Fig. 1: Principle of the approach.

a Conceptual layout of our proposed design. Current polarizing beamsplitter based designs (left) lose half of the light intensity because unpolarized fluorescence is split into a beam dump and the remote focusing objective. Our design (right) uses half the available FOV for the incoming light and a retroreflector in the image plane of the remote objective to fold the image to the other side of the FOV. A knife edge mirror in the primary image plane is then able to direct the refocused light onto the camera. b Light path at the level of the object, retroreflector and image of two point sources located at different z-depths. Moving the retroreflector along the optical axis brings either of them in focus. c Picture of the microscopic retroreflector (R) together with the coverslip (C) and parts of the remote objective (RO). d Layout of the entire microscope for volumetric voltage imaging. A light sheet is generated with a cylindrical lens (CL) and projected onto the sample via a folding mirror (FM), galvo mirror (G), scan and tube lens (SL, TL) and illumination objective (IO). The galvo mirror controls the z-position of the light sheet. Fluorescence is collected via the primary imaging objective (PO) and imaged onto the knife edge mirror (KM) in the primary image plane and directed into the remote focusing path. The remote focusing path consists of a tube lens, remote objective (RO) and voice coil (VC) on which the retroreflector is mounted. After refocusing the image is relayed with a 1:1 relay (RL) onto the camera (C).