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.
Optical Telescope Element
Optical Telescope Element (OTE) is a sub-section of the James Webb Space Telescope, a large infrared space telescope scheduled to be launched early 2021. The OTE consists of some major parts of the telescopes including the main mirror, the secondary mirrors, the framework and controls to support those mirrors, and various thermal and other systems to support the functioning of the telescope. The other two major sections of the JWST are the Integrated Science Instrument Module (holds instruments) and the Spacecraft Element (SE), which includes the Spacecraft Bus and Sunshield. The OTE collects the light and sends it to the science instruments in the ISIM. The OTE has been compared to being the "eye" of the telescope and the backplane of it to being the "spine".