What is a telescope?
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 Array Project
The Telescope Array has three fluorescence detector (FD) telescope stations. As in the previous Fly's Eye and High Resolution Fly's Eye (HiRes) experiments, these detectors work by measuring the air fluorescence light emitted by an extensive air shower. Each FD telescope consists of a primary mirror (made up of 18 smaller hexagonal mirror segments) and a camera. The cameras are made up of 256 PMTs (photomultiplier tubes) which are sensitive to the ultra violet light generated by a cosmic ray air shower.