Atmospheric time constant with MASS and FADE

TL;DR

This paper evaluates the accuracy of atmospheric time constant measurements with MASS, compares it with adaptive optics data, and proposes the FADE method for improved estimation of the AO time constant.

Contribution

It introduces a new approach, the FADE method, for more accurate AO time constant measurements and analyzes the limitations of the single-layer approximation.

Findings

Calibration coefficient C depends on turbulence height, wind speed, and seeing.

MASS measurements agree with AO measurements under typical conditions.

The FADE method improves the accuracy of AO time constant estimation.

Abstract

The approximate nature of the adaptive-optics time constant measurements with MASS is examined. The calibration coefficient C derived from numerical simulations of polychromatic scintillation shows dependence on the height of the turbulence layer, wind speed, and seeing. The previously recommended value of C=1.27 is a good match to typical conditions, while C can vary from 0.6 to 1.6 in other circumstances. For two nights, MASS was compared with the time constant measured with adaptive optics, and the expected agreement was found. We show that the single-layer approximation used in some AO systems to derive the AO time constant can give wrong results. A better approach is to estimate it from the speed of focus variation (the FADE method). The analysis of the speed of scintillation developed recently by V.~Kornilov will lead to more accurate measurements of the AO time constant with MASS.