1. A method comprising: ascertaining a point spread function (PSF), of a focused scanning particle beam, of an observation instrument, the ascertaining the PSF comprising: obtaining a first image, the first image being a reference image obtained based on a reference instrument, and the reference image being an image of an area of a reference standard; obtaining a second image, the second image being an 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 and associated unknown PSF of the observation instrument, the probe size being larger than a pixel size of the reference image; and 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.