Everything you always wanted to know about aspheres. S 1/ Part 2

Effects in the image when imaging faulty aspheres. Season 1/ Part 2?

Plane asphere

b) Let's assume that the asphere is almost perfect and the planar surface is also perfectly flat (which shouldn't be difficult as a rule). Then the imaging performance would be almost diffraction limited. So there would be a Strehl ratio of about 1.

It is now assumed that the aspheric axis is tilted to the plane surface.

This means that an optical wedge takes effect within the lens. This wedge is immanent and can no longer be changed by centering the lens edge geometry.?This is the simplest case of the so-called 'internal centering error' (see also the following parts in next seasons).?If an asphere, which is hyperbolic in the simplest case, is installed unfavorably as a focusing lens or as a collimator, this will produce image errors such as first-order coma (see pictures).?This in turn affects the diffraction-related imaging of the Airy disk and the Strehl ratio.?Depending on the numerical aperture of the collimator (e.g. 0,5), a wedge of 0,3 arcmin can significantly reduce the laser power in the center of the Airy spot (factor 2 or more).?In order to prevent this, the aspheric surface must be irradiated axial symmetrically, which cannot be guaranteed without additional effort such as adjustment during installation.?

This means that even with an additional adjustment process or with an assembly concept with a center-turned element in a socket, the error cannot be eliminated.

Picture shows effect of Coma caused by a wedge in the plano aspheric lens by tilted aspherical axis of 0,3 arcmin. The aberration shows the effect of Coma first order with about 0,32 Lambda wave aberration PV or 0,08 Lambda rms. The Airy disc ist involved and the Strehl ratio is reduced from 1 to 0,767