Tilt angular correlation and tilt sensing techniques with a laser guide star

The two techniques for sensing full aperture tilt with a laser guide star (LGS) are described. The first technique exploits a full aperture beam transmitting through the main optical train. The two auxiliary telescopes, which are separated from the transmitter in transverse directions, are used to measure a laser beacon image motion. The contribution of the down propagation path to the tilt that is measured with an auxiliary telescope is eliminated by averaging LGS image over a laser beacon angular extent. Such averaging requires FOV of the receiver which greatly exceeds the tilt angular correlation scale. A second method exploits a small aperture beam transmitted from behind a portion of a primary mirror of the main telescope. A laser beacon image motion is measured simultaneously with the main and auxiliary telescopes. A full aperture tilt is determined by substracting the tilt measured with the main telescope from that measured with an auxiliary one. This method does not require transmitting laser irradiance through the main optical train, and it might be used for the mesospheric sodium layer. A scheme for measuring uncontrolled motion of the main telescope is also considered.