Adaptative optics

WAVEFRONT PHASE


Adaptive optics is a technique used in modern astronomical observatories to compensate the effect of atmospheric turbulence. The atmospheric turbulence varies so rapidly that the wave front phase measurement must be done in milliseconds.

The incoming light from any celestial object is emitted in every direction, this is equivalent to a spherical wave front. The distance to any deep space object is so large that we, from Earth, receive a small patch of the spherical wave front, this patch can be considered perfectly flat.

This is only true before the light encounters the atmosphere, once the flat wave front propagates through the atmosphere this situation changesdrastically. Optically, the atmosphere is very far from an uniform and crystalline media. The air density, temperature and speed is not constant, it varies rapidly, typically in the order of milliseconds. These effects are commonly known as atmospheric turbulence.

The overall effect of the atmosphere turbulence is that the perfectly flat wave front that we had in free space is now a “rough” surface, known also as aberrated wave front. This means that light rays does not travel in parallel to each other anymore, they are now diverging o convergingin a random way.

What is the effect of this aberrated wave front in the image of an object? The resolution and contrast is severely damaged.

The solution to this is Adaptive Optics, is the solution to achieve extremely high resolution and constrast from Earth based astronomic observatories.

In every Adaptive Optics system exist a deformable mirror, this device can basically correct the aberrated wave front by simply adopting the opposite shape of the wave front. Once the wave front is flattened the light can be focused onto a science instrument for imaging or spectrography purposes.

In this description lacks the most important part of an adaptive optics system: the wave-front sensor. The wave front sensor task is to measure as accurately as possible the wave front in order to command the deformable mirror in an appropriate way.

Most common wave front sensor is the Shack-Hartmann, this device is a “hardware” wave front sensor, this means it needs specialized optical elements along with an imaging sensor to measure the wave front. This element is a micro lens array, each of these micro lenses will produce a small spot whose position is proportional to the mean wave front shape over the micro lens extent.

A Shack Hartmann sensor effectively samples the wave front in as many points as microlenses it has. Meaning that, for example, a Shack-Hartmann comprising 32x32 microlenses will be only capable of measuring a wave front with a resolution of 32x32 points.

This has been the standard in Adaptive Optics for many years, although it has achieved great succes with 4 or 8 meter class telescopes, the next generation of Extremely Large Telescopes needs more resolution. This was our main motivation in Wooptix, to develop a wave front sensor without resolution limitations.

Wooptix WFP only requires a bare imaging sensor to measure the wave front, removing micro lenses has the advantage of virtually no resolution limit. The only limiting factor for the wave front resolution is the pixel count in the imaging sensor. Wooptix WFP can be seen as a “software” sensor, as the wave front reconstruction is done entirely by processing the data and not by an external passive component.

Our technology has been proven with great success in the AOLI instrument, this is a demonstration of the high frame rate and accuracy of Wooptix WFP:

Other applications