The light paths through the NAOMI optical system.
Image of an illuminated mask taken with the NAOMI wavefront sensor CCD.
In the complete wavefront sensor system, each spot is optically matched to a segment of the deformable mirror. The CCD is illuminated by starlight viewed through an 8x8 array of microlenses. The spots all move around independently because of optical turbulence. This motion is measured by the WFS and corrected by the deformable mirror.
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A CAD model of the Wavefront sensor mechanism used in NAOMI. The design allows off-axis guide stars to be used anywhere within the NAOMI field (2.9 arcmin diameter), and provides excellent repeatability on acquisition and for offsetting. |
The Wavefront sensor mechanism being built in the NAOMI lab. |
The 76 Element NAOMI deformable mirror.
The defomrable mirror used in NAOMI is a 76 element segmented mirror (it has 76 square mirrors instead of a continuous flexible mirror). Each mirror is mounted on three piezo elements, each of which has a strain gauge system mounted to it. The strain gauges provide an independent measure of the movement, and are used to reduce the hysteresis of the system to below 1% (piezo actuators typically have a 10% hysteresis).
The NAOMI Fast Steering Mirror, shown with a dummy mirror mounted on the tip-tilt stage (right foreground), drive electronics (right center) and response signals on oscilloscope (left background).
The deformable mirror is designed to have very fast response and be very accurate for small movements (100 nm). Since the overall slope of the wavefront can be quite large, and will be added to be any tracking errors of the telescope or wind shake, a separate tip-tilt mirror is used to remove overall slope from the wavefront.
This mirror, the Fast Steering Mirror, can produce a beam deflection of up to 5 arcseconds at a frequency of 20Hz, and is controlled independently so that it can be driven from accumulated slope on the deformable mirror, or from an external source to facilitate dithering movements.
The control system is based on an array of Texas Instruments TMS320C40 processors, since most of the algorithms used in Adaptive Optics are inherently parallel. The control system is hosted by a Sun workstation, and is designed to operate with a complete sequencing and data display environment on Unix machines.