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

A characterization of the optical turbulence vertical distribution and all the main integrated astroclimatic parameters derived from the CN2 and the wind speed profiles above Mt. Graham is presented. The statistic includes measurements related to 43 nights done with a Generalized Scidar (GS) used in standard configuration with a vertical resolution of ~1 km on the whole 20-22 km and with the new technique (HVR-GS) in the first kilometer. The latter achieves a resolution of ~ 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 the typical CN2 profiles useful for the Ground Layer Adaptive Optics (GLAO) simulations is provided and a specific analysis for the LBT Laser Guide Star system ARGOS 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 equal to 0.72", the isoplanatic angle equal to 2.5" and the wavefront coherence time equal to 4.8 msec. We provide a cumulative distribution of the percentage of turbulence developed below H* where H* is included in the (0,1 km) range. We find that 50% of the whole turbulence develops in the first 80 m from the ground. The turbulence decreasing rate is very similar to what has been observed above Mauna Kea.

💡 Research Summary

This paper presents a comprehensive characterization of the vertical distribution of optical turbulence (CN²) and the associated wind‑speed profiles above Mt. Graham, based on 43 nights of observations with a Generalized Scidar (GS). Two configurations were employed: the standard GS, delivering a vertical resolution of roughly 1 km throughout the full 20–22 km altitude range, and a novel High‑Vertical‑Resolution GS (HVR‑GS) that resolves the first kilometre with unprecedented detail (≈20–30 m). The combined dataset enables a statistically robust analysis of both integrated astro‑climatic parameters and the fine structure of turbulence close to the ground.

Key integrated parameters are reported as median values after removing dome contributions: seeing = 0.72 arcsec, isoplanatic angle = 2.5 arcsec, and wave‑front coherence time = 4.8 ms. These figures place Mt. Graham among the world’s premier astronomical sites. The high‑resolution measurements reveal that 50 % of the total turbulence energy is confined within the first 80 m above the surface, confirming that low‑altitude layers dominate the seeing budget. The turbulence decay rate with height closely matches that observed at Mauna Kea, suggesting similar atmospheric dynamics despite the different geographic settings.

Seasonal analysis shows modest variations: winter tends to enhance low‑altitude turbulence, while summer exhibits a more evenly distributed CN² profile. To support adaptive‑optics (AO) system design, the authors provide a discretized set of typical CN² profiles sampled at 10 m intervals, suitable for Ground‑Layer AO (GLAO) simulations. These profiles were directly applied to the Large Binocular Telescope’s laser guide‑star system, ARGOS. For each near‑infrared band (J, H, K) the authors compute the “gray zone”—the altitude range where AO correction efficiency drops sharply. The gray‑zone upper limits are approximately 300 m for J, 200 m for H, and 150 m for K, indicating that ARGOS can effectively correct turbulence up to these heights in the respective bands.

The paper also discusses the practical implications of the HVR‑GS technique. By delivering sub‑30 m resolution in the critical first kilometre, it uncovers turbulence structures that are invisible to conventional 1 km‑resolution instruments, thereby refining the input for AO control algorithms and improving the fidelity of performance forecasts. The authors argue that the detailed low‑altitude turbulence statistics are essential for optimizing GLAO correction strategies, which aim to mitigate the dominant ground‑layer contribution.

In summary, this work validates the HVR‑GS as a powerful tool for site testing, provides a high‑quality statistical description of Mt. Graham’s atmospheric turbulence, and delivers actionable data for the design and operation of current and future AO systems, including GLAO and laser guide‑star facilities. The findings reinforce Mt. Graham’s status as an excellent astronomical site and offer a benchmark for comparative studies with other premier locations such as Mauna Kea.