Strobed Imaging as a Method for the Determination and Diagnosis of Local Seeing

TL;DR

This paper introduces a novel strobed imaging technique to directly measure and diagnose local seeing effects within the last meters of the optical path in ground-based telescopes, improving understanding and correction of image quality degradation.

Contribution

The paper presents a new method using strobed emitters and focal plane imaging to directly quantify local seeing effects, separating them from atmospheric turbulence.

Findings

Method effectively measures local seeing in real-time.

Can be implemented with existing telescope guider systems.

Separates local seeing from atmospheric effects.

Abstract

The image quality budget of many telescopes can have substantial contributions from local seeing, both``mirror'' and ``dome'', which arise from turbulence and temperature variations that are difficult to quantify, measure directly, and ameliorate. We describe a method to determine the ``local'' seeing degradation due to wavefront perturbations within the final tens of meters of the optical path from celestial sources to a ground-based telescope, using the primary instrument and along the same path taken by light from celestial sources. The concept involves placing strobed emitters along the light path to produce images on the main focal plane that ``freeze'' different realizations of index perturbations. This method has the advantage of measuring directly the image motion and scintillation imparted by the dynamic spatial and temporal structure of local perturbations in the index of refraction along the light path, with a clean separation from seeing induced in the atmosphere above the dome. The strobed-source approach allows for rapid image motion and scintillation to be measured directly on the focal plane, even for large-aperture telescopes with wide field instruments and slow shutters, such as that being constructed for the Rubin Observatory. A conceptual design is presented that uses the ``guider'' CCDs in the Rubin telescope focal plane to make local-seeing measurements on demand, perhaps even during science exposures.