Cambridge University Press 978-0-521-88257-6 - Astronomical Spectrographs and their History John Hearnshaw Excerpt More information 1 • The historical development of astronomical spectroscopes and spectrographs 1.1 COLOUR, REFRANGIBILITY colour, and to the same colour ever belongs the same AND WAVELENGTH degree of refrangibility.’ These experiments were the fundamental starting The concepts of colour, refrangibility and wavelength point for the science of spectroscopy. Newton himself have been crucial for the development of spectroscopes referred to ‘the celebrated Phaenomena of Colours’, and spectroscopy. Indeed the first prism spectroscopes indicating that he was far from the first to study were built partly for measuring the refractive indices the solar spectrum (the word was his term for the or refrangibility of different glasses, while diffraction ‘colour-image’). His apparatus was a primitive spec- gratings were used for early measurements of wave- troscope, and comprised a small hole in his window length. shutters, a glass prism, and a wall 22 feet distant from the prism used as a screen onto which the spectrum was projected. At first no lenses were employed, but 1.1.1 The refraction of light later he inserted a lens after the prism and noted The Dutch astronomer Willebrord Snell (1591–1626) that a spectrum of greater purity could thereby be is usually credited with the discovery of the law of produced. refraction, in the early seventeenth century in Leiden. In his Lectures opticae of 1669 (but published Rene´ Descartes (1596–1650) later included the law posthumously in 1728) Newton used his theory of in his treatise Dioptrics of 1637 (without however colours to account for the colours of the rainbow, which acknowledging Snell), and he used it to account for the contained the primary ‘Colours in order red, yellow, formation of a rainbow, but not explicitly for the colours green, blue and purple, together with all the interme- that the rainbow produces. diate ones, that may be seen in the Rain-bow; whence Isaac Newton’s (1642–1726) first paper, published will easily appear the Production of Colours in a Prism by the Royal Society in 1672, but based on his opti- and the Rain-bow’ [2]. cal experiments undertaken six years earlier, came to It is interesting that Newton observed the same important conclusions on the relationship between phenomenon of a continuous spectrum using the planet refrangibility and colour [1]. Newton studied the Venus, by allowing the planet’s light to enter the eye refraction of the Sun’s rays in a glass prism, and pro- directly from the prism. Even stars of first magnitude jected the resulting spectrum onto a wall of his room. were observed in the same way, and he remarked that a He concluded: ‘. light consists of rays differently telescope would both increase the quantity of light for refrangible, which, without any respect to a difference stellar spectroscopy as well as reduce the undesirable in their incidence, were, according to their degrees of effects of atmospheric scintillation. refrangibility, transmitted towards divers parts of the Newton’s experiments led him to believe that the wall.’ He found that the Sun’s light was composed of dispersive power of prisms was related to the refrac- rays of different primary colours, each with its different tive index of the glass. He concluded: ‘The denser the refrangibility, which gave rise to the dispersive power Matter of the Prism is, or the rarer the incompass- of the prism. He went on: ‘As the rays of light differ in ing Medium is, caeteris paribus, the greater will be the degrees of refrangibility, so they also differ in their dis- Difference of Refraction, and hence the Appearance of positiontoexhibitthisorthatparticularcolour...To the Colours will be more manifest’ [2, see Proposition the same degree of refrangibility ever belongs the same XXIV]. 1 © Cambridge University Press www.cambridge.org Cambridge University Press 978-0-521-88257-6 - Astronomical Spectrographs and their History John Hearnshaw Excerpt More information 2 History of astronomical spectrographs 1.1.2 Wavelength, colour and spectral lines refraction to a prism spectroscope) and wavelength (from a diffraction grating of known groove spacing) Thomas Young (1773–1829) in 1801 was the first per- could be explored. Such experiments laid the basis son to use a simple diffraction grating to demonstrate for the future of spectroscopy and of spectroscope the wave nature of light and to show that the wave- design, as well as the design of telescopes employing length could be obtained from the groove spacing of achromatic doublets for the objective lenses. the grating. His first gratings comprised a series of Fraunhofer’s early prism spectroscope comprised parallel grooves ruled on glass at the spacing of about ◦ ◦ a60 prism mounted in front of a small 25-mm aper- 500 grooves per inch. Using sunlight incident at 45 , ture theodolite telescope with cross wires seen in the he found four bright orders due to the interference of eyepiece. The prism in turn was 24 feet from a narrow the light. The sines of the angles of diffraction (sin β) opening in the window shutters. It was with this appa- increased in accordance with the integers 1:2:3:4, and ratus that he first recorded several hundred absorption from this progression he was able to estimate the wave- lines in the solar spectrum [7, 8, 9]. length of sunlight [3]. According to Young, the visible In 1823, for his more extensive observations of spectrum covered a range of wavelength from 675 down stellar spectra, Fraunhofer used a larger telescope of to 424 nm, with yellow light corresponding to 576 nm. aperture 10 cm, in front of which he mounted a prism Joseph Fraunhofer (1787–1826) continued experi- of apex angle 37◦ 40 to form an objective prism spec- ments with diffraction gratings in the early 1820s. His troscope [6]. With this instrument he was able to first gratings were coarse transmission gratings, made observe the absorption line spectra of six bright stars, by stretching fine parallel wires between the threads as well as of Mars, Venus and the Moon. of two screws – typically with spacings of up to 325 None of Fraunhofer’s spectroscopes employed a wires per inch.1 He was able to measure the angular collimator. For his solar observations the light traversed positions and hence wavelengths of prominent absorp- a narrow slit of width 0.07 inches; at a distance of 24 tion lines in the solar spectrum with such a grating, feet from the prism, the slit subtended no more than an including a value of 5888 A˚ for the orange line that he arc minute, giving a resolving power (R = λ/δλ) prob- labelled D [4]. ably of about 2000, easily sufficient to observe the lines In later experiments Fraunhofer produced grat- in the solar spectrum. ings by ruling with a diamond directly onto glass. Thus In England in 1802 William Wollaston (1766– in 1822 he produced one with 3340 grooves per inch 1828) had earlier observed five lines (a term he intro- and which gave a higher dispersion. He was able to duced) in the solar spectrum, without realizing their confirm Young’s finding that the diffraction orders had true significance [10]. His apparatus was similar to sin β ∝ n (here n is the order number of the diffraction, Newton’s, except that the crevice in his window shut- an integer), instead of simply β ∝ n that he had used ters was just 0.05 inches wide, and his flint glass prism previously [5]. Gratings as finely ruled as 7790 grooves was placed at 10 to 12 feet. Thus the benefit of a nar- per inch were produced the following year [6]. row slit to improve resolving power was established by One of Fraunhofer’s main interests was the deter- the pioneering spectroscopists in the early nineteenth mination of refractive indices for the different glasses century. that were produced by the Benediktbeuern glass works in Bavaria. As refractive index varied with wavelength, he had the foresight to use different emission lines 1.2 THE COLLIMATOR IN as wavelength standards at which refractive indices PRISM SPECTROSCOPES could be measured [4]. Thus the relationship between A collimator was introduced into laboratory spectro- refractive index (measured by applying Snell’s law of scopes by several spectroscopists from 1839, in order to achieve a higher resolving power, and probably as the result of independent developments. The first such use 1 Fraunhofer quoted values in Paris inches. A Paris inch is was most likely in a spectroscope by Jacques Babinet 26.7 mm. They have been converted here to the more familiar (1794–1872), which was presented to the Academie´ des imperial inch of 25.4 mm. Sciences in 1839 by Franc¸ois Arago (1786–1853) [11], © Cambridge University Press www.cambridge.org Cambridge University Press 978-0-521-88257-6 - Astronomical Spectrographs and their History John Hearnshaw Excerpt More information 1.3 Some notable astronomical prism spectroscopes 3 became a model for many laboratory spectroscopes of the mid nineteenth century. In particular, the use of a graduated circle for the telescope and a rotating table to carry the prism permitted great ease of use when measuring angles of refracted rays. In the following years collimators became increas- ingly common. William Swan (1818–94) included one in his astronomical spectroscope of 1856 [13]. By about 1860 the collimator was essentially a universal feature of all astronomical and laboratory spectroscopes. 1.3 SOME NOTABLE ASTRONOMICAL PRISM SPECTROSCOPES OF THE MID NINETEENTH CENTURY The basic arrangement of a prism spectroscope devised by William Simms in 1840 was widely copied. It com- prised a slit, a collimator, a circular prism platform Figure 1.1.