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Other Nairne Instruments
 

Gregorian Reflecting Telescope

Signature Nairne, London

Date circa 1770
Dimensions English Metric
Main Tube Length 25.3in 645mm
Main Tube Diameter 4.5in 115mm
Total Length 29.3in 745mm
Height 30.0in 762mm
Primary Mirror Diameter 4.0in 102mm

Description

This telescope has a large brass cylindrical barrel, with a push-on cap.  The primary speculum mirror is a concave paraboloid, with a central hole, and is kept in place 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 for several inches along the front interior part of the barrel.  This configuration of primary and secondary mirrors is called Gregorian.  The focusing thumbscrew, at the eyepiece end of the barrel, is the long shank design, attaching to the secondary mirror carrier through a slot near the front end of the barrel. The relatively short eyepiece tube screws into a hole in the center of the backing plate, which holds the primary mirror in place.  The two-lens eyepiece is the Huygens type.  There is a slightly flared eyecup attached to a tube which slides over the eyepiece tube, containing a stop with a small aperture.  This sliding tube arrangement permits movement of eyecup/stop from directly behind the eye lens to several inches behind.  Additionally, there are two brass sighting apertures, mounted on the top of the barrel at the front and back.  The Telescope is signed "Nairne  London" in huge script on the top of the barrel, at the back end, with the eyepiece to the left.  The scope mounts on a brass column, with horizontal and vertical swivel joints, to a folding tripod with cabriole legs.

 

 


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