Editing Refracting telescope (section)

===Keplerian telescope===
[[File:Houghton Typ 620.73.451 - Johannes Hevelius, Machinae coelestis, 1673.jpg|thumb|right|Engraved illustration of a {{convert|150|ft|m|abbr=on|order=flip}} focal length Keplerian astronomical refracting telescope built by Johannes Hevelius.<ref>{{cite book|first=Johannes |last=Hevelius |title=Machina Coelestis |volume=First Part |date=1673 |publisher=Auctor}}</ref>]]
The '''Keplerian telescope''', invented by [[Johannes Kepler]] in 1611, is an improvement on Galileo's design.<ref>{{cite book |title= Optics|last= Tunnacliffe|first= AH |author2=Hirst JG |date= 1996|location= Kent, England |isbn= 978-0-900099-15-1|pages= 233–7}}</ref> It uses a convex lens as the eyepiece instead of Galileo's concave one. The advantage of this arrangement is that the rays of light emerging from the eyepiece{{dubious|date=November 2019}} are converging. This allows for a much wider field of view and greater [[eye relief]], but the image for the viewer is inverted. Considerably higher magnifications can be reached with this design, but, like the Galilean telescope, it still uses a simple single element objective lens so it needs to have a very high focal ratio to reduce aberrations<ref>{{cite web|title=Galileo's telescope - Chromatic aberration|url=http://brunelleschi.imss.fi.it/esplora/cannocchiale/dswmedia/storia/estoria5_st.html|publisher=Museo Galileo - Istituto e Museo di Storia della Scienza|accessdate=5 March 2012}}</ref> ([[Johannes Hevelius]] built an unwieldy f/225 telescope with a {{convert|8|in|mm|abbr=out|adj=on|order=flip}} objective and a {{convert|150|ft|m|abbr=out|adj=on|order=flip}} [[focal length]],<ref>{{Cite book |last=Bell |first=Louis |url=https://www.gutenberg.org/files/53740/53740-h/53740-h.htm |title=The Telescope |publisher=McGraw-Hill |year=1922 |location=New York |via=The Project Gutenberg}}</ref>{{page needed|date=July 2024}} and even longer tubeless "[[aerial telescope]]s" were constructed). The design also allows for use of a [[Filar micrometer#Prior devices|micrometer]] at the focal plane (to determine the angular size and/or distance between objects observed).

[[Constantijn Huygens Jr.|Huygens]] built an aerial telescope for [[Royal Society of London]] with a 19&nbsp;cm (7.5″)  single-element lens.<ref name="stjarnhimlen.se">{{Cite web|url=http://www.stjarnhimlen.se/bigtel/LargestTelescope.html|title=Largest optical telescopes of the world|website=www.stjarnhimlen.se}}</ref>