Interferometry and Monochromatic Imaging at the Marseille Observatory

Journal of Astronomical History and Heritage, 24(2), 247– 284 (2021).

INTERFEROMETRY AND MONOCHROMATIC IMAGING AT THE MARSEILLE OBSERVATORY

Yvon Georgelin Observatoire de Marseille/LAM 38 rue Frédéric Joliot-Curie, 13013 Marseille, France E-mail: [email protected]

and

James Lequeux LERMA, Observatoire de Paris-PSL-Sorbonne Université, 61 Avenue de l’Observatoire, 75014 Paris, France E-mail: [email protected]

ABSTRACT: We first give a brief history of the astronomical observatory in Marseilles, which was founded in 1702. Then, we describe the first attempt to measure at this Observatory the angular diameter of stars by interferometry, in 1873–1874. Because the size of the remarkable Foucault telescope that was used by Édouard Stéphan for this program was only 80 cm, none of the bright stars were resolved, and the upper limit to their diameters was given as 1/6 of an arcsecond. This result was however a very significant advance, as only fancy figures had been given previously for stellar diameters. The next incursion in interferometry of the Marseille Observatory took place in 1911–1914, when Charles Fabry and Henri Buisson measured with the same telescope the radial velocity and the temperature of the Orion Nebula, using the Pérot–Fabry interferometer developed at the Marseille University. After World War II, the Observatory underwent a complete renewal. Then Georges Courtès used interference filters to obtain deep photographs of HII regions, and Pérot–Fabry interferometers for measuring their radial velocities. We describe the very important instrumental advances realized for this program, in particular the focal reducers that allowed a considerable increase in sensitivity. The final result obtained by Courtès and his collaborators was a complete Hα survey of the Milky Way, which was the basis for a new description of the structure of our Galaxy, with four spiral arms, and a detailed Hα survey of the Magellanic Clouds. The distribution of HII regions in the closest galaxies was also observed and their velocity fields determined. In 1963, Courtès built the first integral field spectrograph, based on an array of micro-lenses; it had a great success, so that similar instruments are mounted at the focus of the largest present and future telescopes.

KEYWORDS: HII regions, Galactic structure, Magellanic clouds, M 33, M 31, Wide-field camera, Focal reducer, Fabry–Pérot interferometer, Monochromatic imaging, Micro-lenses array, Integral field spectrometer.

Like the companion paper “The Rise of Ultraviolet Astronomy in France” in the March 2021 issue of this journal (Lequeux, 2021), this new paper also is dedicated to the memory of Georges Courtès, who died on 30 October 2019, aged 94.

1 A BRIEF HISTORY OF THE However, the first observatory in Marseil- OBSERVATORIES IN les was only founded in 1702 by the Jesuits, MARSEILLES in their house of the Montée des Accoules,

Astronomy in Provence has a rather glorious through the action of Jean-Mathieu de Cha- early history, thanks primarily to Nicolas Fabri zelles (1657–1710), Professor of Hydrography de Peiresc (1580–1637; Gassendi, 1657). at the Arsenal of galleys, who had worked Peiresc observed the satellites of Jupiter from with Jean-Dominique Cassini in Paris and managed to obtain subsidies from King Louis his house in Aix-en-Provence immediately 1 after their discovery by Galileo, determined XIV. In 1749, this observatory was promot- their period of revolution with a remarkable ed as the ‘Royal Observatory of the Navy’ accuracy, and prepared ephemerides of their after the suppression of the galleys: in this position; before Galileo, he had the idea to way it acquired a national character, preserv- use their eclipses to determine longitudes ed until today. In 1781, it was united with the (Tolbert, 1999), although he realized that this local Academy of Sciences, which renovated method would not be practical at sea. He the building that contained the Observatory, discovered the Orion Nebula at the end of where it held its meetings (Figure 1). After 1610 (Bigourdan, 1916). Peiresc also organ- the suppression of this Academy in 1793, dur- ized a determination of longitudes of several ing the French Revolution, the Observatory Mediterranean harbors using the lunar eclipse was preserved and remained active until its transfer to another location in 1862–1863. The of 28 August 1635, and found the East–West extent of the Mediterranean Sea too large by building is presently an elementary school, but the historical part survives, including the

1000 km (Miller, 2000).

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Figure 1: Marseille Observatory, Montée des Accoules, at the end of the eighteenth century (© Musée de la Marine, Chambre de Commerce et d’Industrie de Marseille-Provence). astronomical tower but without the three The Accoules Observatory was initially domes. directed by Jesuits: Father Antoine-François

In parallel, there was from 1714 another de Laval (1664–1728), another pupil of de observatory in the city: that of Father Louis Chazelles, then after his death Father Esprit Feuillée (1660–1732), of the order of Min- Pézenas (1692–1776), until the suppression imes, a pupil of de Chazelles. Feuillée did not of Jesuits in France in 1763. Guillaume de observe very often, as he was mainly trav- Saint-Jacques de Silvabelle (1722–1801) elling in central and southern America, es- succeeded him, then Jacques-Joseph Thulis sentially as a botanist. However, he deter- (1748–1810) until his death, Jean-Jacques mined the longitudes of several towns using Blanpain (1777–1843) until 1822, Jean-Félix eclipses of the satellites of Jupiter, and some Adolphe Gambart (1800–1836) until his pre- of his observations are of high interest, in mature death and finally Benjamin Valz (1787 –1867), who retired in 1860. particular that of the very rare occultation of a bright star by Jupiter observed at Coquimbo The eighteenth-century Observatory was on 6 April 1710: this gives the most ancient rather well equipped through Royal subsidies, precise measurement of the position of the in particular with a reflecting Gregorian tele- planet. After the death of Feuillée, the Min- scope by Short (it is preserved together with imes were no longer interested in astronomy, other instruments and books of the Observa- Feuillée’s observatory was closed and the tory). However, the personnel were limited to salary attached to his position was transfer- the Director, an astronomer-adjunct (after red to the other observatory. 1777) and a concierge. The activities were

rather classical and of good quality according to Jean Bernoulli (1744–1807), who visited the Observatory in 1774, and Baron Franz- Xaver von Zach (1754–1832). A real break- through occurred when the concierge, Jean- Louis Pons (1761–1831, Figure 2), discovered a comet in 1801 with a telescope he had built himself (Figure 3). He had been trained in astronomy by Silvabelle, who was himself a specialist of comets and had worked on the return of Comet 1P/Halley. Pons, who discovered no fewer than 23 comets from Mar- seilles, was promoted to astronomer-adjunct in 1813. He was invited to Italy to become the first Director of the Marlia Observatory, near Lucca in 1819, where he discovered seven more comets. In 1825, he was appointed Director of the Florence Observatory and dis- Figure 2: Jean-Louis Pons (courtesy: covered seven further comets before his Marseille Observatory). death (Bianchi, 2020). Pons was the most

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Yvon Georgelin and James Lequeux Interferometry and Imaging at Marseille Observatory prolific discoverer of comets ever. two years and was enclosed in a quarter with

For his part, Jean-Félix Adolphe Gambart small narrow streets, whereas Le Verrier wish- discovered 13 comets from Marseilles, and ed for an open site, well away from important he demonstrated that the comet discovered in buildings. The site chosen for the new Mar- 1826 by Biela was in fact periodic and had seille Observatory was on the Longchamp already been observed in 1772 and 1805. Plateau, which was almost entirely surround- Arago says in his eulogy (Arago, 1855: 450): ed by public gardens. In fact, there was already a great deal of construction going on in A great natural facility, and habit, had led the area, but very little industry, and public the young correspondent of the Academy of sciences to make in a few hours lighting had yet to annoy astronomers. A complicated calculations which formerly decree of 1863 established Marseille Obser- would have required several days. (our vatory as “… a branch of the Paris Observa- translation). tory”. (Lequeux, 2013: 121).

The German astronomer Ernst Wilhelm Le Verrier sent Auguste Voigt (1828– Tempel (1821–1889), who between 1860 and 1909) to supervise the on-site operations as 1870 observed from Marseille Observatory, an adjunct astronomer, in replacement of discovered eight comets during his stay (plus Simon. Overloaded with work and pressure seven elsewhere, four asteroids and several from Le Verrier, Voigt resigned in 1866 and tens of nebulae, including those around the Pleiades). Marseilles was at the forefront of cometary and nebular astronomy! As for Benjamin Valz, his main interest was in asteroids, and his student, Jean Chacornac (1823–1873) discovered Phocea in 1853, and then four more asteroids before he trans- ferred to Paris Observatory in 1857.

In spite of this activity, the instrumentation of the Observatory was progressively becoming obsolete, because the Bureau des Longitudes, which was in charge of all French astronomy since the Revolution, preferred to send outmoded Parisian instruments to the provincial observatories rather than pay for new ones. Thus, these observatories were destined to decline; indeed, Marseille Ob- servatory almost closed down in 1860 when

Valz retired, although he was replaced by Charles Simon (1825–1880) for two years. Figure 3: One of the telescopes used by Pons in Marseilles to observe comets (courtesy: Marseille In fact, what saved Marseille Observatory Observatory). was the imaginary belief that observing conditions in Paris were deteriorating, where- was himself replaced by Édouard Stéphan as those in the south of France were much (1837–1923; Figure 4). Le Verrier kept the more favorable. In 1862 the famous optician title of Director until his death in 1877. Sté- Léon Foucault (1819–1868) and the mechan- phan was assisted by two adjunct astron- ic Friedrich Wilhelm Eichens (1818–1884) omers, Alphonse Borrelly (1842–1926) and had produced in Paris a magnificent reflecting Jérôme Eugène Coggia (1849–1919), both of telescope, 80 cm in diameter, which was to whom had discovered several comets and remain for some time the largest modern asteroids during the preceding directorships. reflecting telescope with a silvered glass The land and equipment of the new Obser- mirror.2 Urbain Le Verrier (1811–1877), the vatory belonged to the city of Marseilles, discoverer of Neptune, was then the Director which granted up 15,000 francs annually to- of Paris Observatory, and he decided to ward the costs. install the telescope in the south of France, The Observatory was inaugurated at the and after some hesitation, Marseilles was end of 1864; the architect was Henri-Jacques chosen in 1862 (for details, see Lequeux, Espérandieu (1829–1874), who also design- 2013: Chapter 5). ed the famous church of Notre-Dame de la The mayor of the city was enthusiastic, Garde that dominates Marseilles. The main since the old observatory at the Accoules had building of the Observatory had only two large been completely inoperative for the previous rooms on the ground floor: the Director’s of-

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years, during which instruments of high quality were installed in various buildings:

• The 80-cm reflecting telescope, put in

1864 in a beautiful enclosure designed by

Foucault himself and built by Hubert, but

not without problems (Figures 5 and 6);

• A comet-seeker of Eichens, with an 18.2 cm objective by Foucault (1866) (Figure 7);

• An equatorial telescope by Eichens, with an object-ive of 25.8 cm by Merz of Munich (1872). Its mechanical drive included a governor by Foucault, as did the 80-cm telescope. It was similarly housed in a cylindrical shelter, replaced by a classical dome in the 1960s (Figure 8). It is presently used by amateurs.

• A meridian circle by Eichens (1876; dismantled), with an 18.8-cm objective by Figure 4: Édouard Stéphan (courtesy: Marseille Observatory). Martin; it was placed in a special building (Figure 9). fice and another room for the rest of the staff. • Finally, clocks and various instruments to The first floor was the Director’s living quart- measure the Earth’s magnetic field in the ers (200 m2!), and the second floor contained framework of an international collabora- rooms for the observers. Work lasted 14 tion.

Figure 5 (Left): the 80-cm Foucault-Eichens, Newton-type reflecting telescope at Marseilles, in its shelter designed by Foucault. The observer stands on the Venitian-bridge staircase with access to the eyepiece. This bridge can be moved forward or backward according to the orientation of the telescope and placed to the east of it, or to the west as in this photograph. At the top of the telescope, we see an eyepiece with a wire micrometer. The total reflection prism and two lenses increasing the focal length to 15.95 m (Jonckheere, 1954) are hidden from view by this prism. Right: exterior view of the shelter. Because of rain leakage, it was in 1920 covered by zinc-plated steel plates and unfortunately destroyed in 1964 to make room for new buildings. The telescope itself is preserved as a museum piece (courtesy: Marseille Observatory).

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One may note that the comet-seeker, equatorial and the meridian circle corre- sponded to the specifications spelled out in 1847 by Wilhelm Struve in a letter in French to Le Verrier, in which he described an ideal observatory inspired by that of Pulkovo (the complete translated letter is in Lequeux, 2013: 64):

Figure 6: Side view of the mirror of the 80-cm telescope. The mirror has an unusual shape, being thicker in the center. This ensured less gravitational deformation when inclined. Foucault placed on the back a rubber cushion that could be inflated by the observer to correct for this deformation: an ancestor of active optics (courtesy: Marseille Observatory).

Figure 8: The Merz–Eichens 26-cm equatorial (courtesy: Marseille Observatory).

aperture, perhaps of 9 pouces like that of Dorpat [nowadays Tartu in Estonia; this is the famous Fraunhofer telescope]. This telescope must be equipped with a perfect filar micrometer, in which the threads can be illuminated by reflection in the

dark field of view, so as to render possible the reliable observation of comets.

A comet searcher of the highest quality.

Two pendulums of the highest quality, of which one should be positioned next to the meridian circle and the other destined for use next to the large tele-

scope, set up in a revolving turret.

A good box chronometer which will serve to correlate the two pendulums.

It is probable that Le Verrier, who no doubt would have very much liked to have such a

comet-seeker in Paris, sought to follow Figure 7: The Marseille Observatory comet seeker. This Struve’s recommendations to the letter. telescope has disappeared, but a similar one can be seen at Strasbourg Observatory. The equatorial mounting is designed so that the head of the observer was at the crossing of the two axes (courtesy: Marseille Obse- rvatory).

A good meridian circle equipped with a refractor of at least 4, and if possible 5 or 6 pouces [slightly larger than inches] aperture, in order that planetary observations can be carried out at the same time as stellar observations, and provide a com-

parison with the equatorial observations.

A large equatorial refractor of at least Figure 9: The meridian building (courtesy: Chambre de 6 pouces aperture, or better yet a larger Commerce et d’Industrie de Marseille-Provence).

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Figure 10: A Hubble Space Telescope photograph of Stephan’s Quintet of galaxies. The four redder galaxies form an interactive group, at a mean distance of about 106 Mpc. The bluer galaxy at top left, NGC 7320, is much closer, at 12.6 Mpc. HII regions have been detected and their radial velocities measured with the 3.6 m CFH telescope and the Russian 6 m telescope by Plana et al. (1999) (courtesy: Space Telescope Science Institute).

The new Marseille Observatory remain- the building and the equipment remained ed under the tutelage of Paris Observatory pretty much unchanged until the end of WWII. until 1878, when it became independent and rejoined the ensemble of observatories con- 2 ATTEMPTS TO MEASURE THE structed in France at the end of the nine- ANGULAR DIAMETER OF STARS BY INTERFEROMETRY teenth century in Algeria (then a French col- ony), Besançon, Bordeaux, Lyons and Nice. The 80-cm reflecting telescope in Marseilles An observatory had existed in Toulouse since has been actively used for a whole century. 1733, and it was rejuvenated. A large ob- With it, Stéphan discovered no fewer than servatory was also built in Strasbourg, then a 800 nebulae (mostly galaxies) between 1869 part of Germany, which became French in and 1885 (e.g., see Stéphan, 1884), including 1919, after World War II (WWII). All these the celebrated quintet of galaxies that bears Observatories are still active, although the his name (Figure 10). From these long years observations are generally made in more fav- of observation, Stéphan concluded that out of orable locations; the last one was created in the 420 galaxies he observed, 171 belonged Grenoble in 1985. At Marseille Observatory, to 65 groups. From 1906 to 1962, Robert

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Jonckheere (1888–1974) discovered 3,350 is tilted. visual binaries, many with this telescope, The screen that I use today is proving in this way that reflecting telescopes pierced by two crescent-shaped open- are just as good as refractors for these types ings limited by equal circles, 80 centi- of observations (e.g., see Jonckheere, 1954, meters in diameter; the major axes of which gives interesting details about the tele- these crescents are parallel and their scope). distance is 0.65 m [A drawing in the margin is reproduced here as Figure 11]. The 80-cm telescope was also used by One can hardly exceed this spacing: Stéphan in 1873–1874 in an attempt to mea- beyond, the images will weaken in an sure the angular diameter of stars by interfer- exaggerated manner and lose too much ometry (Lequeux 2020: Chapter 7). This of their sharpness. This drawback arises possibility was proposed as early as 1851 by from the fact that, in the telescope mir- the physicist Hippolyte Fizeau (1819–1896) rors, regardless of the quality of the work, in a manuscript preserved in the Archives of the periphery is somewhat less perfect the French Academy of Sciences and titled than the rest of the surface …

“On a way to derive the diameters of the stars For nine months I have observed from some interference phenomena” (our most of the visible stars, including those translation); but he only published his idea in of the 3rd magnitude and some of the 4th. 1868, as a very short note in a report to the All gave me fringes.

Academy, which mostly went unnoticed (Fiz- eau, 1868: 934, our translation):

There exists ... for most phenomena of interference, such as Young’s fringes, Fresnel’s mirrors … a remarkable and necessary relationship between the size of the fringes and that of the light source, so that fringes of extreme thinness can occur only when the light source has almost imperceptible angular dimensions; hence, to say it in passing, there may be some hope that based on this principle and forming, for example, using two very wide-spaced slits, interference fringes at the focus of the large instruments used to observe the stars, it will be possible to get some new information on the angular diameters of these stars.

We do not know how Stéphan learnt of Figure 11: Drawing by Stéphan of the this idea and came into contact with Fizeau. diaphragm put on the mirror of the 80-cm In any case, it was clearly Fizeau who sug- telescope (courtesy: Archives of the Académie gested that he measure the apparent dia- des Sciences, Paris). meters of stars in this way. In a letter to Thus, the apparent diameter of all Fizeau dated 1 February 1874, preserved at observed stars is considerably less than the French Academy of Sciences, Stéphan 1/6 of an arc second. describes his observations in detail. Here are If I am not mistaken, this is a well- some excerpts of this letter, which we have established concept, the first that has translated into English: been obtained on the matter. Such a re-

After various tests, I opted for a screen sult is not without importance. Moreover, with two crescents placed directly on the it undermines in no way the hope that we mirror. It is with this disposition that the had to determine the diameter of some flexures of the telescope have the least stars. The principle of the method re- influence. Now, this is capital; because, mains, the instrument is too small, that’s for a fringe, it is necessary that the two all.

beams received in the microscope eye- As is well known, Albert A. Michelson piece keep nearly the same intensity and (1852–1931) and Francis G. Pease (1881– it is quite difficult to adjust the relative 1938) made in 1920 the first measurement of positions of the mirror, of the screen, and the total reflection prism [which sends the the apparent diameter of a star, Betelgeuse, beam to the side in the Newtonian mount- using a scheme proposed by Fizeau 69 years ing of the telescope] so that one of the earlier (reproduced as Figure 7.2 in Lequeux, beams does not acquire a more or less 2020). Curiously, Michelson does not cite the great preponderance when the instrument pioneer observations of Stéphan nor does he

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by the Marseilles physicist Jules Macé de Lépinay (1851–1904), to measure accurately the thickness of transparent plates or the distance between two parallel surfaces (Georgelin and Tachoire, 2002: 105–109; Le- queux, 2020: Chapter 8).

Alfred Pérot (1863–1925; Figure 12) and Charles Fabry (1867–1945; Figure 13) were both Professors of Physics at Marseille Uni- versity: Pérot from 1888 to 1908, the date on which he was nominated Professor at the École Polytechnique in Paris and at the same time physicist at the Observatoire de Meudon, where he discovered the gravitational redshift of solar lines (Pérot, 1920; 1921); Fabry from 1893 to 1921, when he became a Professor at Paris University, and in 1926 at the École Figure 12: Alfred Pérot, then a Pro- fessor in Marseilles (photograph: Polytechnique after the death of Pérot. He Jacques Pérot).4 retained this position until his retirement in 1936. In 1919, Fabry had created the Institut make any reference to Fizeau, whom he d’Optique Théorique et Appliquée in Paris knew well however. which he directed until his death, a very suc- cesssful college where most of the bright One cannot underestimate the historical French physicists specializing in optics were importance of Stéphan’s (1874) observat- and are still educated (for details of the life ions. At that time, no one had any idea of the and work of the two men, see Georgelin and real diameters of stars, for which very diver- Tachoire, 2002: 114–131). gent, mostly nonsense values had been proposed. Stéphan was the first Marseilles ast- Pérot and Fabry collaborated from 1894 ronomer to enter the domain of interferomet- in the laboratory of Macé de Lépinay. Pérot ry. Many more were to come, but in a differ- was more of an experimentalist and Fabry a ent way. theorist, so that they were wholly complementary. In 1892, Fabry had the idea that the 3 THE PÉROT–FABRY interference fringes between monochromatic INTERFEROMETER light reflected by two parallel surfaces would be sharper if these surfaces were made semi- Throughout the twentieth century astronomy transparent by a very thin silver deposit. This at Marseilles was characterized by the use of was the basis of the Pérot–Fabry interfero- a novel instrument: the Pérot–Fabry inter- meter (Figure 14). Then, they built several ferometer.3 This instrument derives from such interferometers that immediately had interferometer set-ups used by Fizeau, then many applications in metrology: they made it

possible to measure very accurately the sep-

aration between two reflecting surfaces if the

wavelength was known, or inversely to mea-

sure with high accuracy the wavelength of the

illuminated light if this spacing was known.

Pérot and Fabry measured with their interferometer the wavelengths of many lines relative to the standard meter, with a precision considerably better than obtained previously by Henry Rowland (1848–1901), the reference at the time. Since Rowland had pu- lished his wavelengths in the first issues of The Astrophysical Journal in 1895, Pérot and Fabry published their own methods and results in this same journal (Fabry and Pérot, 1901; 1902; 1904; Pérot and Fabry, 1902; 1904). Figure 13: Charles Fabry, around 1925 (photograph: André Maréchal; courtesy: From 1899 to 1921, Pérot, Fabry, and Marseille Observatory). their collaborators used an interferometer, fol-

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Yvon Georgelin and James Lequeux Interferometry and Imaging at Marseille Observatory lowed by a conventional slit spectrometer with a Rowland concave grating, to measure the wavelengths of the solar absorption lines (Figure 15). The spectral resolution was that of the interferometer and could be extremely good given the high solar flux.

In 1905 another personage appeared, namely Henri Buisson (1873–1944; Figure 16). Professor of Physics at Marseille Uni- versity, he succeeded Macé de Lépinay and started a long collaboration with Fabry, first in metrology, then in geophysics. They showed that the absorption of solar and stellar rad- iation at wavelengths shorter than 290 nm was due to ozone, and measured by spectroscopy its abundance in the upper atmosphere (Fabry and Buisson, 1913). They observed the line shifts due to the rotation of the Sun, and, in 1921, confirmed for many lines the gravitational redshift predicted by Einstein and found previously by Pérot (Buisson and Fabry, 1921). This result was presented by

Einstein himself the next year at the Collège de France. In 1927, Horace Babcock confirm- Figure 14: Principle of the Pérot–Fabry interferometer. ed this result in the near-infrared, using a sim- The light from an extended monochromatic light source ilar set-up (Babcock, 1927). is collimated by a lens, then crosses two parallel surfaces separated by air or vacuum; a second lens produces the In January 1911, Fabry and Buisson in- image of the extended source. Interference rings are – produced on the screen, with a low contrast if there is a stalled a Pérot Fabry interferometer at the single reflection on each surface (bottom). But if the focus of the Eichens–Merz Equatorial at Mar- surfaces are made semi-transparent by a thin deposit of seille Observatory to observe the Orion Neb- silver, or better by dielectric multilayers, the bright rings ula and saw interference rings superposed on become sharper due to the addition of the multiple reflections in phase with each other (middle). The figures the image of the Nebula in the 500.7 nm line. at the top show the interference rings formed on the With a plate sensitive to the blue and ultra- screen, whose contrast (‘finesse’ from the French) is violet, they photographed rings produced by higher if the reflection by the surfaces is increased (right); Hγ at 434.1 nm and by another line at 372.7 then the overall light transmission is reduced. The nm (Fabry and Buisson 1911). We know now interferometer is now close to a resonant cavity. If the extended light source A is not uniform, its image A’ is that the 500.7 and 372.7 nm lines are re- modulated by the interference rings (diagram: James spectively forbidden lines of [O III] and [OII] Lequeux). but they were then attributed to unknown elements. Fabry and Buisson noted that the (1/12) were much too small to obtain a good size of the telescope and its aperture ratio sensitivity, and they soon turned to the 80-cm

Figure 15: A portion of the channeled spectrum of the Sun (positive image), obtained in 1909. Along the abscissae, the great dispersion of the concave grating separates the numerous absorption lines; along the ordinates, the symmetrical Pérot– Fabry rings are seen. Cylindrical lenses deform the image for easier reading. The colored arrows indicate for two lines the positions of the three first rings (green, blue and red, respectively). The widths of these rings indicate the widths of the corresponding lines (see e.g. the difference between the two lines near the arrows on the left), and their positions allow to obtain the lines’ wavelengths with high accuracy (after Fabry, 1938: 192).

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Figure 17 shows the principle of the instrument installed at the prime focus of the telescope, and Figure 18 reproduces a con- temporary photograph.

Figure 19 shows the results of the observation of 12 March 1914. The Director of the Observatory, Henri Bourget (1864–1921), who had replaced Stéphan after his retirement in 1907, had joined Buisson and Fabry

for observing. Rings from the Orion Nebula

were observed simultaneously with those giv-

en by the Hγ lamp, allowing to obtain the radial velocity of the nebula, 15.8 km/s on average at the time of the observation (local vari- ations were noted). The widths of the rings gave an upper limit to the temperature of the emitting gas, 15,000 degrees. Although a part Figure 16: Henri Buisson (courtesy: of the width could be due to turbulence, the Observatoire de Marseille). temperature was certainly of the order of

10,000 degrees, a surprise because scien- Foucault–Eichens telescope. The best tists like Svante Arrhenius (1859–1927), Jo- results were obtained in 1914 and will be seph Norman Lockyer (1836–1920), or Henri described below. Poincaré (1854–1912) were convinced that nebulae were very cold. The width of the rings given by the ‘nebulium’ doublet at 372.6/372.9 nm (Figure 20) was used as an attempt to obtain the atomic weight of the unknown element, about 3 times that of hydrogen. In the same way, the line at 500.6 nm was attributed to another element with an atomic weight of about 2. The authors of the paper published in The Astrophysical Journal (Buisson et al., 1914: 257) wrote:

It is curious to note that the classification of the elements recently given by Ryd- berg leads to the admission, between Figure 17: The Pérot–Fabry interferometer at the prime hydrogen and helium, of two unknown focus of the 80-cm telescope. The focus is in F. In A, a elements having respectively the atomic pair of achromatic lenses of uviol glass give a parallel weights 2 and 3. beam, which crosses the interferometer B. C is an achromatic lens forming the image of the nebula Needless to say, these conclusions vanished modulated by the interference rings on the photographic when Ira S. Bowen (1898–1973) showed in plate P. Filters were inserted into the optical path to 1928 that these lines were forbidden lines of observe only one line at a time (after Buisson et al., 1914: 242). oxygen once and twice ionized, respectively.

4 STAGNATION AND RENEWAL

French and German astronomy were sev- erely struck by WW1 and went into stagnation until the 1930s. In France, the limited means of the observatories were mostly devoted to the endless enterprise of the Carte du Ciel and to routine observations: no really important work was done during this period, except for the results obtained by Bernard Lyot (1897–1952) on the surfaces of the Moon, Mercury and Mars with his polarimeter (1923–1929), then on the Sun with his cor- Figure 18: The interferometer at the prime focus of the 80-cm telescope. On the right, a hydrogen lamp gave a onagraph and monochromatic filter (1930– wavelength reference for the Hγ line (courtesy: Album du 1933). The authorities, and in particular the Laboratoire de Physique de l’Université de Marseille). Minister of Scientific Research, the physicist

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Figure 19: Interference rings in the Hγ line observed for the Orion Nebula (bottom) and from a lamp (top) at the prime focus of the 80-cm telescope (negative photograph). A ring (anneau) with the same interference order (4640) is indicated in both parts of the photograph; the difference in radius is due to the Doppler–Fizeau effect, 42 km/s, which combines the known velocity of the Earth in the direction of the Nebula with the radial velocity of the Nebula, 15.8 km/s; the distance between two successive rings corresponds to 64.6 km/s (courtesy: Marseille Observatory).

Figure 20: Interference rings in Hγ (left) and in the [OII] line at 372.8 nm (right) observed in the Orion Nebula. Buisson, Fabry, and Bourget (1914) note: “The rings show local deformations in certain regions, indicating irregularities of speed which may amount to about 10 km per second. Movements of this sort are manifested in the region to the southeast of the Trapezium [bottom right] in the direction of the star Bond 685.” (after Buisson et al., 1914: Plate VII).

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Figure 21: At the Haute-Provence Observatory, on 8 May 1945, the date of the armistice ending WW2. From left to right, Charles Fehrenbach, Bernard Lyot, André Danjon, Daniel Chalonge and Daniel Barbier (Georgelin Collection).

Jean Perrin (1870–1942, Nobel Prize in 1926), tenure of Henri Buisson, was of the conserva- were aware of the problem, which was not tive species. Astrophysics, which had started limited to astronomy. In 1935 they created with the interferometric study of the Orion the Caisse Nationale de la Recherche Scie- Nebula, was forgotten, and the activities of ntifique that became later the Centre National the Observatory were completely ‘classical’, de la Recherche Scientifique (CNRS), inde- although of good quality thanks especially to pendent from the Universities. The following Robert Jonckheere and David Beloritzky year, the CNRS founded in Paris the Service (1901–1982). d’Astrophysique, which was later called the Things were to change after WW2, due to Institut d’Astrophysique de Paris (IAp), and in the strong personality of Danjon, who com- the South of France the Observatoire de pletely reorganized French astronomy. He Haute-Provence (OHP), both against the will wanted discipline, order and the sense of col- of the Director of Paris Observatory. A num- lective work to replace individualism and leth- ber of iconoclast astronomers appeared dur- argy that, according to him, was the norm ing this period: Jean Dufay (1896–1967), before 1940. In 1946, he wrote in his project Director of the Lyons Observatory from 1933 of reform of the observatories (our translat- to 1966 and also of OHP from 1939 to 1965; ion): Daniel Barbier (1907–1965), Daniel Cha- longe (1895 –1977), the optician André Cou- My focus is that the young French ast- der (1897–1979) and André Danjon (1890– ronomers who are following me, when reaching mature age, do not have the 1967) (Figure 21). In Paris, Henri Mineur feeling of having lived isolated in our Ob- (1899–1964) became the first Director of the servatories, of having wasted their good IAp. Elsewhere in Europe, some contemp- years on inefficient tasks, and of having orary great names were those of Arthur Ed- spent their time in fruitless struggles dington (1882–1944), Bertil Lindblad (1895– against indifference.

1965), Jan Oort (1900–1992), Bengt Ström- Who were these young astronomers? In gren (1908–1987), etc. Marseilles, Charles Fehrenbach (1914–2008) In Marseilles, Jean Bosler (1878–1973), and Georges Courtès (1925–2019). Fehren- who became Director in 1923 after the death bach came from Strasbourg to Marseilles in of Henri Bourget in 1921 and a short interim 1937 as a high-school Professor of Physics

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Yvon Georgelin and James Lequeux Interferometry and Imaging at Marseille Observatory and joined the Observatory in 1942; he was In 1949, Strömgren stayed at OHP for hired in 1943 by Dufay, who did not come some time. He had published ten years ear- very often to OHP, as Adjunct-Director of this lier his fundamental article of the ionization of Observatory. He became Director of OHP in HII regions (Strömgren, 1939) and maintain- 1966, upon the retirement of Dufay, a post ed an active interest in interstellar matter (see that he kept until 1983. In parallel, he e.g. Strömgren, 1948, another very important succeeded Bosler in 1948 as the Director of pa-per). Strömgren worked with Fehrenbach Marseille Observatory. The personnel of this and Courtès on relatively narrow pass-band Observatory were then limited to three (12 nm) interference filters for observing the astronomers, one researcher from the CNRS, lines emitted by HII regions. This resulted in a mechanic, a secretary and a concierge. beautiful Hα photographs of HII regions in Fehrenbach was replaced in 1971 by Guy Cygnus (including the America Nebula) and Monnet, then by the first author of this paper in Perseus, obtained with 75-mm diameter, (YG) in 1976, who was succeeded by the sec- F/1.4 objectives (Fehrenbach, 1951). Many ond author (JL) from 1983 until 1988. new HII regions were discovered in a first survey of the northern Milky Way (Courtès, Fehrenbach played a major role in the 1951a, 1951b; note that the Galactic longi- renovation of Marseille Observatory, where tudes given in these papers are in the old the scientific, technical and administrative system, the Galactic Center being at 327° personnel increased considerably during the instead of 0° in the present system). This 1950s and 1960s. New buildings were con- drew the attention of Otto Struve (1897– structed to house offices, as well as optical 1963) and Bart J. Bok (1906–1983). Struve and mechanical workshops. Fehrenbach (1951a) published an account in Sky & Tele- was also deeply involved in the creation of the scope. Strömgren had a decisive influence European Southern Observatory. on Courtès, who thanked him profusely (and At Marseille Observatory, three activities also Fehrenbach, as needed) in his thesis developed in parallel: (Courtès, 1960: 117). He wrote:

(1) Astronomical optics, with specialties in I am pleased to express to them here my spectrographs (André Baranne, 1932–2021) deep gratitude for the beautiful subject of and aspherical surfaces (Gérard Lemaître); research that they have entrusted to me. (2) The study of stellar populations with (our English translation). objective prisms, especially in the Magellanic These results encouraged Courtès to try Clouds—the favorite topic of Fehrenbach; to reach an apparently impossible goal: to and build an instrument with a 120° field, a very (3) The study of interstellar matter and small F/D ratio, and able to accommodate Galactic structure. a narrow-band interference filter. No photo- We will now concentrate on this latter do- graphic objective could meet these charac- main because it involved a resurrection of teristics, even when putting in front a diverg- Pérot–Fabry interferometry thanks to Georges ing lens as in the hypercinor combination of Courtès. Berthiot or the rétrofocus of Angénieux. The solution involved placing a concave mirror at Courtès studied astronomy at the Uni- a distance from the objective. The first instru- versity of Montpellier under Pierre Humbert ment with a 120° field of view was built on this (1891–1953), a charismatic personality famil- principle in 1949, then a final one with a 150° iar with the history of astronomy, a topic in field, open at F/1.8, in 1951 (Figure 22). The which Courtès developed a life-long interest. same principle had been adopted by Louis G. In 1946, he became an Assistant in Physics Henyey (1910–1970) and Jesse L. Green- at the University, and was recruited by the stein (1909–2002) during WW2 for military CNRS the following year for OHP. He stayed applications, but never published, so that there until the end of 1949, learning the Courtès learned about this only in 1952 techniques of optical astronomy. He built a through Struve (1951b). The 150°-field cam- nebular spectrograph that he used to observe era was little used and was essentially con- the night sky with Jacques Blamont (1926– sidered a test for wide-field space cameras 2020), discovering the forbidden nitrogen line for UV observations. Two versions of these at 519.9 nm and OH bands close to the Hα cameras were constructed and are described line of hydrogen. This spectrograph was then in the companion paper (Lequeux, 1921: Fig- installed on the 1.2 m diameter telescope for ures 8 and 21). observations of comets and novae (Fehren- bach and Courtès, 1949). Another invention of Courtès was the multiple-passband filter (BPM for Bandes Pas-

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santes Multiples), which consisted of a concave grating on which an image of the observed field was projected. It produced images of this field in different colors that could be observed by cameras placed at will on the dispersed final focal surface (Courtès, 1962). Such a device was placed at the focus of the OHP 80-cm diameter telescope (Courtès and Viton 1965) but was not used very much. Conversely, a version placed on the tip of a rocket was built to image the Sun at different UV wavelengths (Bonnet and Courtès, 1962) and launched successfully in 1964: for details and a drawing of the instrument see Lequeux, 2021: Figure 6.

In 1971, a wide-field photographic survey of the whole Milky Way began at OHP and was extended to the southern part in 1972, at the European Southern Observatory. It used a 60° field, F/1 camera with a filter of 1.0 nm bandwidth, on an equatorial mount. Its principle (Figure 23) differs principally from that of Figure 22 by the replacement of the concave mirror by a diverging lens, returning to the wide-field photographic objectives of Berthiot and Angénieux (Courtès et al., 1981). This avoided obstruction of the field. The survey revealed many HII regions, sometimes very faint and extended, most of which are not detectable in unfiltered light (Figure 24).

There was also a need to use large telescopes to make more detailed studies of HII regions, although with a smaller field. The problem was that the long F/D ratio of those telescopes did not allow astronomers to reach faint, extended regions, so Courtès conceived a series of focal reducers. Figure 25 shows one of them, built for the Newtonian focus of the OHP 1.93 m telescope. Figure 22: Principle of Courtès’ 150°-field camera. A spherical concave mirror with the center in c reflects the Focal reducers have been used exten- light from the sky to an objective O, which produces the sively by the Marseilles and Lyons astrono- image on the photographic plate P. A monochromatic mers for monochromatic imaging of HII reg- filter is inserted in F. The black ray tracing is for the edge ions in our Galaxy, especially to find faint of the field, with an incidence angle ω of 75°, Σ is the focal surface of the spherical mirror, whose curvature is ones: 20 fields, 21° in diameter, have been corrected by the Petzval objective O (after Courtès, observed in the Northern Hemisphere with a 1960: 124). small refractor, 5 cm in diameter, and a focal

Figure 23: Principle of the 60°-field camera used for the Hα survey of the whole Milky Way. The optical arrangement was designed by André Baranne. The total length of the instrument is 80 cm. The filter is placed in FI, and a Pérot–Fabry interferometer can be inserted at PF: this complicated the optical design but the interferometer was never used in this set- up. The F/D ratio is 0.7, giving a high sensitivity to extended objects while only stars brighter than V = 6 mag are detected after exposures of several hours (after Courtès et al., 1981: 338).

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Figure 24 (Top): Hα distribution over the entire Milky Way. (Bottom): for comparison, the Milky Way in visible light. From left to right, the region of the Galactic Center, the Vela-Puppis region (above the Large Magellanic Cloud slightly to the right), the spectacular Orion region, and the Cygnus region. The data are from Sivan (1974), and the visible photograph from ESO (courtesy: ESO/S. Brunier).

Figure 25: A focal reducer (F/1, field 1° 12’) correcting the coma of the Newtonian focus of the OHP 1.93 m telescope, calculated by André Bayle and Jean Espiar. An interference filter or a Pérot–Fabry interferometer could be inserted into the optical path. Dimensions in millimeters (after Courtès, 1960: 133). reducer at F/1.5, mounted on an equatorial 5 THE COME-BACK OF THE table (Dubout-Crillon, 1976). Focal reducers PÉROT–FABRY INTERFEROMETER have also been used for monochromatic While searching for the most sensitive way to imaging of nearby galaxies like M 31, M33, M detect the line emission of extended, faint HII 51, M 82, NGC 253, NGC 2997, and NGC regions, Courtès (1960: 133–153) realized 4258. As an example, Figure 26 shows an that the Pérot–Fabry interferometer was ideal unpublished Hα image of M33; for details of for that purpose. Moreover, it allowed mea- observations and a photograph of the instru- surement of the radial velocity of the source. ment, see Boulesteix et al. (1974). Although little used in astronomy since the Courtès also conceived, independently of 1914 observations of the Orion Nebula dis- Olin Wilson (1909–1994) and Guido Münch cussed earlier, the interferometer had not (1921–2020) at Mount Wilson and Palomar, been completely forgotten. Walter Baade a multi-slit nebular spectrograph (Courtès, (1893–1960), Fritz Goos (1883–1968), P.P. 1964: 330), to measure the Hα/[NII] line in- Koch, and Rudolf Minkowski (1895–1976) tensity ratio, an indicator of the degree of had used it in Hamburg to study the intensity excitation in HII regions (see Figures 27 and distribution in the spectral lines of the Orion 28). Nebula, with a rather strange set-up (Baade et al., 1933); this was before Baade emigrated to the USA in 1931 and was

rejoined by Minkowski in 1935. Thanks to the

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Figure 26: The nearby galaxy M 33 photographed in Hα with the focal reducer of Figure 25, at the 1.93 m telescope of OHP (Archives Courtès, Laboratoire d’Astrophysique de Marseille).

Figure 27: The multi-slit spectrograph for the Newtonian focus of the OHP 1.93-m telescope (after Courtès et al., 1969b: 223).

very high wavelength resolution they reach- ed, which allowed them to measure the width of the line, they could fix the temperature of the nebula from 7500 K around the Trapez- ium to 5000 K in the periphery.

A Pérot–Fabry interferometer was also used in 1942 by Dufay, the physicist Jean Cabannes (1885–1959), and the astronomer

Junior Gauzit (1902–1977) to study the emis-

sion lines of the night sky (Dufay et al., 1942).

An interferometer has also allowed to study

the physical parameters and the winds in the

high atmosphere by observing a cloud of lith-

ium delivered by a rocket (Lequeux, 2021: Figure 3). This was known by Courtès, who

borrowed a 30-mm diameter interferometer from Dufay and detected in October 1950 Figure 28: Multi-slit spectrograph of the whole Orion Nebula. The size of the covered field is 45′. Hα is the several faint, extended nebulae in Cygnus. strongest line, with the two unequal [NII] lines on either This interferometer (often designated as side (after Baudel, 1970: 66). étalon, both in French and in English) yielded

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Yvon Georgelin and James Lequeux Interferometry and Imaging at Marseille Observatory better performances than that of Fabry and his collaborators because the semi-reflecting layers of thin silver had been replaced by dielectric multi-layers. But it was too small, and

Courtès struggled to obtain a 60-mm etalon that would keep its parallelism in every position of the telescope. The plates were polished to 1/40 of Hα wavelength by André Cou- der, the covering multilayers of zinc sulfide and cryolite were deposited by the physicist René Dupeyrat in the Laboratory of Physical Research of the Sorbonne University in Par- is, and the invar separators produced by Figure 29: A spectrum of turbulence in the λ Orionis Courtès himself. nebula. The Kolmogorov law of turbulence is verified for relatively short distances (after Courtès, 1953: 379). With this, Courtès could map the velocity field over the Orion Nebula and over the ex- Münch mentioned earlier, he noted: tended, less disturbed faint HII region around Another method is in some respects even Orionis. His purpose was to see if the Kol- λ more powerful … This technique uses a mogorov law of turbulence, as formulated for Fabry–Pérot etalon or interferometer … velocities by von Weiszäcker, applied to these This idea is not new … but in recent years nebulae: “In a homogeneous medium the rel- improvements in the construction of the ative velocity v between two points at a dist- interferometer plates have created an ance l is on the average proportional to l1/3.” enormous gain in the quality of the obser- Figure 29 shows the result, with a remark- vations. The new work that we shall able agreement for relatively short distances describe this month is that of a young (Courtès, 1953). A similar result was obtain- French astronomer, Georges Courtès, a member of the staff of the Marseille Ob- ed for NGC 434 (Courtès, 1960: 207). servatory. His observations were made Courtès, who was clearly at the forefront during the past four years with the 120- of astrophysics of his time, was now well cm reflector of the Haute Provence Ob- known by the American astrophysical comm- servatory near St. Michel. (Struve, 1955: 93). unity. Otto Struve, a regular contributor to Sky & Telescope, first wrote a short paper Then Struve presented the principle and a titled “Glowing hydrogen in the Milky Way” photograph of the equipment, and showed (Struve, 1951a) reporting on Courtès’ Hα some results obviously communicated to imaging, then a more substantial paper to him by Courtès. Those reproduced in Figure describe the novel interferometric method, 30 and not published elsewhere are parti- titled “Motions in gaseous nebulae” (Struve, cularly interesting, as well as Struve’s cap- 1955). After recalling the multi-slit obser- tion, which we also reproduce. Struve also vations of the Orion Nebula by Wilson and mentioned Courtès’ results on the velocity

Figure 30: The original caption by Struve was “Compare the direct Hα photograph of the Horsehead nebula (left) with the Courtès interferometer picture at the right. The bright hydrogen circles are smaller than those in the superimposed strip from a laboratory source; from this difference the radial velocity of the nebula may be measured.” [The fainter line is due to NII] (after Struve, 1955: 94).

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Yvon Georgelin and James Lequeux Interferometry and Imaging at Marseille Observatory

Figure 31: Courtès with students and colleagues in 1965. From right to left: Marie-Hélène Demoulin, Georges Courtès, Marianne Bretz, Gustavo Carranza, Jean Moutonnet, André Baranne, Paul Bastie, André Vuillemin, Yvon Georgelin, Guy Monnet, Jean-Michel Deharveng, and Daniel Lacroix (Georgelin Collection). shifts near the elephant trunks, which are now section of this paper. An important review interpreted as due to the ionization and evap- paper (Courtès, 1964) describes the instru- oration of the trunk neutral gas into the sur- ments used in these observations, gives some rounding space. He concluded, while citing results, and presents many new ideas, in the results of Courtès on turbulence: “The particular the possibility of placing behind a interferometer method in its modern form is Pérot–Fabry etalon an array of small lenses, evidently going to be a powerful and versatile each giving an interferogram. This will be means of investigating nebulae.” (Struve, detailed below, in Section 8.

1955: 94). Courtès (1960) defended his PhD thesis

Probably following this beautiful homage, in 1958 and was promoted to “Maître de Courtès was invited by Nicholas U. Mayall recherches” of CNRS the following year. He (1906–1993), Director of Lick Observatory, was now able to establish a research group for a 6-month stay in 1956 at the University of at Marseille Observatory, which grew rapidly California in Berkeley and at Lick, with a in this very favorable period for scientific research in France (e.g. see Figure 31).6 Fulbright Fellowship. He installed his focal reducer with a Pérot–Fabry etalon on the 90- In 1968, Courtès was awarded two ob- cm Crossley telescope, which was equipped serving sessions with the Palomar 5-m tele- by Mayall with an excellent offset pointing scope, then the largest in the world, and came system, and obtained interferograms of 16 with several students. Courtès had an excel- Galactic HII regions. After his return, he lent relation with Guido Münch, who was a observed many other HII regions with the member of the Palomar staff, and this had interferometer, covering most of the Milky certainly helped to obtain this privilege. One Way: 16 with the OHP 80-cm telescope, 127 of the authors (JL) was then at CalTech, and with the 120-cm telescope, and 70 with two he and his wife took the opportunity to visit refractors, 15 and 10 cm in diameter, in the the 5-m telescope with Courtès and his two Southern hemisphere.5 This material, sup- students (Yvon Georgelin and Guy Monnet). plemented by the identification of the ionizing We remember that the safety rules were not stars of the HII regions also done by Courtès, really respected, as one could (and often did) was to be at the basis of the mapping of the touch bare conductors with 110 volts AC Galaxy that will be described in the next when climbing to the prime focus cabin! Fig-

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Figure 32: Courtès and Georgelin at the prime focus of the 5-m Palomar telescope, ready for an observation night (photograph by Guy Monnet, Georgelin Collection). ure 32 shows Courtès and Yvon Georgelin in this cabin, where they have brought a spec- ially built focal reducer and an interferometer (Courtès: 1960: Figure 4). Another former student of Courtès was Marie-Hélène De- moulin, who at the time was working with Margaret Burbidge at the University of Calif- ornia in San Diego. Both observed with the 3-m diameter telescope at Lick, because Pal- omar was then forbidden to women!

Amongst the results of the Palomar observations, we can cite the measurement of the expansion velocity of NGC 6888, a nebula expelled by a massive Wolf-Rayet star, which was often considered wrongly a supernova remnant (Georgelin and Monnet, 1970b). Figure 33 shows an interferogram of a part of the nebula, where the line splitting due to the expansion is clearly seen.

Several galaxies also were observed at Figure 33: Pérot–Fabry Hα interferogram, obtained with Palomar to map their velocity fields: NGC the Palomar 5-m telescope, of a part of NGC 6888, 253, M 33, NGC 6946 (Monnet, 1971), and M showing the line splitting due to the expansion of the nebula at about 50 km/s. The inter-fringe distance 31, (which also was extensively observed at corresponds to 280 km/s (after Georgelin and Monnet, the OHP; Deharveng and Pellet, 1969; 1975). 1970b: 242).

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The existence of diffuse Hα emission due to 1970) and NGC 4631 (Crillon and Monnet, ionization of the general interstellar medium 1969b). Many details can be found in was discovered thanks to the extreme sensi- Courtès (1973; 1977). Several Galactic HII tivity of the interferometer observations (Fig- regions have also been studied in detail with ure 34). This diffuse ionized gas with its Hα the same set-up, for example, the Orion emission (Sivan, 1974) also exists in our Gal- Nebula by Lise Baudel-Deharveng with high axy where it dominates the low-density inter- spectral and spatial resolutions (Deharveng, stellar medium, but its origin is still poorly 1973). known: leaks of ionized gas out of HII regions A focal reducer, with a Pérot–Fabry etalon through the champagne effect, or ionization and an RCA two-stage image tube, at the foci by isolated hot stars, or to a small extent of the ESO 3.6-m and 1.5-m telescopes ionization by X-rays (see the discussion in has been used to study M 83 (= NGC 5236, Lequeux, 2005: 110). Comte, 1981), NGC 300 (Marcelin et al.,

1985a), NGC 1566 (Comte and Duquennoy,

1982), NGC 1313 (Marcelin and Gondoin,

1983) and NGC 2997 (Marcelin et al., 1980).

At ESO, the velocity field and the rotation curve of the Large Magellanic Cloud have been obtained in a first step with the 10-cm diameter refractor described later in Section 7 (Georgelin and Monnet, 1970a). In a second step, a pioneering work on extragalactic bubbles and super bubbles with direct imaging in the lines of [SII] and Hα as well as Pérot–Fabry kinematics has been completed by Margarita Rosado, Annie Laval, Yvonne Jonckheere-Georgelin, and Guy Monnet. It revealed a type of rapid-expansion bubble now thought to be formed by the combined action of supernova explosions and stellar winds (Georgelin et al. 1983; Rosado, 1986; Rosado et al. 1981; 1982a; 1982b). In a third step, this work on extragalactic super-nova remnants and bubbles was continued with the scanning Pérot–Fabry interferometer described in Section 6 by Margarita Rosado, Annie Laval, and their collaborators (Laval et al. 1989; 1992, Rosado et al. 1990; 1993).

An F/1 focal reducer (without interferometer) was installed at the focus of the 6-m Figure 34 (Top): The diffuse Hα emission in the region of telescope of the Special Astrophysical Obser- the Southern arm of the galaxy M 33, observed with the vatory at Zelentchuk, under an agreement be- Palomar 5-m telescope. Top, a monochromatic Hα tween the USSR Academy of Sciences and photograph. (Bottom): an interferogram of a part, at the same scale. The diffuse emission is not visible in the the French Ministère des Relations Exté- photograph but is detected as faint rings in the inter- rieures. The active cooperation between the ferogram (after Monnet, 1971: 380). USSR and France in space affairs, in which Courtès was involved, was instrumental in Several other galaxies have been ob- reaching this agreement. A new, complete served at the 1.93-m OHP telescope with Hα survey of the HII regions in M 33 was ob- Pérot–Fabry interferometers, often combined tained (Courtès et al., 1987; see Figure 35). with observations with a nebular spectro- The ionized gas of the bulge of M 31 (Bou- graph and other instruments, and their velo- lesteix et al., 1987) and in M 81 (Petit et al., city fields, rotation curves, and physical para- 1988), NGC 2403 (Sivan et al., 1990) and meters determined: M 33 (Carranza et al. NGC 6946 (Bonnarel et al., 1986) was also 1968; Comte and Monnet, 1974), M 51 (Car- observed with the same set-up. These ranza et al., 1969), M 101 (Comte et al., observations are amongst the most spectac- 1979), NGC 2403 (Deharveng and Pellet ular ones obtained with what was then the 1970), NGC 4449 (Crillon and Monnet, largest telescope in the world, which unfortun- 1969a), NGC 4490-85 (Boulesteix et al., ately was located in a relatively poor site.

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Figure 35: Mosaic of Hα images of M 33 obtained with an F/1 focal reducer at the focus of the 6-m Soviet telescope. The coordinates are for 1950.0. Note the numerous shell-like HII regions and the remarkable alignment of HII regions along the southern arm. Compare to Figure 26: in both cases, the observation was seeing-limited, and the F/1 aperture was the same, so the only advantage of the 6-m telescope was in shorter exposures (after Courtès et al., 1987: Figure 1a).

At Córdoba Observatory in Argentina, Clouds (Carranza et al., 1971), NGC 1313 Gustavo Carranza had installed on the 1.5-m (Agüero and Carranza, 1975; Carranza and telescope a 20′-field focal reducer with a Agüero, 1977a), NGC 4945 (Carranza and Pérot–Fabry interferometer, built in Marseilles Agüero, 1983) and NGC 7793 (Carranza and as all similar instruments. It has been applied Agüero, 1977b). to observations of the kinematics of our Galaxy and of other galaxies: the Magellanic

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6 THE CIGALE SCANNING M 100, M 101, NGC 3351, NGC 3938, NGC PÉROT–FABRY INTERFEROMETER 6946, NGC 4395 and M 51 in [NII]), M 81 in

In Maryland, in 1974, Brent Tully, used a [OIII] (see references in Boulesteix et al., Fabry–Pérot interferometer with pressure 1984), and NGC 2535-36 (Amram et al., scanning and an image tube detector, to ob- 1989). The Crab Nebula jet was also studied (Marcelin et al., 1990). tain a detailed velocity map of the spiral galaxy M 51 (Tully, 1974). This is the first known CIGALE was also mounted with an F/2, astronomical use of a scanning Fabry–Pérot 9.6′ field focal reducer at the focus of ESO’s interferometer. The same year, at Imperial 1.5-m telescope. We reproduce in Figure 36 College in London, T.R. Hicks, N.K. Reay and the observations of the N 62B bubble in the R.J. Scaddan developed a piezoelectric scan- Large Magellanic Cloud (Laval et al., 1987) to ning Fabry–Pérot (Hicks et al., 1974). James show how the scanning interferometer works, Caplan (1942–2020) was the first astrono- although it is not easy to interpret immediately mer in France to take an interest in these what is registered. One of the interests of the developments and to collaborate with them. system is to cover all parts of the observed The scanning interferometer was later com- field, without the gaps between the rings of a mercialized by Queensgate Ltd in London fixed etalon. More bubbles, supernova rem- and adopted by all interested astronomers. nants, and other features of the Magellanic

On the other hand, a new detector, the Clouds have been observed later. Hα emis- Image Photon Counting System (IPCS), had sion associated with the HI bridge that con- been developed in 1970–1972 by Alec Bok- nects the two Magellanic Clouds has been senberg and D.E. Burgess at University Col- detected (Marcelin et al., 1985b). lege London (see e.g. Boksenberg, 1972). In The Marseilles observers were also invit- France, a similar system, derived from those ed to mount a scanning Pérot–Fabry inter- used for space UV observations (see Le- ferometer at the focus of the 2.6-m Armenian queux, 2021), was made available by Thom- telescope in Byurakan, to observe the pair of son-CSF in 1976. The first astronomical interacting galaxies NGC 7752-53 (Marcelin application of this system, named COLIBRI, et al., 1987). This was the beginning of a last- was achieved by Jacques Boulesteix in 1978 ing cooperation with Armenian astronomers. (Boulesteix, 1979): he obtained monochromatic images of the NGC 604 HII region in M We stop here the list of galaxies observed 33 in Hα, Hβ, [OIII], [NII] and [SII]. COLIBRI with CIGALE. Many more have been observ- was also used with the Soviet 6-m telescope ed since, including blue compact galaxies in October 1980 with a Pérot–Fabry etalon, to and elliptical galaxies, and also various Gal- observe the general emission of the [NII] line actic objects. A list can be obtained by at 665.84 nm in M 33, and in January 1981 searching for Boulesteix in ADS, because he for the study in Hα of the galaxies NGC 925 has co-authored most of the corresponding and NGC 2903, and NGC 4258 with its curi- papers as the scientist responsible for the ous jet (Boulesteix et al., 1982). instrument. Focal reducers with Pérot–Fabry scanning interferometers derived from In 1980, Keith Taylor and Paul D. Ather- CIGALE have been installed on a variety of ton developed a new instrument that they nam- telescopes: for a list until 1989, see Bland and ed TAURUS, for mapping the velocity field of Tully (1989: 724). More recent ones have extended emission-line sources. TAURUS been installed at the Canadian Mont Megan- involved a home-made scanning Pérot–Fabry tic Observatory (Hernandez et al., 2003), at interferometer and Boksenberg’s IPCS as a the Mexican observatory in San Pedro Martir detector (Taylor and Atherton, 1980). (Rosado et al., 1995), at the William Herschel The Marseilles astronomers could not fall telescope in the Canary Islands (Hernandez behind, so they developed a similar system. et al., 2007) and at the 4-m NOAO-Brazil This was CIGALE, for Cinématique des GAL- SOAR telescope at Cerro Pachon near Cerro axiEs (Boulesteix et al., 1983), with a Queens- Tololo (Mendés de Oliveira et al., 2013). There gate scanning Pérot–Fabry interferometer and is also a Pérot–Fabry scanning interferometer the COLIBRI IPCS detector. It was first mount- called NEFER in the OSIRIS instrument of ed at the Cassegrain focus of the 3.6-m the GRANTECAN 10.4 m Spanish telescope. Canada-France-Hawaii Telescope (CFHT). Margarita Rosado is responsible for this in- NGC 2903 was observed in Hα with 12 scan- strument that has been designed and con- ning steps of the etalon (Boulesteix et al., structed by a multidisciplinarity team from the 1984) and NGC 6946 with 15 scanning steps Instituto de Astronomía of the Universidad (Bonnarel et al., 1988), as well as the southern Nacional Autónoma de México, the Labora- toire d’Astrophysique de Marseille and the In- arm of M 33 (also in Hβ and [OIII]), NGC 1068,

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Figure 36: Observations in Hα of the bubble N 62B in the LMC with CIGALE mounted at the 1.5 m ESO telescope. Images with 20 successive spacing steps of the plates of the Pérot–Fabry interferometer are shown, covering one interference order. Each step corresponds to 18.8 km/s. From this, the velocity structure of the nebula can be obtained as well as its image. The existence of an expansion of the bubble can be seen qualitatively from the presence of an intensity minimum of the inner ring in frame 17 (bottom left) (after Laval et al., 1987: 200). stituto de Astrofísica de Canarias. This dem- presented in Courtès (1977). Amongst them, onstrates the lasting success of this type of the most remarkable is probably a study of HI, instrument. HII, and stars of different ages in the southern

It is impossible here to go through the arm of M33, which presented solid evidence for the density-wave theory of spiral structure. variety of results obtained with the Pérot–

Fabry interferometer. Some early results are

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Figure 37: Graphical summary of the different techniques used for studying the structure of the interstellar matter in the Galaxy: visible interstellar absorption lines (Na I, CaII), 21 cm emission from neutral atomic gas, Hα line emission and radio recombination line emission (the 109α line at a frequency of 5 GHz is an example) from HII regions, photometric distance of exciting stars of HII regions. Lacking: molecular line emission (adapted from Courtès, 1973: 117).

The Hα images and velocities along the ance ambiguity exists in the inner Galaxy. No Galactic plane were at the origin of a deep survey of the molecular component was yet study of Galactic structure, which will be de- available, as the 2.6 mm line of CO was only tailed in the next section. discovered in 1970. As HII regions were observed to delineate rather well the spiral 7 THE 4-ARM STRUCTURE OF THE structure of external galaxies, a study of their GALAXY REVEALED distribution in our Galaxy looked promising.

The Hα survey of the northern Milky Way However, such a study from optical obser- (Courtès, 1951a; 1951b) and the Pérot–Fabry vations alone was still limited by interstellar interferometer measurements of radial vel- extinction, although it could extend to large ocities of HII regions, including faint and dist- distances in some directions. Fortunately, ant ones, were strong incentives for a new radio recombination lines of hydrogen from study of the structure of our Galaxy by the HII regions had been discovered in 1965 by Marseilles astronomers. The previous invest- Bertil Höglund and Peter G. Mezger (1928– igations along the Galactic Plane were limited 2014). As radio waves were not affected by by interstellar extinction to a region of about 3 interstellar extinction, the recombination lines kpc around the Sun, and the only complete were observable throughout the whole Milky map of the Galaxy was obtained by radio ast- Way and the radial velocity of their source ronomers using the 21-cm line of interstellar determined. Figure 37 shows the interrelat- hydrogen (Oort et al., 1958). It revealed a ion between the different techniques to study the structure of our Galaxy. spiral structure, but only in a crude way as the distances were rather uncertain and a dist- Thomas L. Wilson, Peter Mezger, Francis

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F. (Frank) Gardner (1924–2002), and Doug- spectrograph. UBV photometry of these stars las K. Milne carried out a complete Galactic was secured in service observing with the 1- survey of sources of radio recombination m telescope by Patrice Bouchet in 1969, then lines (Wil-son et al., 1970). Unfortunately, by Robert Garnier in 1970 and 1972 (Bigay et they could not solve the distance ambiguity al., 1972). The results were combined with for many sources and their map of the Galaxy those of Wilson et al. (1970). is uncertain for this reason. Combination with Already, in 1969, 4000 radial velocities optical observations was necessary. had been obtained in the North by Courtès

A large optical program of measurement and collaborators and 6000 in the South by of the radial velocities of HII regions was the new survey, as well as many distances of started in 1969 by Yvon Georgelin. His wife exciting stars. From them, a new rotation Yvonne M. Jonckheere-Georgelin was in curve of the Galaxy was derived, in excellent charge of searching for the exciting stars and agreement with Schmidt’s model (1965); the measuring their distances. Beginning with Hα velocities showed good agreement with the southern Milky Way, this work took place those of the main maxima of the 21-cm line at the ESO Observatory at La Silla. They emission, and the first delineation of four spir- were able to observe during 65 consecutive al arms within 4 kpc of the Sun was presented nights! They installed a refractor with a focal (Courtès et al., 1969a). The optical data them- reducer and an Hα interference filter on the selves, including 174 different HII regions, same equatorial mounting as Fehrenbach’s were published by Georgelin and Georgelin refractor that was equipped with an objective (1970a; 1970b), with the addition of three prism, and this allowed them to obtain simul- very distant HII regions in Carina, up to 9 kpc taneously the Hα photography of the field and (Georgelin and Georgelin, 1970c). In the low-resolution spectra of stars in the same North, UBV photometry and spectra of 45 field, in order to detect the exciting stars. exciting stars and radial velocities of 60 new Then interferograms of the HII regions were HII regions were obtained (Georgelin et al., taken during dark sky periods with the 1.5-m 1973). These results associated with those telescope equipped with another focal reduc- of David Crampton allowed one to obtain an er and a Pérot–Fabry interferometer. At Full improved distribution of optical HII regions Moon, higher-resolution spectra of the detect- (Crampton and Georgelin, 1975). ed exciting stars were obtained with this tele- Figure 38 presents a model of the spiral scope equipped with the Marseilles ChiliCass structure of our Galaxy derived from all avail-

Figure 38: 4-arm spiral model of the Galaxy, obtained from high-excitation parameter HII regions (U > 70 pc cm –2). The distance to the Galactic Center was taken as 10 kpc (after Georgelin and Georgelin, 1976: 74).

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able optical and radio observations (Georg- elin and Georgelin, 1976). The distance ambiguity for 67 HII regions without observed exciting stars had been resolved with high certainty for 44 of them, and a larger uncer- tainty for 23 others, using different criteria. This model of the spiral structure was first presented in the unpublished PhD thesis of Yvonne Jonckheere-Georgelin (1975), for which Bart Bok was the external referee.

In 1990, ESO generously constructed a shelter with an opening roof to house a 36-cm Ritchey–Chrétien telescope with a focal reducer, a Queensgate scanning Pérot–Fabry interferometer, and a Thomson–CSF photon- counting detector. This ensemble was named ‘Hα Survey of the Milky Way and the Mag- ellanic Clouds’. Figure 39 shows the system and Figure 40, as an example of the results, an image of the Hα line intensity and velocity field in the Small Magellanic Cloud.

Areas of the Milky Way were selected according to their richness in HII regions (Hα or radio recombination lines) and to their strategic position for determining the position of the spiral arms encountered along the line of sight (for example, see Russeil et al., 1995; 2005). From these new observations and Figure 39: Delphine Russeil filling with liquid nitrogen the dewar of the photon-counting detector attached to a 36- complementary data on interstellar absorpt- cm diameter telescope at ESO-La Silla (photograph: ion lines and molecular gas, an improved Michel Marcelin). model of the spiral structure of the Galaxy has

Figure 40: Color-coded Hα line image of the Small Magellanic Cloud obtained by Étienne Le Coarer from Hα Survey observations. The red parts are receding and the blue ones approaching (after Le Coarer et al., 1993: 368).

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Figure 41 (Left): the final Marseilles 4-arm spiral model of the Galaxy (after Russeil, 2003: 143). The circles represent HII regions with their diameters representing their excitation parameters. The position of the Sun is indicated by a star; the adopted distance to the Galactic Center (at the origin of the coordinates) is 8 kpc. The central bar is schematized, as well as some arm structures. (Right): at the same scale, a recent model from Gaia Data Release 2 (after Khoperskov et al., 2020: 4). The positions of the arms, defined here as over-densities of stars, are shown with error bars. The position of the Sun is indicated by a black circle; the adopted distance to the Galactic Center is 8.19 kpc. The open symbols represent the positions of high-mass star-forming regions with trigonometric parallaxes measured in radio by Very Long Baseline Interferometry (VLBI) of their H2O masers (after Reid et al., 2014: 5).

Figure 42: Mounting to measure the radial velocity of small HII regions. A diaphragm limits the region to be observed. The microscope objective projects on the photographic plate a pupil with an interference ring (after Courtès, 1960: 211). been developed at Marseille Observatory and diaphragm selected a circular portion of the is presented as Figure 41 (Russeil, 2003). It Orion Nebula, 20′′ in diameter; the micro- confirmed the previous model and doubled scope objective collected the corresponding the known lengths of the four arms. As the light and formed on a photographic plate an figure shows, observations of trigonometric image of the telescope mirror, a pupil, with the parallaxes of H2O masers by VLBI and of first interference ring superimposed (Figure stars with the Gaia satellite confirm the prin- 42). The measurement of the diameter of this cipal arm features. uniform ring where all the Hα light entering the hole was concentrated allowed one to 8 TOWARDS MULTI-OBJECT measure the radial velocity with an accuracy SPECTROSCOPY of 3 km/s, better than what was then attain- able with conventional spectrographs. In 1958, Courtès had the idea of inserting behind an Hα filter and a Pérot–Fabry etalon An inconvenience of the fixed Pérot– located in the focal plane of the 1.2-m OHP Fabry interferometer is that the parts of the telescope the objective of a microscope. A image between the successive interference

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Figure 43: Multi-lens mounting. The light from the objective crosses the interference filter Fi, and the Pérot–Fabry fixed interferometer P.F. The focal plane of the 1.93-m OHP telescope is in F. Each of the multi-lens produces at p a pupil (image of the telescope main mirror) with the central interference ring. The F/1 objective O registers the images of these pupils on the photographic plate (after Courtès, 1973: 145). rings are missed. As we have seen, this in- larger telescopes would be available. But for convenience disappears with scanning inter- the moment, it was abandoned. ferometers, but these instruments were not A resurrection occurred a few years later available before 1980. In order to avoid it, with a new device named TIGER (Courtès, Courtès imagined in 1963 “… after long 1982). In this paper, Courtès (1982: 123) nights of insomnia …” extending the micro- wrote: scope device of Figure 42 by inserting a mosaic of micro-lenses (Stanhope lenses)7 at A study of the Faint Object Camera of the the focus of the telescope, behind the filter Space telescope [see Lequeux, 2021 for Courtès’ contribution to this instrument] and the etalon (Figure 43).8 Each of these convinced us that preliminary spectro- lenses acts as a field lens and produces a graphic explorations of 20 arc second small image of the telescope mirror, a pupil, fields would be a necessary preparation on which one central interference ring is sup- for the space observations. The new erposed. A focal-reducing objective projects spectrograph design proposed in this pa- on the photographic plate an image of all per enables one to obtain in one expos- these small pupils with their interference ure simultaneous spectra of an array of 2 rings. In this way, the object is decomposed 1×1 arcsec image elements. The image into a regular mosaic of its different parts, at the Cassegrain focus of a telescope can be enlarged by auxiliary optics and each with its own ring (Figure 44). Of course, projected on a multi-square-shaped lens the angular resolution is limited by the size of array. the micro-lenses. TIGER was one of the first integral-field A few results were obtained for HII re- spectrometers, which are instruments able to gions of M 33, but it turned out that the micro- give simultaneous spectra of all points of an lens solution was less sensitive than the con- astronomical object (Figures 45 and 46). It ventional Pérot–Fabry set-up (see Courtès, derived directly from the instrument of Figure 1973: 148, and note 8). Courtès remarked 43, but without the Pérot–Fabry interferome- that it would however become of interest when ter. However, its interest was not immediately understood by the astronomical com-

munity, even in Marseilles, until Courtès in-

sisted on demonstrating it to his staff. Indeed,

at the beginning of the 1980s, optical fibers

had appeared in astronomical spectrographs,

in both multi-object and integral-field spectro-

scopic modes. Their use was a shock for

Courtès, who was an optical purist. He said:

… the optical fibers bypass a fundamental law of optics which makes cor- respond, to any ray incident on the pupil, a point in the focal plane. (private communication to Yvon Georgelin).

He also thought that it was not necessary for Figure 44: Hα Pérot–Fabry rings produced by the multi- both modes, to impose the passage through lens array of Figure 43 on a part of the Wolf–Rayet nebula NGC 6888. Each ring appears double, because a slit, a useless vestige of the nineteenth cen- Hα and the [NII] line at 658.3 nm have comparable tury. Indeed, no slit is necessary in TIGER, intensities (after Courtès, 1973: 146). which belongs to the integral-field mode, and

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Figure 45: Drawing by Courtès of the principle of the integral field spectrograph TIGER, so named by Courtès because of the stripes formed by the spectra and as homage to The Tyger, a famous poem by William Blake. An array of micro-lenses produces a mosaic of very small pupils. The remaining part is a classical spectrograph based on a Grism (a grating deposited on a prism so that there is no deviation for the central observed wavelength), providing spectra of all the pupils on a CCD. A slight rotation between the dispersion axis and the micro-lenses array, combined with the use of a broad-band filter, avoid overlapping of the spectra (after Courtès, 1982: 124).

Figure 46: Realization of the integral field spectrometer TIGER. The figures on the bottom illustrate the application to observation of a galaxy. The optics has been calculated by André Baranne, and the instrument was automated, with three wheels for changing respectively the enlarger, the lens array and the grism. The detector was a double-density RCA CCD with 640 × 1024 pixels, limiting usually the field to about 5′′ in spectrography and 25′′ in imaging mode. The data reduction procedure was written by Roland Bacon and his young staff at the Lyons observatory (after Courtès et al., 1988: Figure 1). even in multi-object spectrographs: astrono- are drilled in a mask placed in the focal plane, mers from Toulouse have built for the focal and the spectra are obtained by a Grism (Fort reducer at the Cassegrain focus of the 3.6-m et al., 1986; Soucail et al., 1987). Its principle CFHT the multi-object PUMA 1 spectrograph, is thus similar to TIGER, except that the multi- without optical fibers. In this spectrograph, lens mosaic is replaced by a mask. Good- holes at the position of the different targets bye to optical fibers and happy return to imag-

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Yvon Georgelin and James Lequeux Interferometry and Imaging at Marseille Observatory ing! Comte (1995: XIV) wrote:

TIGER was built in the framework of a The purpose of “tridimensional” spectro- collaboration between Marseilles and Lyons scopy (also called spectro-imaging) is to Observatories and installed in 1987 at the obtain spectral information (the third di- mension) on spatially extended objects (2 Cassegrain focus of the 3.6-m CFH telescope space dimensions), such as planets, (Bacon et al., 1988; Courtès et al., 1988). It comets, extended nebulae, galaxies, or enjoyed considerable success. Amongst the fields densely populated with many stellar many results obtained with TIGER at the CFH objects that are simultaneously observ- telescope, mostly in discretionary time, the ed. Extensions of this concept are 2-D observation of the ‘Einstein Cross’, a quad- space reconstructions of point sources by ruple gravitational image of a quasar caused means of Doppler imaging, multispectral by an intervening galaxy, is particularly inter- speckle interferometry, differential interferometry, etc. esting (Adam et al., 1989). Various techniques have been devel- A derivative of TIGER is PYTHEAS, an oped to overcome the basic constraints of integral field spectrograph in which the dis- getting tridimensional information (the so- persive element is a scanning Pérot–Fabry called “data cube”) with finite two-dimen- interferometer followed by a grism (Georgelin sion detectors. Scanning interferometers et al., 1995). Its design is similar to that of and Integral Field Spectrographs now pro- TIGER (Figure 46) except that the interfero- vide complementary spatio-spectral sam- meter is inserted in front of the multi-lens ar- pling performances in the visible range ray. Each micro-lens selects the central part while the Fourier Transform Spectro- Imager, the first near-infrared 3-D instru- of the interferogram. For each step of the ment, is a superb tool for the study of interferometer scan, a channeled spectrum of emission lines in K band. the very small field selected by each lens is A new era now opens for astrophysics registered on the 2-D detector, instead of with the upcoming 8–10 m telescopes. a continuous spectrum for TIGER: maxima On these instruments which will offer occur when twice the spacing of the interfero- adaptive optics, spectro-imaging devices meter plates is an integer number of wave- will allow a full preservation of the imag- lengths. Scanning the interferometer step by ing quality of the telescopes and a very step and registering the result at each step rational use of telescope time. allows it to benefit from the high spectral These advances in optical instrument resolution of the interferometer over a wide design coupled with state-of-the-art digi- wavelength range, while the angular resolut- tal detectors have led to major progress ion is given by the micro-lens array and can in various fields of astrophysics. From also be very high. The micro-lens array can Solar System objects to the most remote be replaced by a mask with holes at the optically visible radio galaxies, 3-D spectroscopy has led to considerable break- image of the objects of interest, as for the throughs in the physical understanding of PUMA device discussed above. PYTHEAS astronomical objects. is the ultimate in integral-field spectrometers, which for example allowed astronomers to Many such observations were reported at obtain simultaneously high-resolution spectra this Colloquium. In a remarkable contribution, of many stars in a globular cluster. Pierre Connes (1928–2019) and Étienne Le Coarer gave a comprehensive review of 3-D Many integral field spectrographs similar spectrometers, stressing that the interferent- to TIGER or PYTHEAS or inspired by them ial color photography technique invented in have been built, for example, MUSE and 1892 and developed by Gabriel Lippmann GIRAFFE-IFU at the ESO Very Large Tele- (1845–1921) can be considered as the first 3- scope (VLT). More are in preparation for the D spectrometer (Lippmann, 1894): the Lipp- giant telescopes presently under construct- mann photographic plate contains the spec- ion, for example, MOSAIC and HARMONI for tral information of the incoming light, with a ESO’s Extremely Large Telescope (ELT). wavelength resolution that depends on the This testifies to the remarkable imagination thickness of the emulsion and can reach a and long-term views of Georges Courtès. resolving power of several thousands. This spectral information can be recovered if need- 9 CONCLUSION ed. For this, Lippmann received the Nobel In March 1994, IAU Colloquium 149 on “Tri- Prize in 1908. Étienne Le Coarer (a pupil dimensional Optical Spectroscopic Methods of Courtès and Connes, who moved from in Astrophysics”9 was held in Marseilles in Marseilles to Grenoble) has proposed with honor of Georges Courtès (see Comte and collaborators a miniature Stationary-Wave Marcelin, 1995). In the Foreword, Georges Interferential Fourier Transform Spectrometer

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(SWIFTS), directly derived from Lippmann’s Hartmann Test on the dismantled mirror, invention (Le Coarer et al., 2007). which showed a surface quality slightly better than λ/20 up to a diameter of 0.74 At the conclusion of the Colloquium, Joss m, but only λ/6 beyond that. The poor Bland-Hawthorn (1995: 379) from the Univer- quality on the edges had been noted sity of Sydney in Australia wrote: already by Stéphan (see his letter of 1 France is arguably the most innovative February 1874 cited in Section 2). nation when it comes to optical design in An 83-cm diameter telescope, almost astronomy. It is clear that, at least from a Marseillaise perspective, this is partly the identical to the Marseilles one, was built legacy of Charles Fabry and Alfred Pérot. in 1875 for the Toulouse Observatory by We heard at the end of the first day the the Henry brothers (optics) and Eichens extraordinary legacy left behind by these (mechanical parts), but it has seen little great pioneers: the first detailed spectro- use. A 120-cm telescope was erected at scopic studies of the ozone layer and the Paris Observatory in 1876 but was a fail- Orion nebula, detection of gravitational ure (Lequeux, 2013: Chapter 7). A more redshift in Solar spectral lines, verification famous glass-mirror reflecting telescope of the Doppler–Fizeau principle, and is the 91-cm Crossley telescope, con- most crucially, the giant leap that became possible in defining internationally accept- structed by Common in 1879 but only put ed measurement standards. into use in 1885 in England, then rebuilt in 1896 for the Lick Observatory. This may be over-enthusiastic, but it is 3. Every time there is a paper dealing with nevertheless clear that the French school of their interferometer, the questions of an optics founded by Léon Foucault, Hippolyte accent in Pérot’s last name and the Fizeau, and Charles Fabry with his collabor- Fabry–Pérot name order are raised by the ators, has always been and is still at the fore- referees and/or the editors. In Marseilles, front of research. Georges Courtès was one it is called the Pérot–Fabry interferometer of the most imaginative and prestigious mem- and we will stick with this designation. In bers of this school. Paris and in foreign countries, the order of the scientists is generally reversed, 10 NOTES probably beginning in the USA with Bab- 1. Documents (in French) about the Obser- cock (1927), because Fabry had become vatory from 1862 are available at more celebrated than Pérot. In France, https://promenade.imcce.fr/fr/pages5/55 the label Fabry–Pérot probably appeared 0.html for the first time in 1934 in the thesis of 2. The Foucault 80-cm diameter telescope Pierre Rouard (1908–1989), who was to is of the Newton-type, with a parabolic succeed in 1944 his advisor Henri Buis- mirror. As for the previous 40-cm Fou- son as Director of the laboratory founded cault telescope, the prism necessary to by Fabry at the University of Marseille. direct the beam to the side was kept small Pérot’s last name has been written with to minimize obscuration, but this meant or without an accent. Although his name that the focus was well inside the tube is written without an accent on his birth and out of reach of a simple eyepiece. certificate, Pérot added it in his PhD Two relay lenses then brought the focus thesis and other publications in order to outside the tube but introduced spherical convey the correct pronunciation. Fabry, aberration, which Foucault eliminated by who wrote most of their joint papers, modifying slightly the shape of the mir- always used Perot, without an accent. ror (Tobin 2003: 217 and 222). When However, in their first two (and funda- Charles Fabry installed his Pérot–Fabry mental) papers, published in 1896 in the interferometer at the prime focus of the Comptes Rendus Hebdomadaires des telescope, he was unaware of the spher- Séances de l’Académie des Sciences, ical aberration thus introduced by the their names are written Ch. Fabry et A. mirror alone, which prevented him from Perot, and A. Pérot et Ch. Fabry, respect- separating the four stars in Orion’s Trap- ively. A similar example is the couple ezium. Still, the optical surface is excel- Stephan-Stéphan: we use Stéphan in this lent: after a ‘Foucault test’ on the sky in paper. As to Fabry’s first name, Charles, we find it abbreviated as C. or Ch. 1954, the famous astronomer and optician André Couder declared: “I could not 4. Jacques Pérot, General Curator of Patri- have done better.” In 1964, Roland Le- mony, is the grandson of Alfred Pérot. He blondet, an excellent and experienced preserves in his family a standard meter optician at Marseille Observatory, did a that has been compared to the official

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one using a Pérot–Fabry interferometer scientific amusements for young people, and has served to measure the wave- in particular by pasting the image of a length of the red line of cadmium in relat- landscape on the flat backside. In Mar- ion to the official meter. For details, see seilles, they were easy to find and de- Georgelin and Tachoire (2002: 132–135). picted the famous church Notre-Dame de 5. These observations were made during a la Garde, soon competing with pin-ups in trip from November 1955 to April 1956 in a bikini. However, Courtès wanted the South Africa, where Courtès and the Bel- rear face had the same curvature as the gian astronomer Jean Dommanget (1924 front one, and the 271 lenses, 2.4 mm in –2014) were in charge of ‘prospection diameter and 15 mm in thickness, were and site study’ for a possible implantation manufactured one by one and assembled of the European Southern Observatory together. Courtès realized later that such (ESO): see Dommanget (1962). mosaics of lenses had already been used

6. Many other students of Courtès are mis- by Gabriel Lippmann and Henri Chrétien sing from the photograph in Figure 31. (1879–1956) in some optical devices. His students were, in chronological order 8. This gives us the opportunity to convey of their entry at the Observatory: Paul how Courtès directed his students, with Cruvellier (1959), Maurice Viton (1962), his willingness to innovate in order to try Guy Monnet (1963), Yvon and Yvonne to catch up with the big American tele- Georgelin (1963), Annie Laval (1963), scopes. In a calm and even paternal tone Renée Crillon-Dubout (1963), Raymond Courtès in 1964 described with simplicity Louise (1963), Marie-France Chériguène- to one of the authors (Y.P. Georgelin) the Duval (1964), Jean-Michel Deharveng procedure to follow in order to build a simple prototype: (1965), Lise Baudel-Deharveng (1968), Jacques Boulesteix (1968), Jean-Pierre Calculate the plate separation of the Sivan (1968), Georges Comte (1971), Pérot–Fabry etalon so that the first inter- James Caplan (1971), Michel Marcelin ference ring projects onto the F/5 pupil of (1978), François Bonnarel (1984), Etien- the 193-cm. Make a very light prototype, 2 kg, and fix it on a 16×16 cm subframe ne Le Coarer (1986), Philippe Amram adaptable on the photographic lens of (1987), Delphine Russeil (1993), Henri Couder: you will then be able to position Plana (1993) and Benoît Epinat (2006). in x,y,θ our 4 Stanhope lenses of 2.4-mm Several of them worked with Courtès in diameter. Ask Agniel, the OHP photo- the Laboratoire d’Astronomie Spatiale grapher, for a contact print of the photo- (LAS) described in the companion paper graph of the galaxy M 33 taken by Tex- (Lequeux, 2021), while being still affiliat- ereau at this focus. Ask Urios (then the ed administratively with the Observatory. only technician assigned to Courtès), to prepare a circular bakelite (a hard, black With the exception of David Cramp- plastic) plate and to drill holes at the ton (DAO, Victoria, Canada), who collab- position of four HII regions of the contact orated with Yvonne Georgelin on the ex- print to insert the Stanhope lenses. On a citing stars of HII regions, the collabor- similar plexiglass plate, he will drill in the ations were essentially around the use of same way small holes at the position of the Pérot–Fabry interferometer: with Gu- these HII regions and of the guiding stars. stavo Carranza, Estela Agüero and Gui- At the telescope you will illuminate lat- llermo Goldès (Observatorio de Cordoba, erally the plexiglass with a small lamp and the small reference holes will be Argentina); with Margarita Rosado and visible; it will be enough to adjust the eye- Patricia Ambrocio-Cruz (Instituto de Ast- glass shade in xyθ to position them on the ronomía, UNAM, Mexico); with Jean- stars. Then you will substitute the bake- René Roy, Claude Carignan, Robin Ar- lite plate and the lenses will be positioned senault and Gilles Joncas (Laval Univer- on invisible HII regions.

sity, Quebec), and Olivia Hernandez (Uni- This direct procedure of Courtès must be versity of Montreal, Canada); and with compared to the heavy indirect proced- Claudia Mendes de Oliveira (Universi- ure used for the operation of the Medusa dade de São Paulo, Brazil). spectrograph; a gnomic projection is us- 7. The focus of these Stanhope ‘magnifiers’ ed to transform the measured positions is formed on the backside of these thick on the Palomar Sky Survey plate to the lenses that are placed in contact with appropriate scale (Hill et al., 1980). This objects. They were invented by Lord first extragalactic interferogram obtained Charles Stanhope (1753–1816), a mem- in October–November 1964 made it pos- ber of the Royal Society, and had great sible to simultaneously measure 4 radial success in medical microscopy but also as velocities with high precision (unpublish-

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ed PhD thesis of Yvon P. Georgelin, gral field mode (Courtès and Georgelin, 1967). In 1965, John C. Brandt and Guido 1967). Thus, the Pérot–Fabry integral Münch had only collected 29 low-preci- field spectrograph (Courtès 1964) pre- sion radial velocities of M 33 obtained ceded the Grism integral field spectro- one by one. graph TIGER (Courtès, 1982).

The following season (October–Nov- 9. The scanned version of IAU Colloquium ember 1965), it was decided to use a 149 is accessible freely via solid Pérot–Fabry etalon (a single plane- https://www.cambridge.org/core/journals parallel silica slide) of 265 μm thickness /international-astronomical-union- and 45 mm diameter. 271 Stanhope colloquium/volume/0ACDB7F1D463D42 lenses were polished one by one and B5E20887CF925A305# assembled without gluing into a hexag- 11 ACKNOWLEDGEMENTS onal insect eye, at a time when industrial resin lens frames did not yet exist. This We thank Michel Marcelin for specifying insect eye made it possible to obtain some uncertain points and for suggesting a multi-interferograms of M 33 in the inte- number of improvements to this paper.

12 REFERENCES

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Dr James Lequeux, born in 1934, started a second career in the history of science when he retired in 1999. Before that, he was an astronomer at Paris Observatory, specializing in interstellar matter and the evolution of galaxies. He was Director of Marseille Observatory from 1983 to 1988 and knew well most of the scientists cited in this paper.

For 15 years James was one of the two Editors-in-Chief of the European journal Astronomy & Astrophysics. Currently, he is an Associate Editor of the Journal of Astronomical History and Heritage (JAHH).

He has published several textbooks and many books on the history of physics and astronomy, and has written nine papers for the JAHH

Dr Yvon Georgelin, born in 1941, began his career as an astronomer-observer in 1963 in the team of Georges Courtès by studying the galactic HII regions and the MagelIanic Clouds. He followed Courtès in his innovative and often highly criticized projects of multi-object and integral field Pérot–Fabry spectrography with Stanhope lenses.

He was Director of Marseille Observatory from 1976 to 1982, and a member of the instrument commission of the 3.6-m Canada-France-Hawaii Telescope. He continued to develop interferential techniques and to observe, particularly in the Southern Hemisphere.

The third part of his career was devoted to conferences, articles, and sites—such as astronomie.regards.free.fr—on astronomical discoveries from Chaldean and Greek antiquity to the present day, which he tried to embellish with cultural elements.

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