Collimated

Low-mass objects have much more collimated flows than high mass sources.

2008 May, A. M. Soderberg, et al., An extremely luminous X-ray outburst at the birth of a supernova, Accepted draft, page 3, Wolf-Rayet stars are also argued⁹ to give rise to gamma-ray bursts, a related but rare class of explosions characterized by highly-collimated relativistic jets.

Although the most collimated outflows look very much jet-like at the highest velocities, their behavior is much more classical at lower speeds.

Closing the aperture results in more collimated light, as only light traveling in the right direction can make it through the smaller opening.