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.
Visible-light astronomy
Hans Lippershey, a German-Dutch spectacle-maker, is commonly credited as being the first to invent the telescope. Lippershey is the first recorded person to apply for a patent for a telescope, however it is unclear if Lippershey was the first to build a telescope. Based only on uncertain descriptions of the telescope which Lippershey tried to obtain a patent for, Galileo made a telescope with about 3x magnification in the following year. Galileo later made improved versions with up to 30x magnification. With a Galilean telescope the observer could see magnified, upright images on the earth—it was what is commonly known as a terrestrial telescope or a spyglass. Galileo could also use it to observe the sky, and for a time was one of those who could construct telescopes good enough for that purpose. On 25 August 1609, Galileo demonstrated one of his early telescopes, with a magnification of up to 8 or 9, to Venetian lawmakers. Galileo's telescopes were also a profitable sideline; selling them to merchants who found them useful both at sea and as items of trade. He published his initial telescopic astronomical observations in March 1610 in a brief treatise entitled "Sidereus Nuncius" ("Starry Messenger").