  Characterisation of atmospheric optical turbulence is crucial for the design
and operation of modern ground-based optical telescopes. In particular, the
effective application of adaptive optics correction on large and extremely
large telescopes relies on a detailed knowledge of the prevailing atmospheric
conditions, including the vertical profile of the optical turbulence strength
and the atmospheric coherence timescale. The Differential Image Motion Monitor
(DIMM) has been employed as a facility seeing monitor at many astronomical
observing sites across the world for several decades, providing a reliable
estimate of the seeing angle. Here we present the Shack-Hartmann Image Motion
Monitor (SHIMM), which is a development of the DIMM instrument, in that it
exploits differential image motion measurements of bright target stars.
However, the SHIMM employs a Shack-Hartmann wavefront sensor in place of the
two-hole aperture mask utilised by the DIMM. This allows the SHIMM to provide
an estimate of the seeing, unbiased by shot noise or scintillation effects. The
SHIMM also produces a low-resolution (three-layer) measure of the vertical
turbulence profile, as well as an estimate of the coherence timescale. The
SHIMM is designed as a low-cost, portable, instrument. It is comprised of
off-the-shelf components so that it is easy to duplicate and well-suited for
comparisons of atmospheric conditions within and between different observing
sites. Here, the SHIMM design and methodology for estimating key atmospheric
parameters will be presented, as well as initial field test results with
comparisons to the Stereo-SCIDAR instrument.
