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Gregorian Reflecting Telescope

Signature James Short, London

Date Mid-1700s
Dimensions English Metric
Main Tube Length 11.0in 280mm
Main Tube Diameter 2.2in 55mm
Total Length 12.7in 320mm
Primary Mirror Diameter 2.0in 50mm


This telescope has a small lacquered brass cylindrical barrel, with a push-on cap.  The primary mirror is a speculum concave paraboloid, with a central hole, and is kept in its position at the rear of the barrel with a screw-in backing plate.  The small secondary mirror is a concave ellipsoid.  It is attached by a slender arm to a screw-driven carrier which slides along the length of the interior of the barrel.  This configuration of primary and secondary mirrors is called Gregorian.  The focusing screw, at the eyepiece end of the barrel, is the short shank design, attaching to the secondary mirror carrier through a slot near the back end of the barrel.  The short eyepiece tube screws into a hole in the center of the backing plate that holds the primary mirror in place.  The two-lens eyepiece is a Huygen's configuration.  There is a slightly flared eyecup that screws into the end of the tube.  It has a very small aperture, intended to block stray light coming from the inside of the barrel.  The telescope is mounted to a short brass column, with a wood screw at the bottom end.  The column is screwed into a hole in the bottom of the storage box or some conveniently placed rail or post.  The scope is signed "James Short London   145/1028=7" on the barrel backing plate.  The numbering in the signature provides a unique identification or serial number for each telescope produced by Short.  It includes a fitted mahogany case with a sliding top.  James Short was the leading maker of reflecting telescopes in this era, and he is considered the first to produce parabolic mirrors in quantity.



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