  Many adaptive optics systems operate by measuring the distortion of the
wavefront in one wavelength range and performing the scientific observations in
a second, different wavelength range. One common technique is to measure
wavefront distortions at wavelengths <~1 micron while operating the science
instrument at wavelengths >~1 micron. The index of refraction of air decreases
sharply from shorter visible wavelengths to near-infrared wavelengths.
Therefore, because the adaptive optics system is measuring the wavefront
distortion in one wavelength range and the science observations are performed
at a different wavelength range, residual image motion occurs and the maximum
exposure time before smearing of the image can be significantly limited. We
demonstrate the importance of atmospheric differential refraction, present
calculations to predict the effect of atmospheric differential refraction, and
finally discuss the implications of atmospheric differential refraction for
several current and proposed observatories.
