# Systems

## Single conjugate adaptive optics (SCAO)

### SCAO

In single-conjugate adaptive optics (SCAO), one guide star is used to get the measurements of the wavefront phase. In Shack–Hartmann SCAO systems the wavefront sensor (WFS) is an array of lenslets that measures the average gradient (slopes) of the phase over each subaperture in the pupil plane. These measurements are then sent to a reconstruction algorithm to estimate the wavefront phase. Currently, the most common method for reconstruction assumes that the WFS responds linearly to the deformable mirror (DM) and then creates an interaction matrix mapping the DM commands to the sensor measurements. The interaction matrix can be obtained theoretically or by measuring the responses of the known commands. The wavefront reconstruction is then essentially achieved by a single matrix vector multiplication (MVM). The inverse of the interaction matrix is multiplied by the measurement signal represented in a vector form. As the number of actuators n increases, the time to compute the reconstruction by means of the MVM method scales as O(n^2) in case of the dense operator matrices. The number of actuators involved in AO systems is expected to increase dramatically in the future. In astronomical applications, this is due to both increasing telescope diameters and new higher-resolution applications on existing systems. This increase in size, from hundreds up to tens of thousands of actuators, requires a faster method for wavefront reconstruction.

### XAO

“Extreme” adaptive optics (XAO) systems are designed for extrasolar planet imaging (detection). Direct imaging detection of photons from the extrasolar planets is extremely challenging. For instance, Jupiter, seen from outside our solar system, is 109 times fainter than the sun; a Jupiter analog in another solar system would be swamped by the light from its parent star, scattered by diffraction, optical imperfections, and atmospheric turbulence. At infrared wavelengths, younger and more massive planets will be brighter but still extremely challenging to detect with current telescopes and adaptive optics (AO) systems. Truly imaging a large sample of extrasolar planets will require an AO system designed specifically for the high-contrast imaging task.

**→ Read more** about the developed reconstruction methods: