William Herschel Reflector

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Fig. 5.4. Original single astatic counterweight described by Lassell in 1842 [5.7] (reproduced from Danjon and Couder [5.1])

Fig. 5.4. Original single astatic counterweight described by Lassell in 1842 [5.7] (reproduced from Danjon and Couder [5.1])

telescope [5.8]. Figure 5.4 is reproduced from Danjon and Couder [5.1] who point out that Lassell was apparently not fully aware of the significance of his own invention. The 1.22 m telescope had a multi-lever astatic support and was, in this sense, the immediate precurser of the majority of support systems of subsequent telescopes. The development of such astatic supports will be discussed in RTO II, Chap. 3. Suffice it to say here that Lassell, underestimating his own invention for lack of proper scientific analysis, erroneously believed (ref. [5.1] p. 687) that the lever arms had to be maintained roughly horizontal. To this end, Lassell designed the telescope tube to be rotatable about its axis, an appreciable mechanical complication. Observational experience rapidly demonstrated to him that this rotation made no difference and that the levers worked equally well in all telescope attitudes. The essential reason for Lassell's error was the unfamiliarity with the new equatorial mount: with the old altitude-azimuth (Alt-Az) mount, lever arms arranged horizontally in the zenith position would have stayed so at all elevations. We shall see (RTO II, Chap. 3) that the recent reversion to the Alt-Az mount requires an inverse adaptation to its simplifications.

Lassell's rotating tube had, in fact, a second and highly practical purpose: to allow easy access from the observing tower (Fig. 5.5) to the Newton focus [5.3]. This is a problem of which amateurs making visual use of Newton telescopes rapidly become aware. If Lassell had used the Cassegrain form, this telescope could have rated, from its opto-mechanical form, as the first

Fig. 5.5. Lassell's 1.22 m telescope set up in 1861 in Malta (reproduced from the original plate of [5.8])

modern reflector. But its speculum mirror still belonged to the pre-modern era.

Another important telescope of this epoch which must be mentioned was built by James Nasmyth in about 1845. It had an aperture of 20 inches (51 cm) and two most important features (Fig. 5.6). These were, firstly, the use of the Cassegrain optical form, although Fig. 5.6 shows that a Newton focus was also available. Secondly, Nasmyth's interest in the Cassegrain optical form was his modification of it to give the Nasmyth focus by adding a Newton-type flat to send the beam through the hollow altitude axis to a fixed focus position, if the observer turns with the azimuth turntable axis. This was a very important invention and makes this Nasmyth telescope the direct precursor of the ESO NTT. This 20-inch Nasmyth telescope, together with a 15-inch Newtonian-Cassegrain built by Thomas Grubb in 1835 for the Armagh Observatory, also represented the first successful manufacture of a

Calver Telescope Mirror
Fig. 5.6. James Nasmyth's 20-inch Cassegrain-Nasmyth telescope about 1845 (reproduced from King [5.2])

convex Cassegrain secondary for a major telescope, almost 200 years after its theoretical invention by Cassegrain! Unfortunately, little clear information concerning the optical quality in the Nasmyth focus compared with the Newton focus or other Newton telescopes seems to be available, since Nasmyth mostly observed the sun and moon rather than star fields.

The last, and potentially the most impressive telescope of the speculum mirror era was the Melbourne reflector of 48-inch aperture. The responsible committee, chaired by Robinson, included Rosse, John Herschel and Lassell, the latter offering his own 48-inch telescope operating in Malta. The latter was refused, to the chagrin of Lassell, because a "more manageable" instrument [5.2] was desired. The basis of this decision was the clear recognition of the advantage of the Cassegrain form because of its telephoto property: a 1.22 m primary with f/7.5 and m2 = -5.54. It is notable that, for the first time, a significant reduction in primary f/no compared with W. Herschel's telescopes was attempted. This feature, combined with the Cassegrain form, presented a new dimension of optical manufacturing and testing difficulty. The contract was given to Thomas Grubb (working with his son Howard), who had invented the whiffle-tree support used by Rosse. He proposed a similar 27-pad support for the Melbourne reflector. No Newton focus was envisaged, which much simplified the visual access to the focus compared with the telescopes of Rosse and Lassell. Apart from the access advantage and that of the shorter, lighter tube, Robinson believed the convex secondary compensated the errors of the primary and produced a flatter image [5.2]. Table 3.3 shows that these statements are incorrect for a Newton and Cassegrain normalized to the same focal length, but the large telephoto effect was justification enough. The mounting, of the so-called English type as an equatorial, was an admirable concept (Fig. 5.7).1 The tube was a lightweight structure allowing full ventilation and reducing windloading. Since observation was in the open, this was an important feature. Note that the telescope had no fixed dome, only a sliding-roof weather protection, again a direction which is ultra-modern today for some of the most advanced telescopes.

Had the committee taken the decision to equip the Melbourne reflector with the newly invented silver-on-glass mirrors, it would undoubtedly have been the first successful modern telescope. In fact, Robinson preferred the cautious approach of retaining speculum, believing that silver films of this size were too risky in the unfavourable climate of Melbourne [5.9]. This proved to be a fatally wrong decision.

Shortly before the first edition of this book was published, my attention was drawn by Peter Hingley, Librarian of the Royal Astronomical Society,

1 S.C.B. Gascoigne has kindly pointed out that the well-known print of Fig. 5.7 does not, in fact, show the Melbourne telescope erected at its final destination in 1869, but a preceding trial erection near Dublin in 1867. This is clear from the angle of the polar axis to the horizontal, which is more than 45°, corresponding to the latitude of Dublin (53.3° N). Melbourne has a latitude of 37.8° S.

Fig. 5.7. The 4-foot (1.22 m) Melbourne reflector erected in 1869 (reproduced from King [5.2])

London, to the existence of a marvellous engraving showing the casting of the first 48-inch speculum blank. I am most grateful to him that this can now appear in the second edition. The original appeared in Vol. XII of the Strand Magazine (London) in 1896, in an article in which Howard Grubb was interviewed. He was responsible for the casting operations in Dublin in 1866, when the original engraving was produced. His account of the casting of the first blank is hair-raising. Two tons of metal were melted in a furnace fired by a mixture of coke and compressed peat. The dramatic nature of the operation is wonderfully shown in the engraving, reproduced in Fig. 5.8.

Fig. 5.8. The casting of the first 48-inch speculum blank for the Melbourne reflector (courtesy Royal Astronomical Society, through Peter Hingley, original engraving reproduced in the Strand Magazine, London, Vol. XII, 1896, p. 372)

Howard Grubb describes it thus: "The metal was poured in about six seconds and the extraordinary spectacle is depicted in the illustration on the next page. Every man wore a large apron and gauntlets of thick felt, with an uncanny-looking calico hood, soaked in alum, drawn completely over his head. This hood was provided with large, glistening talc eyes. These weird figures flitted around in the ghastly light of the intense soda-flame that leapt from the great furnace, and the windows were filled with the eager faces of fascinated spectators". The whole operation in the terrible heat lasted 24 hours, but this blank was a failure. However, the experience enabled the successful casting of the second and third blanks, which were the ones figured and used in the telescope.

The early history of the application of chemical silvering on glass to telescope mirrors is given by King [5.2]. The first successful chemical silvering was displayed in 1851 at the Great Exhibition in London for decorative purposes, following a patent of Varnish and Mellish. This was refined in 1856 by the German chemist Liebig. The process was first applied to small telescopes by Steinheil in the same year, but independently and more strikingly by Foucault [5.10] to a 10 cm telescope (used as a test collimator) in 1857. Foucault then made a number of silver-on-glass reflectors up to 80 cm diameter, the larger ones profiting from his brilliant invention of the "knife-edge test" in 1859 (see RTO II, Chap. 1). The largest, with 80 cm aperture, was completed in 1862 and is shown in Fig. 5.9 2.

Since the decision to use speculum in the Melbourne reflector was only taken in 1862 [5.13], five years after the successful completion of Foucault's first glass telescope, it appears now as an almost inexcusable blunder, particularly as Draper had also had successful results as early as 1858 [5.2]. The tarnishing problem was well-known in speculum and frequent re-polishing of the mirrors was required, an enormous complication which was completely avoided by silver-on-glass. The acceptance tests of the optics in 1868 claimed very high quality. For transport, the mirrors were protected by shellac. Two primaries, A and B, were supplied [5.13] and the incorrect removal of the shellac damaged the surface of primary A from the start. With primary B, the resolution was claimed to be fair and the reflectivity high. The climate of Melbourne, because of extreme temperature changes and damp, was very unfavourable to speculum; also the building gave inadequate dust and wind protection. Over 15 years a programme of observation of nebulae, following J. Herschel's catalogue, was pursued, entirely with hand drawings. A photographic attachment had been supplied but was useless at the very low Cassegrain relative aperture of f/41.6, above all with existing slow emulsions. The committee had failed to recognise the already well-known law in photography of unresolved extended objects whereby exposure time increases with

2 I am most grateful to William Tobin for pointing out that King's Fig. 108 is misidentified as Foucault's 33 cm telescope. A complete account of Foucault's telescopes is given in two papers by Tobin [5.11] [5.12].

William Herschel Reflector
Fig. 5.9. Foucault's largest (80 cm) silver-on-glass reflector, completed in 1862 (reproduced from King [5.2])

N2: the Cassegrain form was totally unsuitable for the astronomical programme pursued. Wind-shake was also a fatal problem with this long focal length.

In 1874, primary A was re-installed and then gave good images: the previous poor images were due to incorrect mounting in the cell, leading to constraints and astigmatic errors of various types. In 1877 the director, Ellery, announced that the mirrors were so tarnished that re-polishing was essential [5.2]. Returning the mirrors to the manufacturer, Grubb, in Ireland was considered impracticable. The technician accompanying the telescope, Le Sueur, who was supposed to deal with the optics, had left in disgust in 1870. Ellery attempted repolishing himself but lacked expertise, above all in testing. In 1890, he claimed success, but the optics never functioned reasonably again.

Much later (1953), the telescope was transferred to Mt. Stromlo where one of the primaries was dropped and smashed: the metal paid for a 50-inch glass blank to be worked into Schmidt-type optics. Subsequently, the remaining metal primary and the mounting were returned to Melbourne, intended to become a museum exhibit [5.13].

The story of the Melbourne reflector is correctly seen as one of the greatest tragedies in the history of the telescope. In 1904 Ritchey [5.2] [5.14] wrote: "I consider the failure of the Melbourne reflector to have been one of the greatest calamities in the history of instrumental astronomy; for by destroying confidence in the usefulness of great reflecting telescopes, it has hindered the development of this type of instrument, so wonderfully efficient for photographic and spectroscopic work, for nearly a third of a century".

Indeed, the lessons of the Melbourne reflector are fundamental and a warning for all subsequent telescope projects:

- The dangers of a design by a committee rather than dedicated individuals. All previous successful telescopes (above all those of Herschel, Rosse and Lassell) were designed and built by enthusiasts who themselves optimized and used them.

- The failure to involve the designer-manufacturer in the erection and optimization of his telescope, in function, on site.

- The failure to give authority and power to an astronomical director of sufficient enthusiasm, vision and astronomical and technical competence to ensure an astronomical programme suited to the nature of the telescope and the necessary technical expertise to maintain it.

5.2 Glass optics telescopes up to the Palomar 200-inch

As a result of the failure of the Melbourne reflector, the pendulum of the rivalry of reflector versus refractor swung back for the last time to the refractor, culminating in the 36-inch Lick and 40-inch Yerkes refractors of Alvan Clark. Nevertheless, the reflector continued to make steady progress without repeating the aperture achieved by Rosse (1.82 m) before the end of the nineteenth century. In 1859 Foucault [5.15] invented the Foucault knife-edge test, the first scientific test of telescope mirrors of high sensitivity (see RTO II, Chap. 1). After 1865 silvered glass dominated mirror technology entirely. In spite of tarnish by sulphur and moisture and relatively poor reflectivity in the far blue, the ease of replacement and average high reflectivity compared with speculum made this one of the most important advances in the history of the reflecting telescope. It favoured the further development of the Cassegrain because 2 reflections were much more acceptable. It was also much lighter than speculum, a heavy alloy. The further rapid development of photography also favoured the reflector because of its complete absence of chromatism. Foucault, Draper, Brashear, H. Grubb, With, Calver, Martin, Eichens, Gautier and Common were the principal successful manufacturers up to 1900. Calver made a 36-inch (91 cm) silvered glass mirror for Common in 1879. Common attempted to make a 5-foot (152 cm) Cassegrain using a blank with a hole cast in it. His work clearly revealed the problems arising from the relatively high expansion coefficient of normal plate (crown) glass (ca. 80x10-7) and the far lower thermal conductivity than that of speculum. The project was abandoned before 1900, but the mirror was used and reworked later by Fecker in 1933.

The most notable reflector of the pre-1900 period used the Calver 36-inch (91 cm) mirror, mounted as a Newton telescope by Common for Edward Crossley's private observatory near Halifax, England. This telescope was presented to the Lick Observatory in 1895 and re-figured and remounted by H. Grubb. The remarkable feature of this Calver mirror was its fast relative aperture of f/5.8, an immense advance on the Melbourne reflector of f/7.5, which itself was a big advance on the values around f/10 - f/9 of Her-schel, Rosse and Lassell. From Eq. (5.1), the asphericity of the Calver mirror compared with similar sized mirrors of Rosse was a factor of about 4 times higher.

The Crossley reflector, because of its high light efficiency at f/5.8 in the Newton focus, was really the first modern reflector and introduced modern astrophysics and cosmology above all through the work of Keeler. Keeler's photographs with this telescope were the first to reveal large numbers of small or distant nebulae [5.2] [5.16]. The Mayall slitless spectrograph made excellent use of the Newton focus of the Crossley reflector for UV spectroscopy applied to faint nebulae. A photograph of the prime focus of this historic telescope, which is still in operation, is given by Dimitroff and Baker [5.17]. Figure 5.10 shows the Crossley reflector at Lick with its new mounting. The rich harvest of astronomical results with this telescope, in spite of its modest size even when made in 1879, demonstrates the advantage of a dynamic and visionary astronomer (in this case James Keeler) in exploiting to the full the potential of a telescope. A beautiful account of Keeler's life and work, evoking the extraordinary productivity of this period in the United States, is given by Osterbrock [5.18].

Fig. 5.10. The 36-inch (91 cm) Crossley reflector, remounted at Lick in 1900 (courtesy Mary Lea Shane Archives of the Lick Observatory, through D. E. Osterbrock)

The turn of the century introduced the work of one of the greatest of all telescope builders, George Ritchey. Excellent accounts of Ritchey's career and achievements, above all due to the remarkable symbiosis with the organisational genius of G.E. Hale, are given by King [5.2], Riekher [5.3] and, again in an admirably complete account, recently published, by Osterbrock [5.19]. Hale was instrumental in getting the 40-inch Yerkes refractor built but fully recognised the inherently greater potential of the reflector. Inspired by Draper, Ritchey had made a 23i -inch (60 cm) mirror with f/3.9 which he set up at Yerkes in 1901 after his appointment by Hale. Ritchey was greatly interested in photography and recognised that higher "speeds" were essential in telescopes intended for nebular photography. In exactly 40 years, the speed (f/no) of large primaries had progressed from Ni = 9.2 (Lassell, 1861) to 7.5

(Melbourne reflector, Grubb, 1869), to 5.8 (Crossley reflector, Calver, 1879), to 3.9 (Ritchey, 1901). This revolution was just as important as silvered glass as the basis for the final triumph of the reflector in the twentieth century. It would not have been possible without Foucault's invention of the knife-edge test. Figure 5.11 shows the 60 cm telescope at Yerkes. We said above, the Crossley reflector was the first modern telescope. This is true in the sense of its impact on astrophysics with f/5.8 in the Newton focus. But Ritchey's 60 cm at Yerkes was, in fact, the first telescope to possess all the following features characterising a modern telescope, albeit with a modest size:

Fig. 5.11. Ritchey's 60 cm telescope at Yerkes, 1901 (courtesy Deutsches Museum, Munich)

- Glass mirrors (silvered)

- A high speed primary (f/3.9) for photography (at a Newton focus)

- A Cassegrain focus for spectroscopy with a fixed spectrograph

- An open frame, completely ventilated tube

- An equatorial mounting (German type with counterweight)

Figure 5.12 shows another view of the 60 cm telescope taken with Ritchey himself observing at the Newton focus. This photograph was discovered by chance in a private house near the Yerkes Observatory late in 1998. D.E. Osterbrock recognized that the figure was Ritchey, had the photograph cleaned and improved and kindly gave it to me for the second edition of this book, for which my grateful thanks. The brimless hat was typical for astronomers observing in the hard, winter conditions of that time (between 1901 and 1904).

Riekher [5.3] shows a reproduction of a famous photograph by Ritchey with a 4-hour exposure of the M31 nebula in Andromeda. Ritchey stopped the primary down to about 4 aperture to reduce the field coma, an experiment which led to his interest in, and understanding, later, of the aplanatic telescope. This photograph showed far more detail than ever revealed before, even with the 40-inch refractor used visually with an aperture 2.2 times as large. (For comparison, the 3.5 m NTT, which went into operation 88 years later, would achieve the same intensity at far higher resolution with a few seconds exposure on a modern CCD detector).

With such results, the way was open for Hale and Ritchey to produce large telescopes of modern form. Hale had procured a 60-inch (1.52 m) plate glass blank of about 20 cm thickness from the St. Gobain glassworks in Paris. Ritchey's optical work for this telescope, both in figuring techniques and testing (see RTO II, Chap. 1), represented a milestone. His method of parabolis-ing was conceived to avoid working the edge of the primary at all. In Eq. (5.2), we showed how the aspherising function can be chosen at will by slightly varying the curvature of the reference sphere cs relative to the vertex curvature cp of the desired paraboloid. If Sz in (5.2) is set to zero for the mirror edge ym, this 2-dimensional function for the difference is a minimum. In three dimensions, however, this does not represent minimum removal of material and some material must be removed right up to the edge. Ritchey chose to give the reference sphere and the parabola the same slopes at the edge. Differentiating zs and zp and inserting cp — cs ~ Sr/r2, where r is the mean radius, gives

for the necessary increase of the reference sphere radius rs compared with rp = 2// for the equal slope (ES) case. Ritchey's mirror had a relative aperture of f/5.0, so Sr/r = 0.125%, a negligible effect on the focal length for a normal telescope. This technique of aspherisation gives maximum removal of

George Willis Ritchey
Fig. 5.12. Ritchey observing at the Newton focus of the 60 cm Yerkes reflecting telescope between 1901 and 1904 (courtesy Yerkes Obervatory, through D.E. Osterbrock)

material at the centre of the mirror, a far better technique than that used by Rosse.

Ritchey's test methods will be discussed in RTO II, Chap. 1. The Foucault knife-edge test was systematically applied by zonal masking and measuring the radius of different zones, the difference between centre and edge zones corresponding to Eq. (5.3). Here we shall mention just one other aspect as an illustration of the care taken. The primary was not only tested in the normal way, at its centre of curvature, but also - as a null test - at its focus in autocollimation and double-pass with a plane mirror of similar size. A plane mirror of the necessary high quality and in a diameter sufficient for a 1.5 m primary was an undertaking in its own right.

Similar care was taken with the support of the mirror. Both for the 60-inch and subsequent 100-inch telescopes, Ritchey systematically applied the astatic support concept of Lassell.

Because of the greater size of the primary, Ritchey no doubt felt it prudent to relax the f/no of the primary to f/5.0 compared with f/3.9 for his 24-inch telescope. The 60-inch telescope had a remarkably versatile optical concept,

Fig. 5.13. Ritchey's 60-inch Mt. Wilson reflector: optical arrangement (courtesy Rolf Riekher)

as shown in Fig. 5.13. It was the first telescope to offer Newton, Cassegrain and coude foci, the coude focus being at the lower end of the hollow polar axis of a fork-type mounting. The Newton focus (b) has f' = 25 feet; the Cassegrain arrangement for photography (c) f' = 100 feet; the Cassegrain arrangement for spectroscopy (f) f' = 80 feet, and the coude arrangement (e) f' = 150 feet. The corresponding relative apertures are f/5.0, f/20, f/16 and f/30, values which have set the style for most of this century. Figure 5.14 shows the telescope, completed in 1908. Ritchey recognised the fundamental importance of avoiding temperature changes in the mirror and strict rules were established to achieve this by control of the conditions in the dome, based on experience with the adjacent Snow solar telescope. It was judged that images were as good as those obtained in the optical shop. The judgment on photographs at the Newton focus after 11 hours exposure was that very round images were obtained whose diameter was not greater than 1.03 arcsec. This set a new standard of optical quality, above all for a telescope of this size and speed. At last, a versatile, manoeuvrable telescope of very high quality was available with a size only surpassed (still) by Rosse's 6-foot telescope of 1845. The 60-inch telescope at Mt. Wilson spelled the definitive death-knell for the large refractor. The reflector was superior to the 40-inch refractor even for visual planetary observation, apart from its potential in photography at both Newton and Cassegrain foci. Together with W. Herschel's 20-foot focus telescope, Ritchey's 60-inch was arguably the greatest relative advance in astronomical observing potential ever achieved. None of the weaknesses of the Melbourne reflector prevented this potential form being fully realised. Ritchey himself was involved in the observing success and maintained close contact with the function of the telescope. With numerous aspects of modernisation, it is still doing good work today, 83 years after its "first light".

Anticipating the success of the 60-inch telescope while it was still to be built, Hale ordered in 1906 a similar blank of 100-inch (2.54 m) diameter which was delivered in 1908. Because of the high glass mass (4.5 t), the melted glass was poured from 3 separate pots leading to bubble concentrations at the joints. Ritchey at first refused to work the blank for fear the bubbles could provoke a breakage. Three further castings failed or were too thin, so Ritchey agreed to work the original blank after a statement by Day that the layer of bubbles strengthened rather than weakened it. Optical work started in 1910 and took over five years. Test procedures were similar to those used for the 60-inch, except that a quantitative Hartmann test was added. The aspect ratio of the blank (diameter to edge thickness) was 8.0 and the relative aperture of the finished mirror f/5.1. The telescope has an English cradle-type mount (Fig. 5.15). This does not allow access to the pole but offered advantages of symmetry and rigidity. The optical forms available with Newton, Cassegrain and coude foci were very similar to the 60-inch (f/5.1, f/16 and f/30).

The first test on the sky in 1917 by Hale and Adams [5.3], observing Jupiter, apparently condemned the telescope as a failure, giving an extremely

Crossley Reflector
Fig. 5.14. Ritchey's 60-inch Mt. Wilson reflector, 1908 (courtesy Donald Osterbrock and the Observatories of the Carnegie Institute of Washington)

poor image. This was due to lack of experience with the thermal inertia of a 4 2 ton primary cast in classical plate glass with relatively high expansion coefficient. The problem was caused by preparatory work during daytime with sunlight and insufficient cooling time for the mirror at night. Just before dawn, an excellent image of the star Vega was observed. Classical plate glass blanks gave an immediate measure of the thermal equilibrium. Later blanks in low- or zero-expansion materials were not sensitive; but we shall see that the consequences for dome seeing could be just as disastrous.

The light grasp of the 100-inch Mt. Wilson (Hooker) telescope was almost 3 times that of the 60-inch. Together, these two telescopes transformed astrophysics and cosmology. The observations by Hubble and Humason in 1924 of M31 in Andromeda, and other galaxies, resolved the outer stars sufficiently well to enable Cepheid variables to be observed and their distances determined. They proved definitively the "island universe" theory of William Herschel that spiral nebulae were external galaxies similar to our own Milky Way system. This led in 1929 to Hubble's redshift law of the expansion of the universe and the Big Bang theory of cosmology.

Recently, the 100-inch telescope has been taken out of service whereas the 60-inch is still working. The pioneer work and greatest achievement of Ritchey was really in the 60-inch, the optics of the 100-inch being the extension of a brilliant concept and professional work of the highest level. Osterbrock [5.19] gives a fascinating account of the collapse of Ritchey's relations with the Mt. Wilson management during and after the completion of the 100-inch and his subsequent, tragic estrangement from the American astronomical establishment.

As after Herschel in 1789 and Rosse, Lassell and Foucault 60 years later, a period of consolidation followed the building of the 100-inch telescope before larger sizes were attempted. Notable telescopes were the 72-inch (1.83 m) Victoria reflector (primary f/5.0), the optics being made by Brashear, completed in 1919; the 69-inch (1.75 m) Perkins reflector, 1932 (Fecker, primary f/4.3); the 74-inch (1.88 m) Dunlap reflector (Grubb-Parsons, primary f/4.9), and the 82-inch (2.08 m) McDonald reflector (Lundin, primary f/4.0). Beautiful and powerful though these telescopes were, their optics represented no significant advance over Ritchey's achievements. The most notable feature was the use of Pyrex (low expansion glass - see RTO II, Chap. 3) blanks for the primaries of the Dunlap and McDonald telescopes, completed in 1933 and 1939 respectively. The McDonald telescope had the fastest large primary (f/4.0) yet made; but this was a fairly modest advance over the 100-inch f/5.1, bearing in mind Ritchey's 24-inch, f/3.9 finished in 1901! Although of smaller size, two telescopes by Carl Zeiss Jena were noteworthy, a 1 m, f/3.0 telescope for Bergedorf in 1911, and a 1.22 m with Newton f/6.9 and Cassegrain f/19.7 in 1915. The latter (subsequently moved to the Crimea) was the largest telescope in Europe for 30 years, but, in contrast to the very

Fig. 5.15. The 100-inch Hooker telescope at Mt. Wilson (1917) (courtesy Deutsches Museum, Munich, and acknowledgement to the Observatories of the Carnegie Institute of Washington)
Fig. 5.16. Pease's concept for a 300-inch (7.5 m) telescope in 1921 (reproduced from Dimitroff and Baker [5.17], courtesy Churchill Livingstone, Edinburgh)

short telescope for Bergedorf, was an outdated optical concept (too long) compared with Ritchey's telescopes.

Ritchey's dream was to build still larger telescopes than the 100 inch. He already understood two central problems: the manufacturing and, above all, thermal problems of larger massive (solid) glass blanks; and the problems of field coma with faster parabolic primaries. In 1921 Pease [5.20] made proposals for a 300-inch (7.5 m) telescope (Fig. 5.16). Ritchey moved in 1924 to Paris [5.19] and worked on lightweighted blanks, taking up the idea in glass proposed originally by Rosse for metal. He produced two 30-inch blanks [5.3] and proposed to use such mirrors (Fig. 5.17) for a vertical siderostat telescope (suggested by Foucault in 1869) using a 6 m plane mirror to feed a 5 m primary. His contact with Chretien had convinced him that the aplanatic form of the Cassegrain, which became known as the Ritchey-Chretien (RC) form, was the optimum for such a giant telescope. In 1927, while working in France [5.3] [5.19], he successfully made the first RC telescope, with the modest aperture of 0.5 m.

Returning to the USA in 1930, he concentrated on the manufacture of a larger RC telescope of 1 m aperture, discussed in Chap. 3. The data, as quoted by Bahner [5.21], reflected Ritchey's deep perception of the photographic

Fig. 5.17. George Willis Ritchey in Paris, 1927, with a built-up cellular mirror disk (courtesy D. E. Osterbrock, photograph by James Stokley)

requirements of the time with f/4.0 to f/6.8 (m2 = —1.7) giving an aplanatic field of 1.5° diameter. The value of Ra was about 0.4, so we have from Eq. (3.114) combined with (2.72)

a very high asphericity because of the low value of m2. This extreme form of RC, giving relatively high obstruction and increased difficulty of figuring and testing, was not chosen without good reason: Ritchey wished to achieve the highest speed possible in the RC focus for photography. Such an eccentric primary at f/4.0 was, at that time, at the limit of even his technological possibilities. The telescope was finished in 1934 and represented yet another brilliant technical achievement, effectively the last in his remarkable career before he retired in 1936 [5.19]. An excellent photograph of Ritchey with this primary is shown by Ingalls [5.22].

The consequences of Ritchey's advanced design were not fully understood at the time and the telescope did not produce notable photographic results until much later [5.19]. In 1971 the original optics were replaced by RC optics of essentially the same design, but made from ULE fused quartz to lower the thermal sensitivity, and with a substantially larger secondary to reduce the vignetting in the field which was, above all, a serious disadvantage for photographic photometry3. The centering tolerances of the Ritchey layout are appreciably more critical than for normal Cassegrain telescopes, which has sometimes led to the assumption that this is a general negative property of RC telescopes. This is not a correct interpretation, as we may see from Eq. (3.364) for the lateral coma (mm) produced by transverse decenter 5 (mm) of an RC telescope:

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