The angular resolution of a telescope is inversely proportional to its aperture. In radio astronomy the technique of interferometry was developed to achieve higher resolution than is possible with a single telescope. In interferometry two or more telescopes connected together are used to give the same resolution as a telescope extending over the distance between them. This powerful instrument is used extensively by Leiden astronomers. If higher resolution is required, observing time can be applied for on other large interferometer arrays such as the VLA in New Mexico, or the European VLBI network.
Dutch astronomy has access to a large number of other observing facilities as well.
The Isaac Newton group of telescopes at the astronomical observatory on La Palma, one of the Canary Islands, is a joint collaboration of the UK and The Netherlands. Leiden staff members are principal investigator in several ESO Key Projects. In addition, Leiden astronomers use observing facilities in space. Several projects on the Hubble Space Telescope are headed by Leiden astronomers. Hipparcos, ISO, and the Compton GRO are other satellite projects in which the Sterrewacht Leiden is actively involved.