This invention relates to a confocal fluorescence correlation microscope with real-time alignment read out. With this instrument it is possible to do confocal imaging together with the particle size determination at a chosen location in the specimen. In particular, this invention relates to a detector module with a fixed aperture and detection electronics that can be conveniently connected to an existing confocal or multiphoton microscope, near the base of the objective lens of the microscope. This detector splits a part of the signal and uses it to generate a spot on the confocal image. This shows the spot where an FCS measurement can be carried out, and the same signal can then be used to perform a fluorescence correlation measurement after parking the excitation beam of the confocal to that spot. No alignment step is necessary for obtaining the measurement.