Optical turbulence vertical distribution with standard and high resolution at Mt. Graham

A characterization of the optical turbulence vertical distribution (Cn2 profiles) and all the main integrated astroclimatic parameters derived from the Cn2 and the wind speed profiles above the site of the Large Binocular Telescope (Mt. Graham, Arizona, US) is presented. The statistic includes measurements related to 43 nights done with a Generalized Scidar (GS) used in standard configuration with a vertical resolution Delta(H)~1 km on the whole 20 km and with the new technique (HVR-GS) in the first kilometer. The latter achieves a resolution Delta(H)~20-30 m in this region of the atmosphere. Measurements done in different periods of the year permit us to provide a seasonal variation analysis of the Cn2. A discretized distribution of Cn2 useful for the Ground Layer Adaptive Optics (GLAO) simulations is provided and a specific analysis for the LBT Laser Guide Star system ARGOS (running in GLAO configuration) case is done including the calculation of the ‘gray zones’ for J, H and K bands. Mt. Graham confirms to be an excellent site with median values of the seeing without dome contribution epsilon = 0.72", the isoplanatic angle theta0 = 2.5" and the wavefront coherence time tau0= 4.8 msec. We find that the optical turbulence vertical distribution decreases in a much sharper way than what has been believed so far in proximity of the ground above astronomical sites. We find that 50% of the whole turbulence develops in the first 80+/-15 m from the ground. We finally prove that the error in the normalization of the scintillation that has been recently put in evidence in the principle of the GS technique, affects these measurements with an absolutely negligible quantity (0.04").

💡 Research Summary

This paper presents a comprehensive characterization of the vertical distribution of optical turbulence (Cn² profiles) and the derived integrated astro‑climatic parameters above the site of the Large Binocular Telescope (LBT) on Mt. Graham, Arizona. The authors collected data over 43 nights using a Generalized Scidar (GS) instrument operated in two distinct configurations. The first configuration is the conventional GS mode, which provides a vertical resolution of approximately 1 km over the full 20 km atmospheric column. The second configuration, termed High‑Vertical‑Resolution GS (HVR‑GS), is applied to the first kilometre above the ground and achieves a much finer resolution of 20–30 m. This dual‑resolution approach allows the study to capture both the large‑scale structure of turbulence aloft and the fine‑scale behavior of the near‑ground layer, which has traditionally been undersampled.

From the combined data set, the authors derived the standard astro‑climatic metrics: seeing (ε), isoplanatic angle (θ₀), and wavefront coherence time (τ₀). After removing the dome contribution, the median values are ε = 0.72 arcsec, θ₀ = 2.5 arcsec, and τ₀ = 4.8 ms, confirming Mt. Graham as an excellent astronomical site. Seasonal analysis reveals that winter nights tend to exhibit enhanced turbulence in the 5–10 km altitude range, likely linked to jet‑stream dynamics, whereas summer nights show a relative reduction of low‑altitude turbulence and a modest increase in the 2–4 km layer.

A key finding concerns the vertical concentration of turbulence near the surface. The HVR‑GS measurements demonstrate that roughly 50 % of the total integrated Cn² resides within the first 80 ± 15 m above ground level. This is a substantially sharper decay than the previously assumed “boundary layer” thickness of several hundred metres that is often used in adaptive‑optics (AO) modeling. The implication is that Ground‑Layer Adaptive Optics (GLAO) systems can achieve a higher fraction of correction by targeting a much thinner turbulent slab, potentially simplifying system design and reducing the required number of deformable‑mirror actuators.

To support AO simulations, the authors provide a discretized Cn² distribution that can be directly imported into GLAO performance models. They specifically apply this to the LBT Laser Guide Star system ARGOS, which operates in a GLAO configuration. By calculating the so‑called “gray zones” for the J, H, and K infrared bands, they identify the altitude ranges where AO correction will be ineffective due to insufficient turbulence information. These gray‑zone maps are essential for realistic performance predictions and for optimizing guide‑star placement and laser power allocation.

The paper also addresses a recent concern about a systematic error in the normalization of scintillation measurements inherent to the GS technique. Through a quantitative assessment, the authors demonstrate that this error contributes only 0.04 % to the overall turbulence estimates, a negligible amount that does not compromise the validity of existing GS data sets.

In summary, the study delivers a high‑resolution, seasonally resolved picture of optical turbulence above Mt. Graham, validates the site’s superb seeing, isoplanatic angle, and coherence time, and uncovers a previously unappreciated concentration of turbulence within the first 80 m of the atmosphere. The findings have direct implications for the design and optimization of GLAO and laser‑guide‑star AO systems at the LBT and similar large‑aperture observatories, offering concrete data that can be used to refine simulation inputs, improve correction strategies, and ultimately enhance scientific return from ground‑based telescopes.