1230-1235 -- Robert Grosseteste describes the use of 'optics' to "...make small things placed at a distance appear any size we want, so that it may be possible for us to read the smallest letters at incredible distances..." ("Haec namque pars Perspectivae perfecte cognita ostendit nobis modum, quo res longissime distantes faciamus apparere propinquissime positas et quo res magnas propinquas faciamus apparere brevissimas et quo res longe positas parvas faciamus apparere quantum volumus magnas, ita ut possible sit nobis ex incredibili distantia litteras minimas legere, aut arenam, aut granum, aut gramina, aut quaevis minuta numerare.") in his work De Iride.
1266 -- Roger Bacon mentions the magnifying properties of transparent objects in his treatise Opus Majus.
1270 (approx) -- Witelo writes Perspectiva -- "Optics" incorporating much of Kitab al-Manazir.
1570 -- The writings of Thomas Digges describes how his father, English mathematician and surveyor Leonard Digges (1520-1559), made use of a "proportional Glass" to view distant objects and people. Some, such as the historian Colin Ronan, claim this describes a reflecting or refracting telescope built between 1540 and 1559 but its vague description and claimed performance makes it dubious.
1570s -- Ottoman astronomer and engineer Taqi al-Din seems to describe a rudimentary telescope in his Book of the Light of the Pupil of Vision and the Light of the Truth of the Sights. He also states that he wrote another earlier treatise explaining the way this instrument is made and used, suggesting that he invented it some time before 1574.
1586 Giambattista della Porta writes "...to make glasses that can recognize a man several miles away"  It is unclear whether he is describing a telescope or corrective glasses.
1608 -- Hans Lippershey, a Dutch lensmaker, applies for a patent for a perspective glass "for seeing things far away as if they were nearby", the first recorded design for what will later be called a telescope. His patent beats fellow Dutch instrument-maker's Jacob Metius's patent by a few weeks. A claim will later be made that another Dutch spectacle-maker, Zacharias Janssen, had a device that sounded like a telescope at the 1608 Autumn Frankfurt Fair.
A replica of Galileo's telescope
1609 -- Galileo Galilei makes his own improved version of Lippershey's telescope, calling it a "perspicillum".
1611 -- Johannes Kepler describes the optics of lenses (see his books Astronomiae Pars Optica and Dioptrice), including a new kind of astronomical telescope with two convex lenses (the 'Keplerian' telescope).
1616 -- Niccolo Zucchi claims at this time he experimented with a concave bronze mirror, attempting to make a reflecting telescope.
1663 -- Scottish mathematician James Gregory designs a reflecting telescope with paraboloid primary mirror and ellipsoid secondary mirror. Construction techniques at the time could not make it, and a workable model was not produced until 10 years later by Robert Hooke. The design is known as 'Gregorian'.
1668 -- Isaac Newton produces the first functioning reflecting telescope using a spherical primary mirror and a flat diagonal secondary mirror. This design is termed the 'Newtonian'.
1672 -- Laurent Cassegrain, produces a design for a reflecting telescope using a paraboloid primary mirror and a hyperboloid secondary mirror. The design, named 'Cassegrain', is still used in astronomical telescopes used in observatories in 2006.
1674 -- Robert Hooke produces a reflecting telescope based on the Gregorian design.
1684 -- Christiaan Huygens publishes "Astroscopia Compendiaria" in which he described the design of very long aerial telescopes.
2003 -- The Spitzer Space Telescope (SST), formerly the Space Infrared Telescope Facility (SIRTF), is an infrared space observatory launched in 2003. It is the fourth and final of the NASA Great Observatories program