Measuring Local Turbulence Along the Optical Path: Multi-beam Optical Seeing Sensor (MOSS)

TL;DR

The paper introduces MOSS, a novel multi-beam sensor that directly measures turbulence-induced image degradation within telescope domes, aiding in thermal and airflow management for improved astronomical imaging.

Contribution

MOSS provides a new direct measurement method for optical path turbulence inside telescope enclosures, capturing both dome and mirror seeing effects.

Findings

Preliminary data constrain optical path turbulence to at least 1.4 arcseconds.

Turbulence varies throughout the night, indicating dynamic environmental conditions.

MOSS successfully characterizes turbulence length scales and coherence properties.

Abstract

Deflection of light along the optical path is a major source of image degradation for ground-based telescopes. Methods have been developed to measure upper atmospheric seeing based on models of the turbulence in the atmosphere, but due to boundary conditions, transmission within telescope enclosures is more complex. The Multi-beam Optical Seeing Sensor (MOSS) directly measures the component of the image quality degradation from inhomogeneity of the index of refraction within the telescope dome. MOSS outputs four near-parallel beams of light that travel along the optical path and are imaged by the telescope's detector, landing like starlight on the telescope's focal plane. By using a strobed light source, we can 'freeze' the instantaneous index variations transverse to the optical path. This system captures both 'dome' and 'mirror' seeing. Through plotting the standard deviation of differential motion between pairs of beams, MOSS enables characterization of the length scale of turbulence within the dome. The temporal coherence of temperature gradients can be probed with different pulse lengths, and the spatial coherence by comparing pairs at different separations across the aperture of the telescope. Optical path turbulence measurements, alongside other telemetry metrics, will guide thermal and airflow management to optimize image quality. A MOSS prototype was installed in the 1.2 meter Auxiliary Telescope (AuxTel) at the Vera C. Rubin Observatory in Chile, and preliminary data constrain the optical path turbulence with a lower bound of 1.4 arcseconds. The optical path turbulence varied throughout the night of observing.