When a beam's edges hit an aperture smaller than the beam, or the edge of a mirror mount, lens tube or iris, part of the power is lost and the sharp edge diffracts, creating rings and intensity peaks downstream. Clipping is a common hidden cause of poor beam quality and surprise optic damage. The rule of thumb is to keep clear apertures at least 1.5 to 2 times the 1/e squared beam diameter.
There's clipping on the Faraday aperture, you can see the fringes on the card, walk it over half a millimeter.
Related terms
Hot spot
A small region of a beam with much higher intensity than the rest, a common cause of optic damage.
IR card
A small card coated with phosphor that glows visibly where an invisible infrared beam hits it, used for alignment.
Beam expander
A pair of lenses arranged as a telescope that enlarges a collimated beam's diameter and cuts its divergence.
Walk the beam
To align a beam onto a path using two steering mirrors adjusted in turn against two targets or irises.