Without ADC Loss of Performance already at 30° Zenith Distance or 60° above Horizon

As example an image of mars (January 2010) has been used: Image made with Zeiss B Optics 180/3310 and Imaging Source31AF03 CCD.
The images simulate the effect of atmospheric dispersion for different zenith distances. On the right column the corresponding spot diagrams with the strehl values are displayed.

With optimal correction of
atmospheric dispersion.

The atmospheric dispersion shifts the images of the different colors. This results not only in a chromatic bleeding but also in a worse point spread function reducing the strehl value.

The diffraction limited performance is defined by Huygens with a Strehl value of 0.8. But experience shows that a telescope with a Strehl value > 0.9 has well distinguished performance advantages.
Atmopsheric dispersion reduces the Strehl value of an optimal telescope below 90% already at
for 60cm aperture and zenith distances greater than 13°
for 30cm aperture and zenith distances greater than 20°
for 20cm aperture and zenith distances greater than 30°