High-resolution lidar observations of mesospheric sodium and implications for adaptive optics

Abstract

Observations of sodium density variability in the upper mesosphere/lower thermosphere, obtained using a high-resolution lidar system, show rapid fluctuations in the sodium centroid altitude. The temporal power spectrum extends above 1 Hz and is well-fit by a power law having a slope that is -1.95±0.12. These fluctuations produce focus errors in adaptive optics systems employing continuous-wave sodium laser guide stars, which can be significant for large-aperture telescopes. For a 30 m aperture diameter, the associated rms wavefront error is approximately 4 nm per meter of altitude change and increases as the square of the aperture diameter. The vertical velocity of the sodium centroid altitude is found to be ~23 ms(-1) on a 1 s time scale. If these high-frequency fluctuations arise primarily from advection of horizontal structure by the mesospheric wind, our data imply that variations in the sodium centroid altitude on the order of tens of meters occur over the horizontal scales spanned by proposed laser guide star asterisms. This leads to substantial differential focus errors (~107 nm over a 1 arc min separation with a 30 m aperture diameter) that may impact the performance of wide-field adaptive optics systems. Short-lasting and narrow sodium density enhancements, more than 1 order of magnitude above the local sodium density, occur due to advection of meteor trails. These have the ability to change the sodium centroid altitude by as much as 1 km in less than 1 s, which could result in temporary disruption of adaptive optics systems.

Increased sky coverage with optimal correction of tilt and tilt-anisoplanatism modes in laser-guide-star multiconjugate adaptive optics

“…TA PSDs were obtained with the formulation given in [21]. The focus error PSD follows ∝ f −1.97 ≈ f −2 [25], excluding low-temporal filtering from the trombone and matched-filtering update. (b) the 1.25 ms loop delay not taken into account in the LQG analysis, creating a model/system mismatch to which the overall performance is sensitive, particularly when the NGS sampling rate approaches the LGS rate.…”

On-Sky Tests of a High-Power Pulsed Laser for Sodium Laser Guide Star Adaptive Optics

“…L was taken to be the di®erence between the mean sodium altitude of 91.6 km Pfroemmer & Hickson (2014) and the LZT altitude of 395 m.…”

High resolution mesospheric sodium properties for adaptive optics applications

“…A general description of the lidar system can be found in Pfrommer et al (2008) and a detailed description, including all subsystems, is given in Appendix A of Pfrommer (2010). The lidar employs a 13 W Nd-YAG frequency-doubled pulsed laser emitting 532 nm radiation, which pumps a dye laser tuned to 589 nm.…”

“…In previous publications (Pfrommer et al 2009;Pfrommer & Hickson 2010) we reported on data obtained using the firstgeneration counting system. Qualitatively, we see the same kinds of structure with the new system, but at higher resolution and with no system-related data acquisition stops.…”

“…When combined with the response function of an AO system, this spectrum determines the direct impact of altitude variations on system performance. A typical (one-sided) sodium density temporal power spectral density (PSD) was shown in Pfrommer & Hickson (2010) (Fig. 2).…”