Aperture - screen
Aperture, often called aperture, is the diameter of the entrance optical part of the telescope (most often the objective lens in a refractor or the mirror in a reflector). It indicates how much light the instrument can capture and thus determines its ability to observe faint objects. Aperture is usually measured in millimetres (e.g. 30 mm, 150 mm, 200 mm) and is one of the most important parameters determining the performance of a telescope.
The larger the aperture, the more light the telescope gathers, allowing faint stars, nebulae, galaxies and fine details to be seen. In addition to the brightness of the image, the larger diameter also has a positive effect on the resolving power of the telescope, i.e. the ability to detect small structures or to separate closely lying stars.
Small apertures are suitable for portable and fast use (e.g. travel telescopes or smart telescopes for wide fields of the sky). Large apertures, on the other hand, are used where detail, range and maximum image brightness are a priority, typically in advanced astronomical instruments. So, in simple terms: larger aperture = more light and more detail.