The Airy disk is smaller at the shorter wavelength of blue or violet light. If we are getting larger spots in that range at a particular setting of the optics, it is defocus, not diffraction.
That's way off-target....
It looks in-focus, then in-focus, thenn not a fatter or smaller disk, but a -->multifringe pattern<----
If you focus for an Airy disk, which is a single visible fringe pattern, at red and green,
there will usually be, in a short barrel, no such thing as a "disk" at violet. It splits into a multi-fringe pattern,
exactly as you see above.
Take a look at the center of the violet pattern in the first post:
it is a smaller disk, sure, but your troubles have only begun....you now have multiple nodes.
"Disk" ceases to describe what is happening....it is 'rings'.
A disk at lamba is
becoming multiple rings at lamba/2 . That's the Physics of it.
That's the truth of diffraction. The "Airy Disk" is just a subset.
A little diffraction science:
The concept of an "Airy Disk" is a monchromatic concept.
It is also a long-barrel approximation.
It is a diffractive concept that does not care what glass you have.
Doing a star test can help show what is going on.
BTW: For the same aperture, the shorter the barrel, the more fringes it can generate
across the spectrum. Look at the entry above and note how the difference
from side to side changes more quickly with a shorter triangle.
..I think I just figured out why this happens...the short barrels.
Edited by MartinPond, 07 April 2021 - 11:44 AM.