Astronomy Telescope Apertures
Large astronomical telescopes, like the upcoming Extremely Large Telescope (ELT) and the James Webb Space Telescope (JWST), are pushing the boundaries of discovery, with massive mirrors to capture faint light, enabling views of the early universe, exoplanets, and dark matter, utilizing cutting-edge designs like segmented mirrors and infrared capabilities to overcome atmospheric blur and see further back in time.
All of the large astronomy telescopes, be they located in outer space or on Earth are very likely to use slit apertures.
This is one of the most demanding applications for precisions slits, being of long length, small width and with a high parallelism and edge definition specification.
They are normally manufactured using electroforming techniques to give this excellent edge definition and straightness without the light scatter you would get with laser cut products.