Rms measured at a specific wavelength other than the optimized in telescopes with any form of chromatism is valid only for that wavelength. Depending on the overall correction for spherical aberration, the error at the wavelength that should be optimized - usually 550nm - can be both, smaller or larger.
For instance, the rms at He-ne line due to chromatism is 0.023 with spherical aberration of the system cancelled. If the system residual is non-zero, that will change. For, say, 1/5 wave p-v (0.059 rms) of overcorrection at 550nm, correction at 633nm goes down to 0.12 wave p-v (0.031 rms), going from under- to over-corrected.
Undercorrection in green would, on the other hand, worsen correction in red and improve it in blue/violet.
If we know whether this other-than-optimized color tested is over- or under-corrected, and what it is supposed to be in a perfectly corrected system, we can obtain the error in the optimized wavelength by simple arithmetics. In the above case, the differential between 0.023 wave rms under-corrected and 0.031 rms over-corrected is 0.054 wave rms of over-correction added to the system in units of 633nm wavelength, which for 550nm translates to 0.062 wave rms, or little over 0.2 wave p-v (a bit more due to rounding off).
What makes this diffcult is that wavefront errors other than spherical are likely to be present. Tested C9.25 has 1/4 wave p-v which, if it would be spherical aberration alone, would result in the rms smaller by a factor of 3.35, or 0.0745. Since it is significantly smaller than that, it indicates the presence of some type of local error, or errors, which inflate the p-v value significantly more than the rms. It is mixed with spherical aberration, making determination of the error in the optimized wavelength more uncertain.
There is nothing wrong with Suiters calculation. It is diffraction based, and still valid since it is still diffraction that determines intensity distribution. His figure for 1/4 wave p-v is for 1/4 wave p-v of spherical aberration, which corresponds to 0.0745 wave rms. The aberration in the C9.25 of a mixed form, with significantly lower rms; it is altogether different animal.
Btw. Strehl ratio is used to express quality of the wavefront (and, by that, quality of optical surfaces). It does not account for the effect of obstructions, except in the part in which they change the rms by taking out portion of the aberrated wavefront. Consequently, the Strehl and the final relative peak intensity of an obstructed system are two different things. If the Strehl (from wavefront quality) is S, and c.obctruction in inits of the aperture "c", the final peak intensity is I=S(1-c^2)^2.
Vla
