# aLIGO ISC Beam Steering - Tip-Tilt Mirror Design

Document #:
LIGO-T1000042-v2
Document type:
T - Technical notes
Other Versions:
Abstract:
This document describes the design of the Tip-Tilt mirrors used for the aLIGO Interferometer Sensing and Control Beam Steering.

The mode-matching telescope elements between the anti-symmetric port (behind the signal recycling mirror) and the output mode cleaner (OMC) require suspended optics which have a sufficient DC pointing range. Also, the Tip-Tilt mirrors~\cite{LIGO-T0900096} have been designed to generate their own alignment control signals. This is generated by dithering the suspended mirror in the pitch and yaw degrees of freedom at a (specific) high frequency ($>$1~kHz). This dither generates a signal on the DC-readout detectors. This signal is then demodulated to provide the control signal. In addition the Tip-Tilt mirrors are used as a beam shutter. This is achieved by applying a large offset in pitch or yaw (on the coils) to steer the beam away from the OMC alignment, and to protect the DC photodiodes.

During the commissioning of eLIGO it has been decided to modify the Tip-Tilt mirrors to include a vertical isolation, as well as to lower the eigenmode frequencies.

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Notes and Changes:
To prevent confusion I changed the title from 'aLIGO ISC Beam Steering - Tip-Tilt Blade Design' to the current 'aLIGO ISC Beam Steering - Tip-Tilt Mirror Design' as this is the full TT design document.
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