Is phase-mask alignment aberrating your STED microscope?

The performance of a stimulated emission depletion (STED) microscope depends critically upon the pupil plane phase mask that is used to shape the depletion focus. Misalignments of the mask create unwanted distortions of the depletion focus to the detriment of image quality. It is shown how the phase errors introduced by a misplaced mask are similar to coma aberrations. The effects are investigated analytically, through numerical modelling and experimental measurement. Strategies for the systematic alignment of the masks are discussed.