A point spread function (PSF) of a focused scanning particle beam of an observation instrument is ascertained by obtaining a first image (reference image) based on a reference instrument, the reference image being an image of an area of a reference standard, obtaining a second image (observed image) of the area of the reference standard, and the observed image obtained using the observation instrument configured with a set of operational parameters that define a probe size for the observation instrument, the probe size being larger than a pixel size of the reference image, and then determining, based on the reference image and the observed image, the PSF of the observation instrument as a component of a convolution of the reference image that provides the observed image.