Photometry of a Sodium Laser Guide Star from the Starfire Optical Range. II. Compensating the Pump Beam

Abstract

Measurements of the brightness and size of a mesospheric sodium guide star pumped by a laser launched through an adaptive optics (AO) system are reported from experiments conducted on 2003 July 22 at the Starfire Optical Range. The guide star was produced by a laser launched through a 0.5 m telescope with (closed loop) and without (open loop) higher order atmospheric compensation using an similar to60 actuator AO system and with various laser polarization states. A 3.5 m telescope (without AO) 50 m away from the launch site recorded the closed- and open-loop images. Calibrated using standard stars, the maximum brightness observed for the 8.5 W of compensated pump laser power out of the top of the telescope for a 96% circularly polarized beam was V-1=7.35(840 photons s(-1) cm(-2) at the bottom of the atmosphere) for the open-loop beam (where V-1 is the V magnitude corrected for the transmission of the V filter at the 589 nm wavelength of the sodium D2 absorption line) and V-1=7.4(800 photons s(-1) cm(-2)) for the closed- loop beam. It appears that circular polarization enhances the brightness of the return by 75%. Open-loop tests produced an integrated guide star that was 11% brighter than closed- loop, but the guide star produced from a closed- loop circularly polarized pump beam was about 50% of the width of either the open-loop beam or the linearly polarized closed- loop beam. At 0."85 FWHM, or 38 cm at 92 km altitude, the guide star is suitable for many astronomical AO applications. The guide star spot sizes were measured from images reconstructed with multiframe blind deconvolution. Because of optical losses, only 8.5 W of laser power was transmitted out of the top of the telescope. Reducing this by atmospheric extinction, the "power through the mesospheric footprint," or peak Gaussian intensity, was 3.9 mW cm(-2) for the closed- loop circularly polarized laser beam. This is about 60% of the saturation intensity I-sat for the optically pumped atom, and 20% of the I-sat for the thermally equilibrated atom.