  We demonstrate partial-transfer absorption imaging as a technique for
repeatedly imaging an ultracold atomic ensemble with minimal perturbation. We
prepare an atomic cloud in a state that is dark to the imaging light. We then
use a microwave pulse to coherently transfer a small fraction of the ensemble
to a bright state, which we image using in situ absorption imaging. The
amplitude or duration of the microwave pulse controls the fractional transfer
from the dark to the bright state. For small transfer fractions, we can image
the atomic cloud up to 50 times before it is depleted. As a sample application,
we repeatedly image an atomic cloud oscillating in a dipole trap to measure the
trap frequency.
