  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.
