  Adaptive optics will almost completely remove the effects of atmospheric
turbulence at 10 microns on the Extremely Large Telescope (ELT) generation of
telescopes. In this paper, we observationally confirm that the next most
important limitation to image quality is atmospheric dispersion, rather than
telescope diffraction. By using the 6.5 meter MMT with its unique mid-IR
adaptive optics system, we measure atmospheric dispersion in the N-band with
the newly commissioned spectroscopic mode on MIRAC4-BLINC. Our results indicate
that atmospheric dispersion is generally linear in the N-band, although there
is some residual curvature. We compare our measurements to theory, and make
predictions for ELT Strehls and image FHWM with and without an atmospheric
dispersion corrector (ADC). We find that for many mid-IR applications, an ADC
will be necessary on ELTs.
