The distortion of the wavefront, and its tip-tilt movement, is measured using the Wavefront Sensor (WFS), which consistes of a Shack-Hartmann wavefront sensor (built at RAL, based on an 80x80 EEV CCD39). The most important lenslet array in the Shack-Hartmann is a 10x10 array, and similarly the DM segments are laid out on a 10*10 grid with the corners missing.
The tip-tilt motion, also caused by the atmosphere, are removed separately by a Fast Steering Mirror (FSM), at the rate of sseveral tens of times per second. Meanwhile, every second, any trend in the tip-tilt motion in one particular direction is corrected by sending guiding packets to the telescope.
Shortcut to Pictures and Diagrams Overview, from NAOMI Web-page at ATC
The NAOMI adaptive-optics facility will provide near-diffraction-limited imaging at the WHT GHRIL Nasmyth focus. The detector will be ING's near-IR imager INGRID. In natural seeing 0.8 arcsec, with a guide star V = 14 lying close to the (on-axis) target, NAOMI will deliver diffraction limited images in K, FWHM ~ 0.15 arcsec. The Strehl ratio (between the peak heights of corrected and diffraction-limited point-spread functions) is predicted to be ~ 0.4.
NAOMI is designed to deliver, under median-seeing conditions, Strehl > 0.25 over 50% of the sky, and Strehl > 0.7 over 5% of the sky. At shorter wavelengths, the performance will not be diffraction limited, but there will be partial correction, probably FWHM < 0.2 arcsec at 0.7 microns. NAOMI will operate with natural guide stars, but has been designed for simple upgrade to laser-guide-star operation.
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