spot diagrams redux

I posted this little issue I have a while back and got not one response. I could say it got swallowed up in the Herb thing, but it actually languished unappreciated for a couple of days before all that hit. I just can't accept that nobody cares, sniff. So, I'm trying again. I have a weak mind and actually wonder about such picayune matters!

Optics buffs,
Color spot diagrams are commonly used to show color error in refractors. 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. 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 5" achro, that region looks dark to me. ( I saw the companion to Propus tonight, plain as could be, right inside the first ring.) But is it, "really"?

So, I suspect that this may be part of the answer to "as terrible as the spot diagrams look, how come you can see anything at all through an achromat?" Namely, there's a bright, neat Airy disc, in any color. The converse is also unfortunately true. Now, did I say something bad about APOs?
Ron

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?

The poorly focused red and blue light will fill-in the
dark spaces between the focused green diffraction rings. You can
show this by making some illustrations with the Aberrator
software. For example, at 1 wave defocus, the pattern center is
dark (where the focused Airy disk would be), and instead there is
a bright ring where the minimum between the disk and 1st ring is
for the focused image. So the chromatic defocus will tend to
fill in the dark spaces between the focused green rings.

The attached plot shows brightness on a slice through the Airy
pattern vs. focus. Vertical direction is along the slice through
the Fresnel pattern, and horizontal direction is defocus from
-2 to +2 waves. If you put a horizontal straight edge
across it, you can see that bright areas fill in dark areas as
the defocus increases.

Attached Image: