The 3.5 m telescope, located in Albuquerque, New Mexico, at the Starfire Optical Range (SOR) routinely images asteroids and moons orbiting around the asteroid. However point-spread-functions (PSFs) with trefoil-like structure make it difficult to detect moons at small angular separations from the parent asteroid. This work investigates whether the low wind effect, such as that reported by VLT/SPHERE, produces trefoil-like PSFs (Sauvage et. al 2016).
Scientists at the Starfire Optical Range have been researching mesospheric sodium beacons for adaptive optics since 1992. Since then, we have developed four different sodium-wavelength lasers, all of which were based on diode-pumped, sum-frequency Nd:YAG oscillators. In 2016 we combined light from two commercial sodium-wavelength lasers, which use resonant-frequency doubling of light from a Raman fiber-amplifier, to form a single beacon. We used standard polarization techniques to combine the two lasers and produce a single beam with both left-hand and right-hand circularly polarized light. To avoid competitively pumping sodium atoms of the same velocity class, we tuned each of the lasers slightly off of the peak of the Doppler-broadened sodium line. However, the return flux from this beacon was lower than we expected, even when we accounted for the effect of detuning. In this paper, we propose an explanation for this phenomenon and show results from computer modeling and on-sky tests to support our claim. We believe the sodium atoms, which were optically pumped by the shorter-wavelength laser between the D2a F=2, m=+2 and F=3, m=+3 states, were down-pumped by the longer-wavelength laser to the D2b F=2, m=+2 and F=1, m=+1 states. Thus, these atoms were no longer available for optical pumping, which reduced the efficiency of the lasers in producing return flux.
Addendum: We hope to show we can mitigate this effect by increasing re-pumping of the sodium D2b line on the shorter-wavelength laser.
To date, three types of laser sources have been used to excite mesospheric sodium atoms to use as a sodium guidestar for adaptive optics (AO). All these sources have inherent challenges and a possible fourth source is to utilize a frequencydoubled Vertical External Cavity Surface Emitting Laser (VECSEL). Such a VECSEL presents output efficiency above 20% with power in excess of 20 W. Modelling is also presented to validate the efficacy of developing VECSEL guidestar systems for use with current guidestar systems or as a stand-alone guidestar. The model agrees with the data collected with the 3.5 m telescope and narrowband laser guidestar at Starfire Optical Range.
It is a widely accepted conjecture that the width of the incoherent halo in an adaptive optics point-spread function (PSF) should decrease with the level of correction. Using end-to-end simulations we prove that this is not the case and the halo is actually increasing in width, albeit at a decreasing overall brightness level as must be the case with increasing correction. The simulations span the cases of: seeing-limited, partial-, and high-order adaptive-optics (AO) correction. We show the relationship between the theory of partially-developed speckle and the observed statistical behavior of on-axis PSF intensity. Finally, we check the results of the simulations with real data obtained using the 3.5m Starfire Optical
Range telescope located in New Mexico, US.
Using a stable single frequency (Δυ < 1 MHz) cw fasor we have characterized the guide star radiance under several conditions,
including routinely measuring the radiance at various launch powers and simultaneously illuminating the same spot
with a second fasor with a range of different frequency separations. Making use of sodium's hyperfine energy diagram and
allowed transitions it is shown that some transitions do not contribute to the radiance after a short time period thus greatly
reducing the number of states whose populations need to be tracked in a simple rate equation model. An offshoot of this
view is the importance of the pump source's spectral content for efficient sodium scattering. Accounting for atomic recoil,
which causes atoms to be Doppler shifted out of resonance, we obtain model curves for photon return flux versus launch
power for both linear and circular polarization, both agree with measurements; the only free parameter being the sodium
column density on the single night both sets of data were taken. We attempted to measure the sodium velocity distribution
due to recoil using two Fasors in a pump-probe arrangement. We have measured some subtle phenomena that this simple
model does not explain and these will be discussed. These may imply the importance of understanding the collision rates
for sodium atoms to re-equilibrate through velocity changing collisions, spin relaxation and coherent beam propagation
under various atmospheric conditions.
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.
We present astronomical results from K-band adaptive optics (AO) observations of the wide binary system σ Corona Borealis with the Lick Observatory natural guide star adaptive optics system on 2004 August 27-29. Seeing conditions were excellent and the AO compensation was very good, with Strehl ratios reaching 50% at times. The stellar images were reduced using three different analysis techniques: (1) Parametric Blind Deconvolution, (2) Multi-Frame Blind Deconvolution, and (3) the MATPHOT stellar photometry code. The relative photometric and astrometric precision achievable with these three analysis methods are compared. Future directions that this research can go towards achieving the goal of routinely obtaining precise and accurate photometry and astrometry based on near-infrared AO observations are described.
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.
Adaptive optics (AO) is useful in correcting the blurring effects of the atmosphere responsible for most energy in the halo of the point spread function. A coronagraph can further enhance faint companion searches by reducing the diffraction rings surrounding a well corrected image peak. Here, we use a coronagraph on the Advanced Electro-Optical System (AEOS) at the Maui Space Surveillance Complex (MSSC) to test these benefits. The spatial characteristics of the scattered energy produced at AEOS are explored from the viewpoint of searching for faint stellar companions. The benefit of using AO is found to be 3 to 4.5 stellar magnitudes for a ten second integration time at 1 to 2 microns, with the most benefit at radial distances less than one arcsecond. More advanced AO/coronagraphic systems should be able to produce even better results. Using AO and a coronagraph should be strongly considered when attempting to image faint companions.
Adaptive optics images of Iota Cassiopeia taken in the I and H bands with the 3.63m AEOS telescope on Haleakala show a strong waffle-like pattern. The Parametric Blind Deconvolution (PBD) point spread function is modified to include such an optical aberration pattern, and position angles, separations, and magnitude differences are obtained for the components of the quadruple star system. The H band image of the astrometric companion is the first ever, the I band image is the first to show all four components, and by combining our data with previous visual and speckle measurements, the orbits and masses of components A and Aa are derived for the first time. The faint sub-solar mass component is 2.3, 2.9, 3.7, and 4.2 magnitudes
fainter than its nearby (0.4") companion at H, I, V, and B bands, respectively.
A Lorentzian shaped point spread function (PSF) for adaptive optics is demonstrated. In some cases, a Lorentz atop an Airy pattern is a better description of the PSF. If the object can also be described with an analytic function, then Parametric Blind Deconvolution can be applied as a self referencing method to extract information about both the object and PSF simultaneously. The technique is used to derive the triaxial shape and rotational pole of the asteroid Vesta from observations made on the night of October 18, 1997, marking the first time the high order loop was closed for science with the Starfire Optical Range 's 3.5m telescope.
Preliminary results are presented from Rayleigh beacon closed loop images of the large asteroids Ceres, Pallas, and Vesta obtained with 1.5 m telescopes of the USAF Phillips Laboratory's Starfire Optical Range. Measurements of the apparent diameters of two post-processed images of Ceres lead to excellent true diameter, and of two images of Pallas to good true values. Measurements of images of Vesta on two occasions lead to the full triaxial ellipsoid solution and rotational pole direction, all in excellent agreement with previous speckle interferometric results. A preliminary iterative blind deconvolution image of Vesta obtained from three 20 sec integrations is presented.
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