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William Herschel and the “Front-View” Telescopes

2017; Springer International Publishing; Linguagem: Inglês

10.1007/978-3-319-32826-3_4

2509-310X

Roger Ceragioli,

History of Science and Natural History

William Herschel is rightly regarded as one of the greatest telescope makers in history. His instruments vastly exceeded in aperture and light-gathering power all those that came before his time, and established the reflecting telescope as a formidable instrument of astronomical research. To understand his research successes – and the limits of his success – we must understand the performance of his telescopes. The present chapter attempts a comprehensive evaluation of the optical performance of Herschel’s telescopes. This is possible because of the vast documentation he left behind relating to his building and usage of telescopes, and also because a substantial number of his surviving mirrors have been tested using modern methods. After an introduction the chapter proceeds with a review of how the reflecting telescope developed before Herschel, and a consideration of engineering and materials problems in building reflectors. It then turns to optical matters, considering the types of imaging errors (aberrations) found particularly in Newtonian telescopes, the design form favored by Herschel. Further on the chapter considers how the theory of those errors (aberration theory) developed before Herschel and into the nineteenth century. Study of the forms of aberration and the history of aberration theory is necessary for understanding what optical errors Herschel saw in his telescopes, and what he could know – or not know – about them. Following this we consider Herschel’s methods of fabrication and optical testing, in particular concentrating on the small elliptical flat mirrors (“secondaries” or “diagonals”) that are a critical part of any Newtonian telescope, even today. These petite mirrors form a relatively neglected area of study compared to Herschel’s more physically imposing primary mirrors, which have often been tested optically. It is ironic, therefore, that these small mirrors, precisely because they are used at a 45° tilt, called “oblique incidence” – versus the primaries used at “normal incidence” – have the potential to affect the final telescopic image just as gravely as the primaries, unless they are rigorously flat. Yet before the late nineteenth century there was no accurate method of assessing the surface profile of flat mirrors. The question, therfore, arises: How could Herschel make optically usable diagonal mirrors for Newtonian telescopes? The analysis and solution of this question forms one of the most important results of the present chapter. The upshot is that because Herschel (and other eighteenth-century telescope makers) could not make accurate flat mirrors – or any usable diagonal mirrors of large dimension – Herschel turned to the so-called “front-view,” or “Herschelian,” telescope for his largest instruments. With all this as background, the next sections present a detailed discussion of the performance of eighteenth and early nineteenth-century Newtonian telescopes, and a similar consideration of “front-views.” Attention is also devoted throughout the chapter to explaining how Lord Rosse’s giant Newtonian telescopes of the mid-nineteenth century consituted a dramatic improvement on what came before and paved the way for the final triumph of the reflecting telescope over the refracting in astronomical research at the beginning of the twentieth century. The final section of the chapter discusses the protracted cover-up and controversy surrounding the failure of Herschel’s most famous telescope, his so-called 40-ft front-view – by far the most massive telescope constructed before the mid-nineteenth century – containing a 48-in. diameter mirror. The cover-up of this monumentally expensive failure ultimately ricocheted onto William Herschel’s son, John, who became embroiled in an acrimonious exchange of letters in the 1840s with Thomas Romney Robinson, director of the Armagh Observatory and ardent admirer of Lord Rosse’s grand achievements. Robinson emerged from the exchange victorious, but John, the dutiful son, although scathed, appeared the nobler figure. The tragic clash between these great men closes out the chapter.