Accession No

0132

Brief Description

reflecting telescope, Gregorian, tentatively attributed to William Robertson, circa 1750

Origin

Maker

Robertson, William [tentative attribution]

Class

astronomy; optical

Earliest Date

1750

Latest Date

1750

Inscription Date

Material

metal (brass, speculum metal); hide (leather); glass

Dimensions

height 530mm; length 720mm; breadth 285mm

Special Collection

Robert Whipple collection

Provenance

Purchased by Robert Stewart Whipple from T.H. Court on 14/10/1923. A “new cap” was made by G. Greene in 07/1925.

Inscription

‘WR’ (inside primary mirror cell)
‘WR’ (beneath stand)

Description Notes

Gregorian reflecting telescope, brass. Leather covered barrel with screw-in cover. Screw-in eye-end (mirror cell) with screw-in eyepiece. Eyepiece has single draw tube and screw-in mount for third eyepiece lens (now missing); both existing lenses in threaded brass mounts. Speculum metal primary mirror held by 3 brass springs. Focus by long screw from eye end moving speculum metal secondary mirror. Two wing nuts secure barrel to altazimuth mount on pillar and folding tripod stand with decorated cabriole feet.

References

Events

Description
Isaac Newton built the first reflecting telescope in 1669. Unlike a refracting telescope a reflecting telescope uses concave mirrors to focus light. The great advantage of using mirrors is that all colours of light are reflected equally. This was an improvement on the refracting telescope which used lenses to focus light. Here the light refracted at different angles to create a distorting fringe of colour around the image being observed.

At first high quality reflecting telescopes were difficult to make as the mirrors available to use degraded quickly and were unable to transmit enough light. But by the eighteenth century methods of making mirrors had improved and instrument makers such as James Short had created hundreds of reflecting telescopes for sale.

There are three types of reflecting telescope: the Gregorian, the Cassegrain and the Newtonian. Both the Gregorian and Cassegrain telescopes are pointed directly at the object being viewed. But the Newtonian has a secondary mirror set at 45° to the objective so the observer stands at right angles to the telescope. Today modern telescopes (with apertures greater than one metre) are reflecting telescopes. Various innovations make them more powerful and accurate. Computer controlled deformable mirrors can correct against changes due to atmospheric conditions or faults in the material.

Created by: Jenny Downes

FM:43516