Optical telescope
"Observable world" describes what can be seen using a telescope, when viewing an object or range the observer may use many different techniques. Understanding what can be viewed and how to view it depends on the field of view. Viewing an object at a size that fits entirely in the field of view is measured using the two telescope properties—focal length and aperture, with the inclusion of an ocular eyepiece with suitable focal length (or diameter). Comparing the observable world and the angular diameter of an object shows how much of the object we see. However, the relationship with the optical system may not result in high surface brightness. Celestial objects are often dim because of their vast distance, and detail may be limited by diffraction or unsuitable optical properties.
William Herschel Telescope
The telescope is housed in an onion-shaped steel dome with an internal diameter of , manufactured by Brittain Steel. The telescope mount is located on a cylindrical concrete pier so that the centre of rotation is above ground level, which lifts the telescope above ground-layer air turbulence for better seeing. A conventional up-down 6m-wide shutter with wind-blind, several large vents with extractor fans for thermal control, and a capacity crane (used for moving the primary mirror e.g. for aluminising) are all incorporated. The size and shape of the shutter allow observations down to 12° above the horizon, which corresponds to an airmass of 4.8. The total moving mass of the dome is , which is mounted on top of a three-storey cylindrical building. The dome was designed to minimise wind stresses and can support up to its own weight again in ice during inclement weather. The dome and telescope rest on separate sets of foundations (driven down into the volcanic basalt), to prevent vibrations caused by dome rotation or wind stresses on the building affecting the telescope pointing.