The Early Development of the Achromatic Telescope Revisited

Abstract

Abstract The emergence in the Netherlands of a flint forward achromatic telescope with a provenance going back to the English optician William Eastland (1702–1787), known for his opposition to the Dollond patent on the achromatic telescope, prompted us to review the early history of the achromatic telescope, especially with regard to Eastland’s role and testimony during the Dollond lawsuit. An investigation of this Eastland-related telescope revealed that the configuration of its doublet lens differed from the design Eastland had sketched in court. However, the close fitting of the two lenses constitutes a plausible next step in the improvement of the achromat. This configuration appeared to be identical in design to the one used in the earliest known flint-forward configuration made by the Dollond firm. An analysis of the spherical aberration of other early designs of the achromat, using the theory only available at the time, indicates that probably several designs steps were deliberately taken. To explain the developments that emerge from the presented curvatures, we propose a sequence of designs for the configuration of doublet lenses. Starting with the presumed Moor Hall design of the 1730s, we discuss the flint forward designs of the early achromats, as well as the crown forward designs of the later period. Most English achromatic telescopes were the result of a process of ‘trial and error,’ designed almost with no influence from the theory developed by several European scholars since 1760. However, in France – and from 1774 onwards in Holland – dioptrical theory was the leading one for most of the optical practitioners.