  We present the current optical design for the IRIS Atmospheric Dispersion
Corrector (ADC). The ADC is designed for residual dispersions less than ~1 mas
across a given passband at elevations of 25 degrees. Since the last report, the
area of the IRIS Imager has increased by a factor of four, and the pupil size
has increased from 75 to 90mm, both of which contribute to challenges with the
design. Several considerations have led to the current design: residual
dispersion, amount of introduced distortion, glass transmission, glass
availability, and pupil displacement. In particular it was found that there are
significant distortions that appear (two different components) that can lead to
image blur over long exposures. Also, pupil displacement increases the
wavefront error at the imager focus. We discuss these considerations, discuss
the compromises, and present the final design choice and expected performance.
