Portable equatorial refractor telescope

This is a brass, portable, equatorial, refracting telescope called a heliometer as it is designed for viewing the Sun.

Its size makes it portable; its stand makes it equatorial; its optics makes refracting. As an equatorial telescope its 'equatorial plate' can be adjusted for latitude, aligning it with the equator thus allowing the telescope to move easily with the Earth's rotation and so follow a star (or Sun) as it rises and sets. It is a refracting telescope because it uses all glass lenses (rather than mirrors) to focus the light.

The telescope is simply signed 'Dollond' making its exact maker difficult to indentify since the Dollond family made telescopes over several generations.

Solar observing had a certain attraction to the general public in the 19th century. It was an astronomical activity that could be done in the day and with the advent of 'eclipse tourism' towards the end of the century it had an added element of adventure to it as people would travel around the globe in search of eclipses to view.

Telescope: The brass barrel and equatorial mounting are supported by three folding legs. The telescope lens is focused by means of a handle on the side of the tube. The focal length of the achromatic object glass is, c. 17in, aperture, 5.0cm (2.0in). It is fitted with object-glass divided image micrometer.

Stand: Starting from the bottom, at the foot of each of the three folding legs is a leg screw. These leg screws bring the lower plate of the mounting (horizontal plate) into a horizontal position determined by two spirit levels set on it at right angles to one another. The plate is divided by half degrees and rotated by moving a handle at the side. It also carries a compass.

The upper plate (equatorial plate) turns on the horizontal axis. It is divided into twice 12 hours and rotated by turning a handle. Latitude is shown on the latitude arch, which passes throught the balancing weight between the 2 plates.

Above the equatorial plate fixed to an axis is the declination semi-circle upon which the telescope is supported. It is subdivided into twice 90 degrees and moved by means of a handle.

Object Details

ID: AST0908
Collection: Astronomical and navigational instruments
Type: Telescope
Display location: Not on display
Creator: Dollond, Peter
Date made: circa 1780
Credit: National Maritime Museum, Greenwich, London
Measurements: Overall: 520 mm

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