Accurate seeing measurements with MASS and DIMM

TL;DR

This paper evaluates the accuracy of seeing measurements using MASS and DIMM instruments, analyzing their biases and proposing modifications to improve measurement precision in astronomical observations.

Contribution

The study introduces a modified MASS data processing method to correct for scintillation saturation and assesses the combined use of MASS and DIMM for accurate atmospheric seeing measurements.

Findings

DIMM and MASS can achieve about 10% accuracy with bias control

Optical aberrations significantly bias DIMM measurements

Achieving 1% accuracy is unrealistic due to the stochastic nature of seeing

Abstract

Astronomical seeing is quantified by a single parameter, turbulence integral, in the framework of the Kolmogorov turbulence model. This parameter can be routinely measured by a Differential Image Motion Monitor, DIMM. A new instrument, Multi-Aperture Scintillation Sensor (MASS), permits to measure the seeing in the free atmosphere above ~0.5km and, together with a DIMM, to estimate the ground-layer seeing. The absolute accuracy of both methods is studied here using analytical theory, numerical simulation, and experiments. A modification of the MASS data processing to compensate for partially saturated scintillation is developed. We find that the DIMM can be severely biased by optical aberrations (e.g. defocus) and propagation. Seeing measurements with DIMM and MASS can reach absolute accuracy of ~10% when their biases are carefully controlled. Pushing this limit to 1% appears unrealistic because the seeing itself is just a model-dependent parameter of a non-stationary random process.