DIAGRAMS for a 10" f/4 f/20 D-K
The circles represent the Airy disk, theoretically and practically the smallest concentration of light that can be seen. If the blur spot of an optical system can be contained at or within the airy disk the system will perform at maximum resolution. The numbers under the Airy disk images represent the actual distance or position off-axis at the focal plain. It can be seen in the picture above that the blur spots are contained within the Airy disk with the exception of the .300" position spot (a .6" diameter circle) position. In practicality, this represents the extreme edge of the field and even at this point no coma could be sensibly perceived. This is why my D-Ks need no additional corrector lenses. In order to achieve this, I had to make the primary mirror at f/4 (rather than a faster f/3) and have an overall effective focal ratio of 20 in order to suppress the size of the secondary mirror to a 25% overall physical obstruction. Many people would like an EFR of about 15 or 12 but the secondary obstruction would be larger and the goal here is an optimized planetary and lunar scope. Just two reflecting surfaces and nothing else. It matters.