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.
Haploscope
A haploscope is an optical device for presenting one image to one eye and another image to the other eye. The word derives from two Greek roots: "haploieides", single and "skopeo", to view. The word is often used interchangeably with stereoscope, but it is more general than that. A stereoscope is a type of haploscope, but not vice versa. The word has more currency in the medical field than elsewhere, where it refers to instruments designed to test binocular vision. These instruments include Worth's amblyoscope and the synoptophore.