The stellar scintillation on large and extremely large telescopes

TL;DR

This paper investigates how the outer scale of optical turbulence affects stellar scintillation noise in large telescopes, showing significant reductions in noise estimates for extremely large telescopes and long exposures.

Contribution

It introduces the impact of the outer turbulence scale on scintillation noise estimates for large telescopes, refining previous models and including effects of central obscuration.

Findings

Outer scale reduces scintillation noise for 30-40 m telescopes.

Scintillation noise can be decreased by more than a factor of 10 with outer scale effects.

Estimated scintillation noise for future large telescopes is around 10 μmag for 60 s exposures.

Abstract

The accuracy of ground-based astronomical photometry is limited by two factors: photon statistics and stellar scintillation arising when star light passes through Earth's atmosphere. This paper examines the theoretical role of the outer scale $L_0$ of the optical turbulence (OT) which suppresses the low-frequency component of scintillation. It is shown that for typical values of $L_0 \sim 25 - 50$ m, this effect becomes noticeable for a telescopes of diameter around 4 m. On extremely large, $30 - 40$ m, telescopes with exposures longer than a few seconds, the inclusion of the outer scale in the calculation reduces the scintillation power by more than a factor of 10 relative to conventional estimates. The details of this phenomenon are discussed for various models of non-Kolmogorov turbulence. Also, a quantitative description of the influence of the telescope central obscuration on the measured scintillation noise is introduced and combined with the effect of the outer scale. Evaluation of the scintillation noise on the future TMT and E-ELT telescopes, predicts an amplitude of approximately $10 \mumag$ for a 60 s exposures.