This report will describe the progress towards modeling the radiance of a mesospheric atomic sodium guidestar pumped
with a continuous-wave, narrow-linewidth source. We will model the cases of pumping only the D2a line and pumping
both the D2a and D2b lines simultaneously. The simulation is named the sodium guidestar simulation or SGS.
A computer-automated cw sodium guidestar FASOR (Frequency Addition Source of Optical Radiation) producing a
single frequency 589-nm beam with up to 50 W for mesospheric beacon generation has been integrated with the 3.5-m
telescope at the Starfire Optical Range, Kirtland AFB, New Mexico. Radiance tests have produced a peak guidestar V1
magnitude = 5.1 (~7000 photons/s/cm2 at zenith) for 30 W of circularly polarized pump power in November 2005. Estimated
theoretical maximum guidestar radiance is about 3 times greater than measured values indicating saturation due
to atoms possibly becoming trapped in F'=1 and/or atomic recoil. From sky tests over 3.5 years, we have tracked the
annual variation of the sodium column density by measuring the return flux as a function of fasor power and determining
the slope at zero power. The maximum occurs on October 30 and the minimum on May 30, with corresponding predicted
returns of 8000 (V1 = 4.8) and 3000 (V1 = 5.8) ph/s/cm2 with 50 W of fasor power and circular polarization. The
effect of the Earth's magnetic field on the radiance of the sodium laser guidestar (LGS) from various azimuths and elevations
has been measured. The peak return flux over our observatory occurs at [az=198o; el=+71o], compared with the
direction of the magnetic field lines at [190o; +62o], and it can vary by a factor of 3 over the sky above el = 30o. First
results for non-optimized sodium LGS adaptive optics (AO) closed-loop operation have been obtained using binary
stars. Strehl ratios of 0.03 have been measured at 850 nm and a 0.14 arc second binary star has been resolved during
first closed loop observations. Guidestar characteristics, including radiance, size, and Rayleigh backscatter, the sodium
LGS wavefront sensor (WFS) AO system, and recent closed-loop results on binary stars are presented.
Mesospheric sodium guidestar radiance is plotted vs. wavelength, fasor power, fasor polarization and date. Peak radiance for circular polarization was about 7000 photons/sec/cm2 (V1 magnitude = 5.1) for 30 watts of pump power in November of 2005. Pumping with circular polarization at high power produces about 2 times more return than linear polarization. Pumping D2a at high power produces about 12 times more return than pumping D2b. A lidar equation is used to determine column density. Estimated maximum possible guidestar radiance is about 3 times greater than measured values. Guidestar radiance may be saturated by atoms becoming trapped in F'=1 and atomic recoil.
A CW Na guidestar excitation source has been constructed and installed on the 3.5-m telescope at the Starfire Optical Range. This device is comprised of injection-locked Nd:YAG ring lasers operating at 1064 nm and 1319 nm and a doubly resonant cavity where sum-frequency generation of these wavelengths in LBO produces a diffraction-limited linearly-polarized 589-nm beam. Up to 50 W of 589-nm light for mesospheric guide-star generation has been produced. The injection-locked Nd:YAG lasers are capable of operating at up to 100 watts at 1064 nm and 60 watts at 1319 nm.
Three sets of sky tests have been conducted at the Starfire Optical Range with a continuous-wave, single-frequency, 20-W laser in preparation for a 50-W facility-class laser. Brightness measurements were made of the sodium guidestar produced with and without adaptive optics (AO) correction to the outgoing laser beam when it was either linearly or circularly polarized. Correcting for the transmission of our V filter at the sodium wavelength, a circularly polarized laser beam of 12 W out the telescope produced a guidestar of V=7.1 (1015 ph/s/cm2 at the top of the telescope). In general, a circularly polarized beam produces a guidestar between 75 and 100% brighter than a linearly polarized beam, indicating a significant degree of optical pumping of the sodium D2-line magnetic sublevels. However, guidestars produced with beams launched with tip-tilt correction only were 11% brighter than with beams launched with full AO correction. From deconvolved images of the guidestar taken with the 3.5-m telescope, the smallest spot, produced from a beam with 8.5 W of power out the telescope, circular polarization, and launched closed loop, had a Gaussian FWHM of 0.85 arcsec, or 38 cm at an altitude of 92 km. This corresponds to a peak Gaussian intensity of 3.8 mW/cm2.
We report on the development of a 50-W, continuous-wave, sodium wavelength guidestar excitation source for installation on the azimuth gimbal structure of the 3.5-m telescope at the Starfire Optical Range. The laser is an all solid-state design employing two diode-pumped Nd:YAG sources operating at 1064 and 1319 nm that are combined to generate 589-nm radiation using a lithium triborate non-linear crystal. Key features of the system include single-frequency, injection-locked high-power oscillators, a doubly resonant sum frequency generator cavity, a short-term 10 kHz wide 589 nm spectrum, excellent beam quality and power stability, and turn-key operation using computer control and diagnostics. The laser beam is projected from the side of the 3.5-m telescope. A novel elevation beam dither approach is employed to determine range to the centroid of the guidestar formed in the column of mesospheric sodium and maintain focus of the wave front sensor.
The multi-conjugate adaptive optics (MCAO) system design for the Gemini-South 8-meter telescope will provide near-diffraction-limited, highly uniform atmospheric turbulence compensation at near-infrared wavelengths over a 2 arc minute diameter field-of-view. The design includes three deformable mirrors optically conjugate to ranges of 0, 4.5, and 9.0 kilometers with 349, 468, and 208 actuators, five 10-Watt-class sodium laser guide stars (LGSs) projected from a laser launch telescope located behind the Gemini secondary mirror, five Shack-Hartmann LGS wavefront sensors of order 16 by 16, and three tip/tilt natural guide star (NGS) wavefront sensors to measure tip/tilt and tilt anisoplanatism wavefront errors. The WFS sampling rate is 800 Hz. This paper provides a brief overview of sample science applications and performance estimates for the Gemini South MCAO system, together with a summary of the performance requirements and/or design status of the principal subsystems. These include the adaptive optics module (AOM), the laser system (LS), the beam transfer optics (BTO) and laser launch telescope (LLT), the real time control (RTC) system, and the aircraft safety system (SALSA).
We extend our prior work on continuous-wave and pulsed excitation of sodium guidestars. We discus the effects of atomic recoil, saturation of absorption, and optical pumping on guidestar brightness for the cases of continuous-wave (CW) single-mode and multimode excitation.
We compare the effectiveness of various laser systems for producing a sodium guidestar in the mesosphere for large aperture telescopes. We discuss the requirements for two applications at two sites: satellite imaging at the Starfire Optical Range in Albuquerque, New Mexico (SI) and infrared astronomy at the Steward Observatory in Tucson, Arizona (IR). SI may use either a hybrid system employing a rayleigh and a sodium guidestar or a system employing only a sodium guidestar. IR will use only a sodium guide star. Our results are based on analysis and computation that have been compared to 5 different experiments. Parameters included in our comparison are pulse format, polarization, center frequency, and bandwidth. The infrared astronomy application power requirements are low enough that state of the art laser technology can meet them. But the satellite imaging application is problematic. Required powers are sufficiently high that thermo-optic effects in materials can be a problem.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.