Design and testing of a dynamic refocus system for Rayleigh laser beacons

We present the design, laboratory tests and preliminary field tests of a dynamic refocus system for 351nm Rayleigh beacon laser guide stars. The purpose of dynamic refocus is to increase the beacon signal from a pulsed laser, by maintaining focus in a fixed plane while the laser pulse travels through the atmosphere over an extended height range. The focusing element in our system is a moving concave mirror. The optics have been designed and built to focus on a ring of 5 beacons at 1 arc minute radius at the 6.5 m MMT, covering the range 18-40 km. Laboratory tests of image quality resulted in 0.5 arcsec refocused images corresponding to the height range 22 - 28 km, free from spherical aberration. Preliminary field tests were performed on the Mt. Bigelow Observatory 1.5 m telescope, with a frequency-tripled, Q-switched YLF laser beam projected from a 25 cm telescope. To simulate an off-axis sub-aperture of the MMT, the laser and telescope axes were set 3 m apart and reimaging optics were placed ahead of the refocus unit to image at the same plate scale as the MMT (500 μm/arcsec). Returns from different heights were selected by gating the detector with a Pockels cell. Returns over a 10 km height range from 8km to 18km were brought into focus for a total mirror motion measured to be 900 μm. The system is now ready for testing dynamic refocus, which will be accomplished by attaching the mirror to a metal resonator tuned to the laser pulse frequency. The range from 23 to 35 km to be used will require a motion of 500 μm.