The History of Telescopes and Binoculars: an Engineering Perspective

Plenary Paper The History of Telescopes and Binoculars: An Engineering Perspective John E. Greivenkamp∗ and David L. Steed College of Optical Sciences, University of Arizona, 1630 E. University Blvd, Tucson, AZ 85721 ABSTRACT The design of the refracting telescope advanced rapidly following its invention in 1608, reaching its modern configuration in about a century. Even though the development of binoculars began almost simultaneously, nearly three hundred years elapsed before practical prismatic binoculars became available. The impediments to practical binoculars were not only in optical design, but in mechanical design, manufacturing, and materials. This paper will document the history of telescopes and binoculars from an engineering perspective looking at the evolution of basic optical system layout as well as some of the mechanical issues faced. This development will be illuminated using examples from the Museum of Optics at the College of Optical Sciences at the University of Arizona. Keywords: Binoculars, Telescopes, History of Optics, Antique Optics, Optical Design, Optical Instrumentation 1. INTRODUCTION The development of practical handheld refracting telescopes and binoculars is much more than the story of optical design. It must be told in conjunction with advances in mechanical design, manufacturing technology and materials development. A strong argument can be made that the path to modern optical instrumentation is as dependent on the availability of precision brass tubing in the mid-1700s as it is on the innovations in the optical designs of the instruments. Because of this, collections of antique optical instruments are often referred to as “glass and brass.” While the most obvious differences between handheld telescopes and binoculars and astronomical telescopes are the diameters of the objective lenses and the system size, the more significant difference may be that handheld telescopes and binoculars were produced in far greater numbers. This trend continues to the present. These instruments are mostly intended for terrestrial applications and produce erect images. The early uses were primarily nautical and military; consumer use including sports and theater followed later. These telescopes and binoculars were not one-off instruments and the design and engineering had to be consistent with a production environment. Hans Lipperhay of the Netherlands is credited with inventing the telescope in 1608. His patent application was followed a week later with a request from the examining committee “to ascertain … whether he could improve it so that one could look through it with both eyes” [1]. Within several months, Lipperhay constructed and demonstrated this first pair of binoculars. The development of telescopes and binoculars began almost simultaneously, yet the telescope reached its modern design configuration in about a century while it took almost three hundred years for the modern form of prismatic binoculars to become practical. This delay was due to the fact that binoculars are more than just two telescopes. There are significant engineering challenges in the requirements that the optical axes of the two telescopes be parallel or collimated and that the two magnifications must be matched [2]. The ability to adjust the interpupillary separation to match that of the user is another important design feature. This paper will document the development of handheld telescopes and binoculars using examples from the Museum of Optics at the College of Optical Sciences at the University of Arizona. ∗ [email protected] www.optics.arizona.edu/museum Novel Optical Systems Design and Optimization XIV, edited by R. John Koshel, G. Groot Gregory, Proc. of SPIE Vol. 8129, 812902 · © 2011 SPIE CCC code: 0277-786X/11/$18 · doi: 10.1117/12.904614 Proc. of SPIE Vol. 8129 812902-1 Downloaded from SPIE Digital Library on 02 Jul 2012 to 150.135.248.41. Terms of Use: http://spiedl.org/terms 2. REFRACTING TELESCOPES Following the demonstration of the Galilean or Dutch form of the telescope (a positive focal length objective lens with a negative focal length eye lens) in 1608, the telescope evolved rapidly. Some of the important dates in the development of the refracting telescope are [1, 3-8]: - 1611: proposal by Johannes Kepler of the astronomical telescope consisting of two positive lenses. - 1617: first recorded construction of a Keplerian telescope by Christoph Scheiner - Circa 1625: introduction of a single erecting lens in the Keplerian telescope to produce an erect image by Christoph Scheiner. - 1645: Anton Maria Schyrle de Rheita used a two element erecting couplet to produce a practical terrestrial telescope with an erect image and acceptable magnification and field of view. - 1662: two-lens eyepiece incorporating both an eye lens and a field lens was invented by Christian Huygens. This combination of a Schyrle erecting system and a Huygens eyepiece represents the basic optical system layout of the terrestrial or relayed-Keplerian telescope that survives to this day. Figure 1 schematically illustrates the evolution of the refracting telescope. In most situations, all of the optical elements except the objective lens are mounted with fixed spacing in the last draw or eye draw of the telescope. Additional improvements that were incorporated into the design are a field stop placed between the two elements of the Huygens eyepiece, and a glare stop at the intermediate pupil location between the two erecting lenses. The intermediate pupil is the image of the objective lens aperture formed by the first erecting lens. Telescopes incorporating the Schyrle erecting system (three-lens eyepiece) were into production and available in the last half of the 1600s. There is some evidence that the four-lens eyepiece was in use in the late 1600s, however it fell out of favor until the mid-1700s [7]. At that time, telescopes using Schyrle-Huygens erecting systems with both four and five- lens eyepieces were in production. While there were improvements to follow, most notably the achromatic objective in the mid-1700s, the modern design form of the terrestrial refracting telescope was firmly established by this time. 2.1 Chromatic Aberration and the Achromatic Doublet The primary limitation of the image quality that was produced with early telescopes was chromatic aberration of the objective lens. Because of the color dispersion of the glass in a simple objective lens, different wavelengths have different focal lengths and are brought to focus at different distances. The longitudinal blur associated with chromatic aberration limits the angular resolution of the telescope. For a given focal length lens and glass type, the only method to reduce the blur was to use a lens with a small diameter or to use an aperture to reduce the effective lens diameter. This situation is shown in Figure 2. In the early 1700s, it was believed that chromatic aberration was fundamental and could not be corrected. There was a tradeoff between resolution and light collecting ability, and most handheld telescopes of this era had very small objective lenses, often with an effective aperture diameter of 10-15 mm. An example of a pre- achromatic telescope with a Schyrle erecting system is shown in Figure 3. An interesting and unusual configuration that occurred with pre-achromatic telescopes is the reverse taper telescope. The angular field of view largely determines the required diameters of the erecting lenses and especially the field lens. In higher magnification telescopes, the objective lens could be smaller than the lenses in the eye draw. The objective end of the telescope is smaller in diameter than the eyepiece end. A reverse taper telescope is shown in Figure 4. Achromatic objectives correct chromatic aberration by combining a positive lens made from a low-dispersion glass with a negative lens made from a high-dispersion glass. The original inventor of the achromatic objective is Chester Moor Hall, a barrister in London [9]. In 1733, he commissioned two different opticians, Edward Scarlett and James Mann, to each make one of the lens elements. By chance, both opticians subcontracted the work to the same man, George Bass. Chester Moor Hall then continued to keep his invention secret. Around 1750, Bass told optician John Dollond about the achromatic lens he had made. Dollond’s son, Peter, saw the commercial advantages and once they had made test lenses, patented the invention in 1758. Chester Moor Hall twice attempted to challenge the patent. He lost his case on the grounds that the person who should profit by the invention is the one who benefits the public by it, not one who keeps it locked in his desk drawer. This was a landmark decision in patent law that remains in place to this day. Proc. of SPIE Vol. 8129 812902-2 Downloaded from SPIE Digital Library on 02 Jul 2012 to 150.135.248.41. Terms of Use: http://spiedl.org/terms The achromatic doublet was a significant optical improvement for telescopes. It allows for improved resolution with much larger objective lenses and more light-gathering capability. Most handheld telescopes could have objective lenses with diameters of 25-50 mm or larger. Dollond went on to become the dominant manufacturer of telescopes in the late 1700s and early 1800s. The name “Dollond” became a synonym for a telescope. Scheiner Erecting System Exit Pupil fObjective 2fRelay 2fRelay fEye Objective Erecting or Relay Lens Eye Lens Schyrle Erecting System Exit Glare Field Pupil Stop Stop fObjective fRelay 2fRelay fRelay fEye Objective Erecting or Relay Couplet Eye Lens Schyrle Erecting System with Huygens Eyepiece Glare Field Exit Stop Stop Pupil Field Eye Lens Lens fObjective fRelay 2fRelay fEye Objective Erecting or Relay Couplet Eye Piece Figure 1. The evolution of the design of the terrestrial or relayed-Keplerian telescope. The separation between the elements in the erecting couplet was often twice the lens focal length, but this is not required. The Schyrle erecting system is often referred to as a three-lens eyepiece, and the Schyrle-Huygens erecting system as a four-lens eyepiece.

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