  We present an alternative Corrector-ADC design for GMT. The design consists
of just 3 silica lenses, of maximum size 1.51m, and includes only a single
low-precision asphere for 20' field-of-view, and none for 10'. The
polychromatic (360nm-1300nm) image quality is d80<0.043" at zenith and
d80<0.20" for ZD<60 degrees. The monochromatic image quality is d80<0.1"
everywhere, and typically ~0.05". The ADC action is achieved by tilt and
translation of all three lenses; L1 and L2 via simple slide mechanisms each
using a single encoded actuator, and L3 via a novel 'tracker-ball' support and
three actuators. There is also a small motion of M2 via the hexapod,
automatically generated by the AGWS system. The ADC action causes a small
non-telecentricity, but this is much less than the unavoidable chromatic
effects shared with the baseline design. The ADC action also changes the
distortion pattern of the telescope, but this can be used positively, to reduce
the maximum image motion due to differential refraction by a factor of three.
The transmission is superb at all wavelengths, because of the reduced number of
air/glass surfaces, and the use only of fused silica.
