Aberration theory considers defects in optical imagery to be described by a polynomial expansion in which the ray displacement from an ideal image point, or the wavefront displacement from an ideal reference sphere, is written in terms of object and pupil coordinates. The accuracy of predictions made on the basis of aberration theory depends on the order of the expansion polynomial and the particular system under study.

OSLO computes three groups of aberrations, called chromatic, Seidel, and fifth-order aberrations. A single term, the spherical aberration, is computed to seventh order. The Seidel aberrations are available for both the axial ray (image aberrations) and the chief ray (pupil aberrations). Fifth-order aberrations are available in either Seidel or Buchdahl format.

Aberration coefficients can be computed using only paraxial ray data. This means that the aberrations of a system can be computed regardless of whether or not it is possible transmit real rays through it. Because of this, aberration theory is extremely useful for studying the general nature of design forms, as well as for preliminary optimization. No matter how the system changes, it is always possible to compute the derivatives of aberration coefficients with respect to constructional parameters.

In OSLO, the paraxial chromatic aberrations are called PAC and PLC, which stand for primary axial color and primary lateral color. The terms SAC and SLC in OSLO stand for secondary axial color and secondary lateral color, and are similar to PAC and PLC, but use wavelengths 2 and 1, instead of 2 and 3. OSLO uses the convention that the middle wavelength is 1, the short wavelength is 2, and the long wavelength is 3.

All aberrations are computed in the current wavelength, as set in the Wavelengths spreadsheet. If you want to compute the chromatic differences of the Seidel aberrations, reset the current wavelength before computing the aberrations.

The interpretation of aberrations, while very important to optical design, is well described in most optics textbooks and the Optics Reference manual, and is not repeated here. The various aberration commands in OSLO are found in the Calculate >> Aberrations menu.