What is NAOMI?

NAOMI is the Adaptive Optics (AO) system for the WHT, installed at the GHRIL Nasmyth focus.
NAOMI stands for `Natural-guide-star AO for Multi-purpose Instrumentation'. The Adaptive correction is performed by a 76-element (228 degrees of freedom) segmented deformable mirror. It is similar to that used in ELECTRA, but with strain gauges on each actuator which help to measure the position, and allow each segment to be returned to the starting position reliably.

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

Why use segmented DM?  What performance will NAOMI give?

Most of the deformable mirrors used in astronomical adaptive-optics systems have continuous facesheets. Segmented mirrors allow better correction at short wavelengths, at the cost of higher IR emissivity from the gaps between the segments, and loss of 2% of the incoming light to the same. Positioning of the individual elements of the deformable mirror is unusually accurate, thanks to strain-gauge feedback which ensures a linear response to the requested wavefront corrections.

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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