Adaptive Optics

WAVEFRONT PHASE


Adaptive optics is a technique used in modern astronomical observatories aimed at compensating the effects of atmospheric turbulence. The atmospheric turbulence varies so quickly that the wavefront phase must be measured milliseconds.

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

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

The overall effect of the atmosphere turbulence is that the perfectly flat wavefront that we had in the open space is now a “rough” surface, also referred to as aberrated wavefront. This means that light rays do not travel in parallel to each other anymore, now they diverge or converge randomly.

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

Adaptive Optics is the solution: it can achieve extremely high resolution and constrast from Earth based astronomic observatories.

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

In this description, the most important part of an adaptive optics system is missing: the wavefront sensor. The wavefront sensor measures as accurately as possible the wavefront in order to command the deformable mirror properly.

The most common wavefront sensor is Shack-Hartmann, this device is a “hardware” wavefront sensor. This implies that the sensor needs specialized optical elements in cooperation with an imaging sensor to measure the wavefront. This element is a microlens array. Each microlens will produce a small spot whose position is proportional to the main wavefront shape over the microlens extent.

Shack Hartmann sensor effectively samples the wavefront in accordance with the points of the microlenses. This implies that, for example, a Shack-Hartmann comprising 32x32 microlenses can only measure a wavefront with a resolution of 32x32 points.

For many years, this was the standard in Adaptive Optics, despite its great success with 4 or 8 meter class telescopes, the next generation of Extremely Large Telescopes needs more resolution. This, , developing a wavefront sensor without resolution limitations was our main motivation in Wooptix.

Wooptix WFP only requires a bare sensor imaging to measure the wavefront. Removing microlenses has the advantage of virtually breaking down the limits in terms of resolution. The only limiting factor for the wavefront resolution is the pixel count in the imaging sensor. Wooptix WFP can considered a “software” sensor, as the wavefront reconstruction is done entirely by processing the data and not by an external passive component.

Our technology has been tested successfully in the AOLI instrument, acknowledging the high frame rate and accuracy of Wooptix WFP:

Other applications