Below is data compiled by using the same specs from the f12 CC and RC but now it is a f12 Dall-Kirkham Cassegrain.


RMS spot size at edge of 2.0" diameter field is 71.3 microns.

Notice how quickly the Dall-Kirkham design passes out of the airy diameter, lower left corner on the above chart. Optically even at f12 this design is very poor compared to the CC and RC designs, unless it is used on-axis only.

Point Spread Function illustating the star at the edge of the field.


f12 Classical

f12 Ritchey-Chretien

f12 Dall-Kirkham

22.8 microns

22.3 microns

71.3 microns


As noted earlier, defocus can help reduce the off-axis size of the stars but there is almost no benefit of defocus to the Dall-Kirkham design.