Symmetry in Star Testing

No lens has perfect color correction, and there is always some variation in focus across the spectrum. For simplicity, assume we have a lens corrected for C and F. The green light focuses slightly closer to the lens than either the blue or red.

What happens when we focus for our eye? We are focusing quite close to the green minimum focus, where our eye is most sensitive. When focused in the green, the blue and red are slightly out of focus. If we did a spot diagram, the blur circle for the red and blue would be larger than the one for green. For an achromat, the blue and red blur circles would generally be larger than the Airy disk (depending on aperture and focal length).

Now move slightly inward of the visual focus. The green is now slightly out of focus, and the blue and red are ever farther out of focus. The green blur circle has grown, as has the red and blue blur circles, and blue and red are still larger than green.

Now move slightly outward from the visual focus. The green is now slightly out of focus, but the red and blue are closer to focus because their focal point is farther away from the lens than the minimum green focus.

Based on this, it is hard to see why there would be symmetry in white light. Different things are going on inside focus than outside focus. This will always be true, unless all colors come to exactly the same focus (reflector, anyone?).

If you want to see what the correction for spherical aberration is, you really need to isolate the green light.

Clear skies, Alan

See
http://aberrator.astronomy.net/scopetest/html/news.html

I don't see a lot of symmetry in the white light images of the APOs, and notice a lot of comments about how they test in green light.

Clear skies, Alan

Alan say :No lens has perfect color correction, and there is always some variation in focus across the spectrum. For simplicity, assume we have a lens corrected for C and F. The green light focuses slightly closer to the lens than either the blue or red.

Markus : sorry but shall a real apo not show 3 wavelenghtes at excact same focuse ???????????

Alan say :.....Based on this, it is hard to see why there would be symmetry in white light. Different things are going on inside focus than outside focus. This will always be true, unless all colors come to exactly the same focus (reflector, anyone?).

Markus:if the optics is superb polished you will see a perfect symmetrie even in achromats with same contrast, only the colors you see on each side of focuse are diffrent.
The diffrence in a perfect corrected lenses between inside of focuse and outside of focuse is nothing than the diffrent colors, but the fressnell rings and here contrast must be 100% equal if the optics is perfect polished

Alan : If you want to see what the correction for spherical aberration is, you really need to isolate the green light.

Markus: Alan here above you are correct , but with a real superb Fraunhofer achromat design or other semiapo or apo design, all optimized for green , usual the spherical aberration on outer wavelenghte is very similar. Only photographic corrected systems show diffrent wavelenght optimisations.

Look again the the white lioght startest images of perfect corrected, achros, semiapos and apos and see its true, simply because its a physical law, that it have to be this way

Markus