Dorpat Great Refractor
First modern achromatic great refractor with German equatorial mount.
The Dorpat Great Refractor, also known as the Fraunhofer 9-inch, was a large refracting telescope built by Joseph von Fraunhofer in the early 1820s at what was then Dorpat Observatory in the Governorate of Estonia (later Tartu Observatory in southern Estonia). It is notable as the first modern, achromatic, refracting telescope of the great refractor era, and it introduced the first modern equatorial mount type called a 'German equatorial mount,' which became standard for most large refractors thereafter.
- Aperture
- 9 Paris inches (about 9.6 in / 24 cm)
- Focal length
- 4 m (13.4 ft)
- Location
- Dorpat Observatory, Governorate of Estonia (later Tartu Observatory, southern Estonia)
- Builder
- Joseph von Fraunhofer
- Mount type
- German equatorial mount (first modern equatorial mount)
- Era
- early 1820s
Lore & Background
The Dorpat Great Refractor was built by Joseph von Fraunhofer in the early 1820s, marking the start of the era of great refractors. It had a 9 Paris inch (about 9.6 in or 24 cm) aperture achromatic lens and a 4 m (13.4 ft) focal length. The telescope was equipped with the first modern equatorial mount type, called a 'German equatorial mount,' developed by Fraunhofer, which became standard for most large refractors from then on.
The telescope was located at Dorpat Observatory in the Governorate of Estonia (Estland), which later became Tartu Observatory in southern Estonia. A similar Fraunhofer '9-inch' (24 cm) telescope at Berlin Observatory was later used by Johann Gottfried Galle in the discovery of Neptune. The article notes a tendency to round apertures to the nearest large figure, which can create drift in conversions; the Fraunhofer '9-inch' were nine Paris inches (pouces), about 9.6 in or 24 cm, not exactly nine English inches, and closer to ten inches.
Reader's Guide
The Dorpat Great Refractor is significant as the first modern achromatic refracting telescope of the great refractor era, built by Joseph von Fraunhofer in the early 1820s. It introduced the German equatorial mount, which became the standard mount for most large refractors thereafter. This telescope exemplifies the beginning of the era of great refractors, which dominated observational astronomy in the 19th and early 20th centuries. Its design and technology influenced subsequent large refractors, including the Fraunhofer 9-inch at Berlin Observatory used for the discovery of Neptune. The Dorpat Great Refractor represents the technological advances in lens making and mounting that allowed refractors to become practical and precise instruments for astronomy, despite the challenges of fabricating large achromatic lenses. Its legacy includes setting the pattern for later great refractors, which were often the centerpieces of new observatories and used for observing double stars, astrophotography, and other pioneering work.
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