Diagnosing spatial-coherence outer scale, seeing, and dome-induced turbulence from simultaneous multi-wavelength imaging of point-like sources

Abstract

Atmospheric turbulence causes refractive-index fluctuations that distort the wavefront of light from astronomi cal sources. As starlight propagates through multiple turbulent layers, the cumulative effect sets the achievable image quality (IQ) of observations obtained with ground-based telescopes. Global parameters such as seeing (ϵ0), coherence time, isoplanatic angle, and the wavefront spatial-coherence outer scale (L0) characterise site performance, but L0 remains poorly constrained, with estimates limited to sparse campaigns. Turbulence in side telescope enclosures further degrades IQ; although typically much smaller than the atmospheric seeing contribution, dome seeing is still not well characterised, particularly its wavelength dependence. We present a simple method to estimate L0 from multiwavelength point-source imaging in integral-field spectroscopic (IFS) data. The approach also retrieves seeing and dome contributions as a function of wavelength, enabling long-term monitoring. We illustrate the method with MUSE/VLT data and outline its extension to non-IFS instruments.