
General Information
NAOMI, the WHT's adaptive-optics (AO) system, is mounted on an optical bench at the Nasmyth focus. The key components are the 10*10 Shack-Hartmann wavefront sensor (WFS), the 76-element Thermotrex segmented deformable mirror (DM), and the Zeiss tip-tilt mirror. The wavefront sensor measures the distortion of the incoming wavefront, and the deformable mirror is configured (many times per second) to cancel these distortions, yielding a near-diffraction-limited image at the science camera (currently INGRID).
The DMs of most AO systems have continuous face-sheets. NAOMI is unusual in having a segmented DM. Segmented mirrors allow better correction at short wavelengths, but have higher IR emissivity (from the gaps between the segments), and the afternoon setup (mirror-flattening) is complicated. Positioning of the individual segments of the NAOMI DM is particularly accurate, thanks to the unique strain-gauge feedback which ensures a linear response to the requested wavefront corrections.
NAOMI is also unusual in having a moveable pickoff mirror to direct light from a guide star anywhere in the field, to the WFS, allowing the science object to be on-axis, which is an advantage for IR observing.
NAOMI was designed to deliver, under median-seeing conditions, Strehl
> 0.25 over 25% of the sky, in K band (2.2 microns), 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. NAOMI is not expected to
provide good correction in seeing worse than 1.2 arcsec (when there are
large phase changes across individual mirror segments). NAOMI will operate
with natural guide stars, but has been designed for simple upgrade to operation
with single laser guide stars.
Links contained introductory information to different NAOMI components are tabulated below.
| Created by Stephen Goodsell
sjg@ing.iac.es |