Editing Refracting telescope (section)

===Galilean telescope===
<!--- The article Galileo Galilei links here. Please do not change the title of this subsection without making the appropriate amendments to linking articles --->

{{stack|[[Image:Galileantelescope.png|thumb|400px| '''Optical diagram of Galilean telescope'''
'''y''' – Distant object; '''y′''' – Real image from objective; ''' y″''' – Magnified virtual image from eyepiece;
'''D''' – Entrance pupil diameter; '''d''' – Virtual exit pupil diameter; ''' L1''' – Objective lens; ''' L2''' – Eyepiece lens '''e''' – Virtual exit pupil – '''Telescope equals'''
]]}}
The design [[Galileo Galilei]] used {{Circa|1609}} is commonly called a '''Galilean telescope'''.<ref name=":3">{{Cite web|date=2008|title=Galileo's telescope - The instrument|url=https://brunelleschi.imss.fi.it/esplora/cannocchiale/dswmedia/esplora/eesplora1.html|access-date=2020-09-27|website=Museo Galileo: Institute and Museum of the History of Science}}</ref> It used a convergent (plano-convex) objective lens and a divergent (plano-concave) eyepiece lens (Galileo, 1610).<ref>Sidereus Nuncius or The Sidereal Messenger, 1610, Galileo Galilei ''et al.'', 1989, pg. 37, The University of Chicago Press, Albert van Helden tr., (History Dept. Rice University, Houston, TX), {{ISBN|0-226-27903-0}}.</ref> A Galilean telescope, because the design has no intermediary focus, results in a non-inverted (i.e., upright) image.<ref name=":6">{{Cite web|date=2008|title=Galileo's telescope - How it works|url=https://brunelleschi.imss.fi.it/esplora/cannocchiale/dswmedia/esplora/eesplora2.html|access-date=2020-09-27|website=Museo Galileo: Institute and Museum of the History of Science}}</ref>

Galileo's most powerful telescope, with a total length of just under {{convert|1|m|in|spell=us}},<ref name=":3" /> [[Magnification|magnified]] objects about 30 times.<ref name=":6" /> Galileo had to work with the poor lens technology of the time, and found he had to use aperture stops to reduce the diameter of the objective lens (increase its [[focal ratio]]) to limit aberrations, so his telescope produced blurry and distorted images with a narrow field of view.<ref name=":6" /> Despite these flaws, the telescope was still good enough for Galileo to explore the sky. He used it to view [[impact crater|craters]] on the [[Moon]],<ref>{{cite book|last=Edgerton|first=S. Y.|title=The Mirror, the Window, and the Telescope: How Renaissance Linear Perspective Changed Our Vision of the Universe|date=2009|publisher=Cornell University Press|isbn=9780801474804|location=Ithaca|page=159}}</ref> the four [[Galilean moons|largest moons of Jupiter]],<ref>{{cite book|last=Drake|first=S.|title=Galileo at Work|title-link=:IArchive:galileoatwork00stil|date=1978|publisher=University of Chicago Press|isbn=978-0-226-16226-3|location=Chicago|pages=153}}</ref> and the [[phases of Venus]].<ref>{{Cite web|date=2019-06-02|title=Phases of Venus|url=http://intellectualmathematics.com/blog/phases-of-venus/|access-date=2020-09-27|website=Intellectual Mathematics|language=en-US}}</ref>

Parallel rays of light from a distant object ('''y''') would be brought to a focus in the focal plane of the objective lens ('''F′ L1 / y′'''). The (diverging) eyepiece ('''L2''') lens intercepts these rays and renders them parallel once more. Non-parallel rays of light from the object traveling at an angle '''α1''' to the optical axis travel at a larger angle ('''α2 > α1''')  after they passed through the eyepiece. This leads to an increase in the apparent angular size and is responsible for the perceived magnification.{{fact|date=September 2024}}

The final image ('''y″''') is a virtual image, located at infinity and is the same way up (i.e., non-inverted or upright) as the object.{{fact|date=September 2024}}