Accession No

2703

Brief Description

reflecting telescope, Gregorian, by George Adams Snr., 1750 (c)

Origin

England; London; Fleet Street; Tycho Brahe’s Head

Maker

Adams, George (Snr.)

Class

astronomy; optical

Earliest Date

1750

Latest Date

1750

Inscription Date

Material

metal (brass, speculum metal, steel); glass

Dimensions

telescope length 442mm; breadth of base 248mm; height 450mm; diameter of barrel 83mm

Special Collection

Provenance

Purchased from Sotherby's, London, England; Lot 72, 18/12/1980. Purchased with grant from Preservation Fund, Science Museum (PRISM).

Inscription

‘Made by Geo: Adams at Tycho Brahe’s Head in Fleet Street London.’ (on eye-end)
‘Ad:’ (on eye-end)

Description Notes

Brass gregorian reflecting telescope. Barrel with plain sights, no cover. Screw-in eye-end holds speculum metal primary mirror by 3 brass springs. Screw-in eyepiece with screw-on lens mount and eye-stop. Focus by long screw from eye-end, moving speculum metal secondary mirror. 2 wing-nuts (one broken) secure barrel to altazimuth mount, with wing-nut clamp for vertical motion, on turned pillar stand. Stand screws into folding tripod base with cabriole feet. Lion decoration

Condition fair; incomplete (cover missing)

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:45484