Occasionally users reach out to us for help on understanding large wavefront error observed in their systems. For example, in this optical system below, there is one Paraxial lens used to collimate the beam. In this case we would expect the output beam to be perfectly collimated with a wavefront error of 0, but here the wavefront map shows a very large RMS wavefront error and the wavefront has a distinctive spherical shape. Why is this the case?

This has something to do with how wavefront calculation works in OpticStudio. The Wavefront Map you see is not the raw accumulated optical path length of each pupil ray, but rather it’s the referenced wavefront map, or wavefront error between a pupil ray and a reference optical path length. The construction of the wavefront reference will have a direct impact on the wavefront map. By default, the Afocal Image Space box is left unchecked under System Explorer\Aperture. This assumes your system is in Focal mode and OpticStudio constructs a reference sphere as the optical path length reference, or in other words the wavefront error is referenced to a spherical wavefront. You can imagine when referencing a perfectly flat wavefront to a spherical reference, the wavefront map will appear spherical, which is why you see such spherical wavefront map in the case above when the output is perfectly collimated.  

This means when dealing with afocal system with collimated output, you need to check the Afocal Image Space box in System Explorer. This tells OpticStudio you intend to work in afocal image space and the wavefront reference should be planar instead of spherical. Once you check the box, the wavefront reference is a plane and you will now observe a perfectly flat wavefront map with 0 RMS wavefront error.

You can find a description on this Afocal Image Space setting in the help file at The Setup Tab > System Group (the Setup Tab) > System Explorer > Aperture (System Explorer) > Afocal Image Space.

The wavefront calculation can become quite involved depending on how the reference is constructed. This is one of the areas we see a lot of customer questions. Therefore, we have published a Q&A describing how wavefront computation works under the Wavefront section. Please feel free to check it out.