[5.3] [5.44] [5.45]
3 The original blank of the Isaac Newton Telescope (INT) primary was made by Corning in 1936 for the Michigan Observatory in the course of work initiated for the Palomar 200-inch telescope, but the proposed telescope was never built. The blank was presented to the INT project in 1949 [5.33]. I am extremely grateful to Dr. R. Bingham for information about this blank and confirmation that it was massive. He also kindly drew my attention to the publication [5.34], the most complete on the original INT.
meant the photon efficiency of a 50 cm telescope equipped with a CCD can rival the Palomar 5 m with classical plates!
Site selection also made a major advance. The sites in Chile established 1 arcsec as the criterion of "good" seeing. As a result, the specification of all the Bowen-type telescopes was d80 ~ 0.5 arcsec at the Cassegrain focus. Polishing and test techniques were developed to deliver this quality even with primaries in the range f/3.0 - f/2.5.
Apart from the WHT, because of its more advanced Alt-Az mount, none of the 6 standard Bowen-type telescopes listed in Table 5.2 has any major feature, either optical or mechanical, which distinguishes it from the others. However, such telescopes allowed the Europeans to recover their position, in both telescope technology and observation, which they had lost to the USA at the turn of the century. As arbitrary examples, two of these telescopes are shown here, the 4.0 m Kitt Peak telescope (Fig. 5.21; see also Pasachoff [5.49]) and the 3.6 m ESO telescope (Fig. 5.22). The building-dome concept of all these telescopes was also similar, following closely the Palomar 5 m tradition (see Fig. 5.23).
The Bowen telescopes [5.30] were originally conceived for photography with plates at the Cassegrain (RC) focus. In fact, although most of them had provision for such photography, very little of it has been done at these foci because photography at the PF at about f/3 was some 6 to 10 times more rapid for extended objects. PF correctors have therefore had much more significance for these telescopes than Cassegrain correctors. As we discussed in detail in Chap. 4, PF correctors are rendered easier by the overcorrection of the RC primary; but modern RC designs with high values of m2 have only small departures from the parabolic form, so this advantage is no longer very significant. This situation has led to statements, e.g. by Learner [5.50], that the RC telescope had run its course by 1980 after a relatively brief 50 year period of partial supremacy over the classical Cassegrain. This is a view I personally do not share, since the RC form is no more difficult to manufacture with modern techniques and the field quality has technical advantages in active telescopes (see RTO II, Chap. 3), apart from the astronomical significance of better correction in the field for rapidly increasing electronic detector sizes.
The 4.2 m WHT (Fig. 5.24) followed this trend in the Anglo-Saxon world and has the classical Cassegrain form. It was the last, and most notable telescope to be built by the famous firm of Grubb-Parsons, which ceased operations after its completion about 1985, thus ending a tradition going back to William Parsons (Lord Rosse) and Thomas Grubb.
A number of large Schmidt telescopes were also built in this period. The Palomar Schmidt with 1.22 m aperture of the corrector was completed in 1948 and was a notable advance in size. The mirror has a diameter of 1.80 m and works at f/2.5. In 1960 the 1 m Schmidt telescope for the Bjurakan
Observatory in the USSR was completed with a mirror working at f/2.1. The nominal fields of these telescopes were 5° to 6° diameter.
The largest Schmidt telescope yet built emerged from the difficult postwar period in the (then) GDR. The concept was laid down by Kienle [5.3] in 1949 for a "Universal Telescope" combining the properties of Schmidt and classical telescope conceptions, since the means for building separate instruments were not available at that time. Figure 5.25 shows the original concept. The Schmidt corrector plate has a diameter of 1.34 m free aperture, the largest yet made. The glass is UBK7 and the thickness 40 mm. The spherical primary has a focal length of 4 m giving an f/3 Schmidt telescope with a field of 5° diameter. The Schmidt plate can be removed allowing, in the original concept, an f/2 PF with a doublet corrector (never realised)
for the spherical primary, an f/11 Nasmyth focus and an f/46 coude. The telescope became operational in 1960 and has been very successful within the climatic limitations of its site. Since the spherical form of the primary was determined by the Schmidt concept, the two Cassegrain foci are of the "spherical primary" (SP) type treated in § 22.214.171.124. This solution has very large field coma, but is acceptable for use with spectrographs provided the slits are accurately placed on the optical axis.
Complex mechanics was required to keep the telescope in adjustment in spite of the radical changeovers operated. This subject will be dealt with in connection with active telescopes (RTO II, Chap. 3). Such a universal concept was a legitimate design in its time but is unlikely to be repeated, although active control would make its realisation easier.
The optical data of Table 5.2 and otherwise in this chapter is intended to give the reader a guide to the larger and most notable telescopes. The literature includes many lists of telescopes, many of them in older books such as Danjon-Couder [5.1] and Dimitroff-Baker [5.17]. An excellent general source of up-to-date information is Riekher [5.3]. A very useful listing of basic data is given by Gochermann and Schmidt-Kaler [5.51]. The most complete listings by far, above all for the optical properties, are by Bahner [5.52] and Wolf [5.53], also Kuiper and Middlehurst [5.31]. Bahner lists all existing reflectors of aperture > 0.9 m for normal telescopes, > 0.5 m for Schmidt telescopes and > 0.4 m for meniscus telescopes. The follow-up work by Wolf lists all normal telescopes > 1.2 m and wide-field telescopes > 0.6 m. Another very useful listing from the optical viewpoint is given by Schielicke [5.54].
The MPIA 3.5 m telescope, which went into operation in 1985 after 15 years of development and manufacturing time [5.43], was the last of the equatorially-mounted Bowen-type telescopes. With its Palomar type mounting, it was also the most massive and most expensive. In spite of the beauty of the optics and mechanics of this telescope, it was by that time generally agreed that telescopes of this size could be built more cheaply and effectively with more modern mounts (usually Alt-Az) and new optical and building concepts made possible by modern electronics. These modern developments are the subject of RTO II, Chap. 3. In fact, a copy of the MPIA 3.5 m telescope was built by Carl Zeiss for the proposed observatory in Iraq. For obvious reasons, this instrument has never been installed.
The development of the reflecting telescope in the period covered by this chapter is beautifully illustrated by the work of Fehrenbach [5.55]. This work only covers developments in France and has no pretensions to scientific or technical comprehensiveness; instead, it deals with the background and personalities involved, including the period Ritchey spent in France, in a personal and most revealing way.
A. List of mathematical symbols Chapter 2
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