  A defocused image of a bright single star in a small telescope contains rich
information on the optical turbulence, i.e. the seeing. The concept of a novel
turbulence monitor based on recording sequences of ring-like intrafocal images
and their analysis is presented. It can be implemented using standard
inexpensive telescopes and cameras. Statistics of intensity fluctuations in the
rings and their radial motion allow measurement of the low-resolution
turbulence profile, the total seeing, and the atmospheric time constant. The
algorithm of processing the images and extracting the turbulence parameters is
developed and extensively tested by numerical simulation. Prescriptions to
correct for finite exposure time and partially saturated scintillation are
given. A prototype instrument with a 0.13-m aperture was tested on the sky. The
RINGSS (Ring-Image Next Generation Scintillation Sensor) can be used as a
portable turbulence monitor for site testing and as an upgrade of existing
seeing monitors.
