What is the maximum flat image circle or radius in the regular (non-EdgeHD) C14?
July 24, 2013
For the C14 SCT (at f/ratio = 11.2703), the total diffraction limited depth-of-focus is 0.27944 mm. With this depth-of-focus, the image’s flatness lies within a diameter of 20.46 mm, when the critical star focus is set for the center of field (on-axis) [i.e. 0.13972 mm focus depth, both behind and in front of the sensor].
If the star focus is set at the edge of this diameter (20.46/2 = 10.23 mm). then the image’s maximum diffraction limited flatness is enlarged to lie within a maximum diameter of 28.93 mm. This is equivalent to focusing at the 71% zone of the maximum image circle diameter.
However, since there is some coma present in SCT telescopes, the star images will gradually reveal increasing elongation, as the stars approach the edge of field.