Accession No

1076

Brief Description

reflecting telescope, 24"/5" Gregorian, by James Short, English, 1763

Origin

London; England

Maker

Short. James

Class

astronomy; optical

Earliest Date

1763

Latest Date

1763

Inscription Date

1763

Material

glass; metal; brass; wood; ivory

Dimensions

l (barrel) 850 mm; d (primary mirror) 112 mm; box length 972mm; breadth 425mm; height 326mm

Special Collection

Provenance

On loan from St. John's College, University of Cambridge from 08/1951.

Inscription

JAMES SHORT LONDON 51/1262=24”
1763 (on cover)
St John’s Coll
Camb. (ivory plate on box)

Description Notes

Reflecting telescope, 24"/5" Gregorian, by James Short, English, 1763.

Gregorian reflecting telescope, brass. 24-inch focal length; 5-inch aperture. 850mm-long barrel with screw in eye end and screw on aperture stop. Brass finder telescope mounted above. Long screw with knurled end moves speculum metal secondary mirror. Speculum metal primary mirror (d:112mm) held by 3 steel clips on brass end ring of barrel. Divided object glass micrometer with linear scale, divided 0-7 inches in brass, subdivided to 1/16” with vernier. Reverse half divided around aperture 90-0-90 by 1 deg. 2 steel squares for lateral and rotary movement, with long wooden handles. Brass cover. Barrel mounted by 4 screws on semi circle, on altazimuth mount with 2 slow motions moved by screws with square ivory handles. Vertical motion has clamp by knurled screw. Telescope rotates around pillar stand, with clamp. Folding tripod base. Fitted box with handles and castors has additional eye end with screw on stop, screw in cover for eye end of barrel, inner box for micrometer, small box of small brass screws, tin box with alternative speculum metal secondary mirror.
good condition. Telescope only on display

Key for this instrument may be on bunch of keys for St. John’s instruments. See location note and notes fields for more information.

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:39921