Below is a comparison of the Point Spread Function of a f10 RC that is first focused on center, then focused to make the on-axis near the size of the airy diameter and the third example is focused to yield similar RMS sizes both on and off-axis. Again, this is at the edge of a 2.0" diameter field. RMS spot size (in microns) at the 2.0" diameter edge is given for each PSF just below the image as well as the in-focus travel amount needed to yield the below results.
 

23.6

Focused for on-axis.

Off-axis PSF.

Best focus position for planetary or other narrow field work.
     
     

17.0

0.23mm

Focus of on-axis is optimized to be no larger than the airy size.

Off-axis PSF.

Best position for all around use, even planetary since the on-axis is extremely seeing limited.
     
     

~11.7

0.39mm

Focus optimized to give similar RMS spot sizes both on and off-axis.

Off-axis PSF.

Best position for wide field imaging, say with a 1.0" x 1.5" CCD chip.

 
 

Continue on to the next page to see an analysis of Classical, Ritchey-Chretien and Dall-Kirkham Cassegrains compared at f12.