  The region surrounding the Llano de Chajnantor, a high altitude plateau in
the Atacama Desert in northern Chile, has caught the attention of the
astronomical community for its potential as an observatory site. Combining high
elevation and extremely low atmospheric water content, the Llano has been
chosen as the future site of the Atacama Large Millimeter Array. We have
initiated a campaign to investigate the astronomical potential of the region in
the optical/infrared. Here, we report on an aspect of our campaign aimed at
establishing a seeing benchmark to be used as a reference for future activities
in the region. After a brief description of the region and its climate, we
describe the results of an astronomical seeing campaign, carried out with a
Differential Image Motion Monitor that operates at 0.5 micron wavelength. The
seeing at the Llano level of 5000 m, measured over 7 nights in May 1998,
yielded a median FWHM of 1.1". However, the seeing decreased to 0.7" at a
modest 100 m gain above the plateau (Cerro Chico), as measured over 38 nights
spread between July 1998 and October 2000. Neither of these represents the best
seeing expected in the region; the set of measurements provides a reference
base for simultaneous dual runs at Cerro Chico and at other sites of interest
in the region, currently underway. A comparison between simultaneous
measurements at Cerro Chico and Cerro Paranal indicates that seeing at Cerro
Chico is about 12% better than at Paranal. The percentage of optically
photometric nights in the Chajnantor region is about 60%, while that of nights
useful for astronomical work is near 80%.
