Before any investigation of the firmament, Galileo begins with an investigation of the device itself. The investigation brings his astronomical studies back to “the complete theory of this instrument” and turns a two-lens lead-and-paper pipe into an object almost as unfathomable as the stars.
Thus Galileo begins by experimenting on and with the device, testing its performance, its magnification and measurement of angles. The view through the telescope is supplemented by a view of the laws that govern that view. Everything the view makes visible also allows seeing itself to be seen. The telescope here appears as constructed, materialized theoria (in the Greek sense of the word), as vision. The device is no longer simply for enlarging, for bringing things closer or reproducing them. The telescope is not just an extension of the senses nor an auxiliary device to improve or correct the senses, one whose usefulness would ultimately lie in the “advantages of the instrument … on land and at sea.”
Rather, the telescope creates the senses anew: it defines the meaning of vision and sensory perception, turning any and all visible facts into constructed and calculated data. Ultimately, all the phenomena and “messages” it produces bear the mark of theory. The sensory evidence transmitted by these messages is conveyed alongside the procedure by which that evidence was established. This experimentalization of seeing relates to the fact that the eye and its natural vision are now merely parts of a single optical case among many others. The telescope does not enlarge any more than the eye makes smaller, and the telescopic view is no less natural than the eye’s vision is artificial. Galileo’s telescope thus erases the coordinates of natural vision, the natural view, and the natural eye.
The telescope’s “self-referentiality” means three things. First, the telescopic view pinpoints the observer as much as the object observed. Second, any relation to the object in Galileo’s observations is also a relation of observation to itself. Finally, the telescope’s medial character is also revealed in its self-referential structure. The surprising turn taken by Galileo’s viewing of the sky through the telescope is that when Galileo looks through his telescope at the planets, and in particular at the moon, what he sees above all is the Earth.
Joseph Vogl: Becoming Media: Gallileo's Telescope