Chromatic aberration of eyepieces in early telescopes

Summary

The twofold objective of this study is (1) to identify and give a brief review of the historical development of the various designs of early (pre-1850) telescope eyepieces, and (2) to determine by measurements and calculations the axial and lateral chromatic aberrations of a number of extant eyepieces from that period in order to provide basic data on which to judge the relative quality of different eyepiece forms. Eight distinct types of eyepieces containing one to five lens elements are discussed and illustrated. The second objective was addressed by measuring the focal lengths of the individual lens elements and the lens spacings in each eyepiece. The data were processed by a ray-tracing program that yields the chromatic aberration (CA) and focal length of each of the eyepieces. Twenty-one telescopes from that period were studied in this way. Similar calculations were also made using data available in the literature on several additional historic telescopes. The conclusions drawn from the data are: (1) Telescopes with Galilean eyepieces have a smaller CA than those with Keplerian or Huygens eyepieces. (2) The two-lens Keplerian terrestrial eyepieces are demonstrated to have larger axial and lateral CA than any of the other eyepiece types. (3) Calculations from examples of telescopes by Divini and Campani for which data were available indicate that while the axial CAs of the two were the same, Campani's instrument had a much smaller lateral CA. (4) Five element eyepieces were found to be better corrected for CA than the early four-lens systems that replaced them. (5) This study tends to confirm the speculation that the reason makers of early achromatic telescopes overcorrected the objective lenses was to compensate for the CA of the eyepieces.

This study was made as part of the DIOPTRICE project to study early refracting telescopes sponsored by the Adler Planetarium Astronomy Museum with support from the National Endowment for the Humanities (Grant # RZ-50206-04). I wish to thank my son, Eric Rudd, for writing the ray-tracing program that was a vital part of this study and for his continuing expert advice. Thanks go to Marvin Bolt, for his careful reading of the manuscript and his valuable suggestions. Duane Jaecks continues to provide a welcome stream of ideas, information, and inspiration. The instruments pictured in the figures are all from the author's collection.

Notes

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²Albert Van Helden, ‘The development of compound eyepieces, 1640–1670’, Journal of the History of Astronomy, 8 (1977), 26–37.

³Rolf Willach, ‘The development of telescope optics in the middle of the seventeenth century’, Annals of Science, 58 (2001), 381–98.

⁴Henry C. King, The History of the Telescope (New York, 1955).

⁵Deborah Jean Warner, ‘Telescopes for Land and Sea’, Rittenhouse: Journal of the American Scientific Instrument Enterprise, 12 (1998), 33–54.

⁶Reginald J. Cheetham, Old Telescopes (Southport, UK, 1997).

⁷Thomas H. Court and Moritz von Rohr, ‘A history of the development of the telescope from about 1675 to 1830 based on documents in the Court Collection’, Transactions of the Optical Society, 30 (1929) 207–60.

⁸Maria Luisa Righini Bonelli and Albert Van Helden, ‘Divini and Campani: A forgotten chapter in the history of the Accademia del Cimento’, Supplement to Annali dell’ Istituto e Museo di Storia della Scienza (Florence, 1981), 7.

⁹R.T. Gunther, Early Science in Oxford, Vol. 2, Astronomy (Oxford, 1923), 296.

¹⁰Schyrl de Rheita, A.M., Oculus Enoch et Eliae save Radius Sidereomysticus (Antwerp, 1645).

¹¹Sir John F.W. Herschel, The Telescope (from the Encyclopedia Britannica) (Edinburgh, 1861), 52.

¹²Peter Louwman, ‘Christiaan Huygens and his telescopes’, in Titan: From Discovery to Encounter, ed. Karen Fletcher (Noordwijk, the Netherlands, 2004), 103–14 (113).

¹³J. Ramsden, ‘A description of a new construction of eye-glasses for such telescopes as may be applied to mathematical instruments’, Philosophical Transactions of the Royal Society 73 (1782) 94–99.

¹⁴Willach (note 3), 390.

¹⁵Willach (note 3), 392–94.

¹⁶Herschel (note 11), 59.

¹⁷John Dollond, ‘A letter from Mr. John Dollond to Mr. James Short, F.R.S. concerning an improvement of refracting telescopes’, Philosophical Transactions of the Royal Society, 48 (1753), 103–7.

¹⁸Herschel (note 11), 60.

¹⁹William Kitchiner, The Economy of the Eyes, Part II. Of Telescopes (London, 1825), 200–209.

²⁰Dollond (note 17).

²¹Willach (note 3).

²²Balthasar de Monconys, The voyages of M. de Monconys (Paris 1665), appendix.

²³M. Eugene Rudd and Eric P. Rudd, ‘A new method of measuring chromatic aberration in lenses’, Journal of the Antique Telescope Society, 25 (2003), 3–8.

²⁴Kitchiner (note 19), 52–66.

²⁵William Herschel, ‘A series of observations of the satellites of the Georgian planet … with an introductory account of the telescopic apparatus that has been used on this occasion …’, Philosophical Transactions of the Royal Society, 105 (1815), 293–362 (297).

²⁶Kitchiner (note 19), 62.

²⁷Willach (note 3), 383.

²⁸Bonelli and Van Helden (note 8).

²⁹Willach (note 3), 394, 397.

³⁰Dollond (note 17).

³¹Court and von Rohr (note 7), 217.

³²See, e.g., H. Dennis Taylor, The Adjustment and Testing of Telescope Objectives, 5th edition (Bristol, 1983), 44.