The point spread function (PSF) describes the response of an imaging system to a point source or point object. As such, the PSF is important for colocalization. To check the PSF, it is recommended to acquire Z-stacks, with 10-20 nm steps and use, for example, 100 nm molecule repeats; allowing the user to measure PSF in the sub-resolution area.
It is also recommended to always use Olympus immersion oil with Olympus lenses. This is because both the refractive index (RI) and the Abbe number (also termed the V number) are important. The Abbe number is an approximate measure of the material’s dispersion, which describes how the RI changes with wavelength. A high Abbe number indicates low dispersion. Using Olympus immersion oil avoids chromatic aberrations caused by dispersion but do check the immersion oil bottle to ensure the expiry date has not passed.
Finally, ensure the coverslip is 1.5H cover glass and consider the RI of the mounting medium.