Optical auto-focus unit and a method for auto-focus

A charged particle evaluation system that may include a column that includes an opening; an illumination unit that is configured to scan an area of a sample with an electron beam that passes through the opening; and an optical auto-focus unit that is configured to (i) illuminate the sample with an optical beam that is proximate to the electron beam, during the scan of the area with the electron beam; (ii) receive a reflected optical beam from the sample, (iii) determine a focus status of the electron beam, and (iv) participate in a compensating of an electron beam misfocus.