Ronchigram is an image at infinite magnification with zero defocus, which looks smooth and featureless for a very thin sample. Ronchigram is the shadow image (“Gabor hologram”) cast by sample (Projection) in the diffraction disc. Discovered by Ronchi (1948) during the investigation of the Spherical aberration of optical lenses.

A Ronchigram of an amorphous sample is often used to visualize (1) electron’s optic alignment and (2) lens aberration. A trained microscopist can use the patterns and symmetries to adjust the STEM alignment using the multipole lenses in the aberration corrector.

We choose the largest aperture possible so that the STEM probe is not compromised by diffraction broadening while excluding the high-angle electrons. If you insert the aperture and find the Ronchigram becomes distorted, then it could be condenser aperture is charging up due to contamination, adding its own electrostatics aberration.