Editing Reflecting telescope (section)

====Stevick-Paul====

Stevick-Paul telescopes<ref>[http://www.amsky.com/atm/telescopes/spscopes/spt.html Stevick-Paul Telescopes by Dave Stevick]</ref> are off-axis versions of Paul 3-mirror systems<ref>{{cite journal
 |last=Paul |first=M.
 |year=1935
 |title=Systèmes correcteurs pour réflecteurs astronomiques
 |journal=[[Revue d'Optique Théorique et Instrumentale]]
 |volume=14 |issue=5 |pages=169–202
}}</ref> with an added flat diagonal mirror. 
A convex secondary mirror is placed just to the side of the light entering the telescope, and positioned afocally so as to send parallel light on to the tertiary.
The concave tertiary mirror is positioned exactly twice as far to the side of the entering beam as was the convex secondary, and its own radius of curvature distant from the secondary. Because the tertiary mirror receives parallel light from the secondary, it forms an image at its focus.
The focal plane lies within the system of mirrors, but is accessible to the eye with the inclusion of a flat diagonal.
The Stevick-Paul configuration results in all optical aberrations totaling zero to the third-order, except for the Petzval surface which is gently curved.
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