in the number of sensed stars and adopting manual stages for alignment and acquisition purposes, is conceptually identical to the ones we plan to implement on 8m class telescopes in the near future. The experiment is described extensively in Farinato, J. et al, 2002. The prototype is composed of two sub-systems: the atmosphere-telescope simulator part and the wavefront sensing part. Using fibers we simulate 9 guide stars.

The telescope simulator is made by two telescope lenses and a diaphragm. The atmosphere is simulated by inserting plastic screens in front of the diaphragm. Such a holder allows to insert up to seven screens at differents altitudes ranged between the ground and 10 km.

The beams illuminating the turbulence screens are collimated, simulated a real situation where the reference sources are at infinite distance. Furthermore, the exit pupil appears at infinity. This sub-system delivers to the wavefront sensing unit a F/32 telecentric focal plane, where the images of the light sources are blurred by the turbulent screens.

The wavefront sensing module consists of

four star enlargers, each ending with a pyramid prism,

a re-imaging objective and

a CCD detector to record the intensity on the re-imaged pupils.

The star enlargers can be moved along two axes orthogonal to the system optical axis, in order to pick up the light of the reference sources. From the optical point of view, the star enlargers effect is to magnify the images of the reference sources, increasing the focal ratio of each beam separately; the net effect is the shrinking of the pupil size on the detector plane.

Each star enlarger ends with a pyramid prism which splits the light into four beams; the beams corresponding to different sources are optically combined by an objective, which produces in total four pupil images (one for each face of the pyramid prism) onto the detector.
The perfect overlapping of the pupil images corresponding to the four reference sources actually occurs when the detector is conjugated to the telescope pupil diaphragm; when it is conjugated to a different screen, in fact, each pupil image splits into four slightly displaced pupils (one for each reference source), which can be accommodated into a circle called meta-pupil.

Results

A property of the layer-oriented system is to be more sensible to the nearest perturbing layer from the focusing altitude even if it measures the wavefront variation of several layers. If the atmosphere is composed by a single layer, the measurement precision of the layer depends on how much it is far away from the focusing altitude of the system. This distance introduces a smoothing of the layer with a known amplitude and this smoothing can be numerically computed knowing the relative layer altitude with respect to the focusing altitude.
We verified quantitatively the smoothing of the screen as a function of its distance to the focusing altitude by comparing the measured wavefront and the computed one. The system is focused on the ground. The screen 4 is successively placed at different altitudes going from layer 1 to layer 5. Using the wavefront obtained when the screen is on the ground layer, we compute the wavefronts which should be obtained at different altitudes ranging from layer 2 to layer 5 by numerically blurring it. We tried to reproduce the actual situation in which the screen is moved farther and farther from the conjugation altitude and it appears more and more blurred, due to the smoothing effect associated to the field of view.
The numerical smoothing has been accomplished by convolving the reference wavefront by a circular dish, whose diameter, computed on the basis of geometrical considerations on the system optical layout, increases with the distance of the screen from the conjugation altitude: going from a given altitude to the adjacent one the diameter of the dish changes by approximately 5 pixels.
We verify also that non-conjugated layers are perturbing marginally the sensing of the conjugated ones whatever is the altitude of sensing and that the turbulent layers are properly recognized if the WFS is conjugated to them. We measure the composite wavefront for different altitudes of sensing and compare it with the three screens used to simulate the atmosphere.

Comparison between the measured and the theoretical wavefronts for different positions of the atmospheric screen

Composite wavefront obtained when the WFS is focused on the ground.

Composite wavefront obtained when the WFS is focused on the altitude 3.

Screens 6, 1 and 3 are respectively placed on layers 1, 3 and 5 and the system is successively focused to the same layers. They have respectively RMS values equal to 1.7, 2.5 and 1.8 radians. We can see that the composite wavefront has always the highest correlation coefficient with the conjugated screen. The atmospheric layers are properly recognized if the WFS is focused at their altitude.
For the first time we built in the laboratory a wavefront sensor measuring collectively the light coming from four different sources. There are no conceptual limit to the number of the observable stars, other than optomechanical issues. The light carrying information upon the turbulence or the wavefront from their origin to the entrance of the wavefront sensor has been successfully co-added optically and fed into a single detector. The data analysis performed shows that the system behaves qualitatively and quantitatively as predicted.

Board system has allowed us to verify the layer-oriented concept. Especially,

The correct behaviour of the LO WFS has been demonstrated by observing several configurations with up to 3 layers at different focusing altitudes;

The proper smoothing of the layers has been checked and using a uniform disc for the smoothing (condition not true in the real set-up) gives anyway correlation coefficients always higher than 0.92 even with the highest altitude layer.

The proper re-imaging of a specific layer, as seen through two additional screens, has been accomplished with correlation coefficients in the range 0.62...0.92.