Fig. 1

Fig. 1

Setup: a A collimated laser beam is delivered into the setup by a beam splitter (BS) and onto a galvanometric scanning mirror (GSM), which is imaged into the back focal plane of an air objective (OBJ1). Scanning the GSM translates the focus in one dimension, as shown by the double-headed arrow in the boxed front focal space of OBJ1. A step mirror reflects the light with different amounts of defocus back into the objective, which then travels through the 4F system onto the GSM, which removes the lateral-scan motion, leaving only defocus in the wavefront. The GSM is again imaged onto the back focal plane of a water dipping objective (OBJ2). As OBJ1 and OBJ2 are pupil matched, OBJ2 forms an aberration-free image of the focus (as formed by OBJ1) in the sample space. b Zoomed-in view of the boxed region from a. The panel on the left shows the focus of the light at its nominal focus. Black arrows show returning marginal rays after reflection. Each step on the mirror results in a focus spot in the sample plane with a displaced axial position. c Alternative configuration with a tilted mirror that allows continuous axial scanning. Here, the remote objective OBJ1 is slightly shifted off the optical axis to create a tilted focus that is perpendicular to the mirror surface. Scanning this focus laterally results in a change in focus, as illustrated by the black arrows