  After proposing a new method of deriving the atmospheric time constant from
the speed of focus variations (Kellerer & Tokovinin 2007), we now implement it
with the new instrument, FADE. FADE uses a 36-cm Celestron telescope that is
modified to transform stellar point images into a ring by increasing the
central obstruction and combining defocus with spherical aberration. Sequences
of images recorded with a fast CCD detector are processed to determine the
defocus and its variations in time from the ring radii. The temporal structure
function of the defocus is fitted with a model to derive the atmospheric seeing
and time constant. We investigated by numerical simulation the data reduction
algorithm and instrumental biases. Bias caused by instrumental effects, such as
optical aberrations, detector noise, acquisition frequency, etc., is
quantified. The ring image must be well-focused, i.e. must have a sufficiently
sharp radial profile, otherwise, scintillation seriously affects the results.
An acquisition frequency of 700 Hz appears adequate. FADE was operated for 5
nights at the Cerro Tololo observatory in parallel with the regular site
monitor. Reasonable agreement between the results from the two instruments has
been obtained.
