Spot diagrams are commonly used to show color error in refractors. They are very useful, at least for their main purpose of selling apos! But I have a problem with them. They are comparing apples and oranges.
The little circle at the center represents the size of the Airy disk, usually in green, and as such recognizes the diffraction behavior of light. The green ring pattern is not shown on the usual diagram, but we are supposed to know it's there. It is my understanding that the color spots, on the other hand, result from geometric ray-trace alone, and show how the light would be arranged in the absence of diffraction.
I'll pose a question that is relevant to us achromat victims. How does a red or blue color spot that would be, say, 4x the size of the Airy disc in the absence of diffraction, really look when it arrives at the focal plane? Is there smeared light between the diffraction rings of the well-focused green image? Or, as I suspect, does the poorly focused red and blue actually arrange itself in rings nearly coincident with the yellow-green rings, making those rings brighter than in a perfect telescope, but still well defined?
This has important implications for double star observing, where one is often trying to spot a dim companion between the central, visible part of the Airy disc, and the first ring. In my achro, that region looks dark to me. But is it, really?
Ron
