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.
Telescope (goldfish)
Black telescopes are descendants of a type of wild carp known as Silver Prussian Carp or Gibel Carp, this fish is believed to originate from China in the 1400s. In the 1500s they were traded in Japan, and lastly, in the 1800s, they made their way to the USA. It is widely accepted they were a result of selective fish breeding by Chinese who first called them Dragon Fish or Dragon Eyes.