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The image quality budget of many telescopes can have substantialcontributions from local seeing, both``mirror'' and ``dome'', which arise fromturbulence and temperature variations that are difficult to quantify, measuredirectly, and ameliorate. We describe a method to determine the ``local''seeing degradation due to wavefront perturbations within the final tens ofmeters of the optical path from celestial sources to a ground-based telescope,using the primary instrument and along the same path taken by light fromcelestial sources. The concept involves placing strobed emitters along thelight path to produce images on the main focal plane that ``freeze'' differentrealizations of index perturbations. This method has the advantage of measuringdirectly the image motion and scintillation imparted by the dynamic spatial andtemporal structure of local perturbations in the index of refraction along thelight path, with a clean separation from seeing induced in the atmosphere abovethe dome. The strobed-source approach allows for rapid image motion andscintillation to be measured directly on the focal plane, even forlarge-aperture telescopes with wide field instruments and slow shutters, suchas that being constructed for the Rubin Observatory. A conceptual design ispresented that uses the ``guider'' CCDs in the Rubin telescope focal plane tomake local-seeing measurements on demand, perhaps even during scienceexposures.

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