Telescopes, Astronomical and navigational instruments, Optical equipment

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Transit instrument


Object connections:

Collection Telescopes, Astronomical and navigational instruments, Optical equipment
ExhibitionsShips, Clocks & Stars: The Quest for Longitude
User collections Captain Cook's Third Voyage by NMMCollections
William Wales by NMMCollections
Captain Cook's Second Voyage by NMMCollections
Gallery locationNot on display

Object details:

Object ID AST0981
Description This transit instrument has engraved on the central bosss 'Fait par Ramsden pour Dollond London', made by Ramsden for Dollond London. Ramsden married into the famous optical instrument making Dollond family in 1765 when he married John Dollond's youngest daughter, Sarah. The inscription indicates that Ramsden was making this for Dollond to help him complete an order rather than for Dollond's own use. Ramsden's expertise was in creating the accurate metalwork and scales, Dollond's was in the optics. Once Ramsden completed his work on this telescope it is thought he then passed it onto Dollond to complete the optics (lenses and eyepieces). Why the inscription is in French is unclear though it may indicate the nationality of the intended recipient. This portable transit instrument is similar to that used on James Cook’s Transit of Venus expedition of 1769. A very similar instrument, made by John Bird and belonging to the Royal Society, was taken in the 'Resolution' on Cook’s second voyage and in the 'Discovery' on the third voyage. The instrument shown here was probably made a few years after Bird’s, which does not seem to have survived. The 112cm telescope tube is made of brass with an objective lens of 38mm diameter. The central supporting arms (which form part of the telescope tube) are 787mm across. In all, the telescope is built in a cross shape, one part the telescope, the other the supporting arms which are cones tailing away from the telescope. The ends of the cones fit into the mount for the telescope. There is a cap for the objective with a slide off cover. Also attached to the cap is a light reflection shield whose shield can turn to an angle with respect to the telescope tube. One side of the shield is brass and the other is paper or card. There is a place to slide a rectangular cover over the eyepiece, but there is no cover. There is no apparent focusing system for the telescope. The original mounting is missing for this telescope but a replacement has been built from square metal tubes and painted black. The original brackets for the mount remain and have been incorporated into the new mount, AST0981.2. There are three adjustable feet at the base of the mount for levelling. On one of the supporting brackets there is a large semicircle with a scale in degrees and there is an index arm with a vernier which attaches to this and the telescope to give an altitude reading. The arm is separate and comes away easily once the telescope is removed. The bracket has a fine adjustment mechanism for moving that side of the telescope up and down over a small area; this is for accurate aligning of the mount so that the telescope readings are correct. The fine adjustment rod/knob is missing so that this cannot easily be used. There is a similar mechanism on the opposite bracket but this finely adjusts the telescope in azimuth over a small area so that the mount gives accurate readings. Again a rod/knob is missing to control this and so it is difficult to use. Accompanying this telescope is a large spirit level, marked AST0981.1, which rests on the telescope supports. There are also six wall plugs, bolts and spare bolts (possibly missing?).
Date made circa 1780

Artist/Maker Ramsden, Jesse
Dollond & Aitchison
Place made London
Credit National Maritime Museum, Greenwich, London, Caird Collection
Measurements Replica mount: 735 mm x 905 mm x 521 mm
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