Geometry of Gregorian Telescope
This Demonstration shows the principle of a telescope designed by the Scottish mathematician and astronomer James Gregory in the seventeenth century; it was first built in 1673 by Robert Hooke. The parameters and are the semiaxes of the ellipse, and the equation of the parabola in polar coordinates is The ellipse and parabola have a common focus.
For the construction of a Gregorian telescope see .
The photo was taken from [2, p. 94].
 Wikipedia. "Gregorian Telescope." (Nov 29, 2016) en.wikipedia.org/wiki/Gregorian_telescope.
 J. Gregory, Optica Promota, London: Thomson, 1663. books.google.com.mx/books/about/Optica_promota.html?id=W2U_AAAAcAAJ.
 King's College London. "James Gregory's Telescope." (Nov 29, 2016) www.kingscollections.org/exhibitions/specialcollections/to-scrutinize-nature/scrutinizing-the-skies/james-gregorys-telescope.
 I. Bruce. "James Gregory's Optica Promota." (Nov 29, 2016) www .17centurymaths.com/contents/James%20 Gregory/Prop1OP.pdf.