Modern telescope developments: pupil segmentation and techniques to reduce mass
In Chap. 5 of RTO I an account was given of the evolution of the reflecting telescope from the optical point of view from Lord Rosse, about 1830, up to about 1980. From about this time, the evolution of telescope optics, which had retained certain essenti
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3.1 Evolution and revolution in telescope optics In Chap. 5 of RTO I an account was given of the evolution of the reftecting telescope from the optical point of view from Lord Rosse, about 1830, up to about 1980. From ab out this time, the evolution of telescope optics, which had retained certain essential features ever since Galileo's telescopes in 1610, was supplanted by a revolution. A summary of this process was recently given by the author [3.1]. Up to about 1980, telescopes retained the following basic characteristics: - A nominally rigid and monolithic primary element (objective or mirror) - A nominally rigid or (following the Palomar 5 m telescope) a passively compensating structure holding the optical elements - A generally "passive" nature, whereby adjustments could only be made by off-line interventions Within this global framework, the evolution of telescope optics was above all represented by the physical appearance of the telescope. Figure 3.1 shows seven major telescopes corresponding to the state of the art at the time see Table 3.1. Their appearance depends essentially on the f/nos of their primaries. This evolution is shown graphically in Fig. 3.2. The systematic reduction after 1800 occurred over aperiod of 200 years because of progress in figuririg and testing techniques. Apart frorn aperiod of Table 3.1. Evolution of primary f/no in reflecting telescopes Telescope W. Herschel 48-inch Melbourne (Grubb) 48-inch Mt. Wilson (Ritchey) 60-inch ESO 3.6 m ESO NTT 3.5 m ESO VLT 4 x 8 m Columbus (now called LBT) 2 x 8m R. N. Wilson, Reflecting Telescope Optics II © Springer-Verlag Berlin Heidelberg 1999
Date of completion 1789 1869 1908 1976 1989 1998 ... 2001 ...
Primary f/no f/lO f/7.5 f/5.0 f/3.0 f/2.2 f/l.8 f/1.14
170
3. Modern telescope developments: segmentation and mass reduction
Fig. 3.1. The evolution of primary f/no in reflecting telescopes and its effect on
their appearance. From top to bottom and left to right as in Table 3.1 [3.1]
stagnation around 1940-1960, the fall in f/no has been an essentially monotonie function reaching the two values shown of about f/l.5 and f/l.O for the year 2000. Will this development go further? Opinions differ where the useful limit lies. The aspheric figuring required for a given Schwarzschild constant increases with the inverse cube of the f/no, while the space and mechanical stability gains diminish rapidly at extreme values. At f/0 .25 a spherical primary has an edge zone parallel to the axis. Beyond that it becomes theoretically impossible to obey the sine condition unless, with an aspheric primary, the final f/no is increased, as in the Cassegrain form. Probably Galileo would not have recognised the last three telescopes of Fig. 3.1 as telescopes at all,
3.1 Evolution and revolution in telescope optics
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Fig. 3.2. The evolution of primary f/no in reflecting telescopes as a function of time [3.1J
certainly not the last two. The first f
