# Chapter

2.1 The ideal optical system: the principal planes and unit magnification between them 23

2.2 Geometrical construction of ideal image formation 24

2.3 Geometrical wavefronts and rays 27

2.4 The relationship between the focal lengths 29

2.5 Derivation of the Lagrange Invariant 31

2.6 Aperture stop, entrance and exit pupils 32

2.7 Telecentric aperture stop 34

2.8 Gaussian optics of a conventional refracting telescope with ocular (afocal in both object and image spaces) 36

2.9 Image principal plane in the defocused telescope of Fig. 2.8, producing a real image at 12 39

2.10 Prime focus forms of reflecting telescope 41

2.11 Gaussian optics of a Gregory telescope 42

2.12 Gaussian optics of a Cassegrain telescope 43

2.13 Exit pupil position E in the Cassegrain form with the entrance pupil E at the primary 51

2.14 Entrance pupil position E in the Cassegrain form with the exit pupil E at the secondary 52

2.15 Limit case of a Cassegrain telescope with a plane secondary mirror 53

Chapter 3

3.1 Wavefront, longitudinal and lateral aberration 58

3.2 The Abbe sine condition 86

### 3.3 Normal representation of spot-diagrams 88

3.4 (a) Spot-diagrams for a classical Cassegrain telescope with the geometry of the ESO 3.5 m NTT (f/11; m2 = -5) for an optimum field curvature rc = -1955 mm

(concave to the incident light) 99

3.4 (b) Spot-diagrams for an RC aplanatic telescope with the geometry of the ESO 3.5 m NTT (f/11; m2 = -5)

for an optimum field curvature rc = -1881 mm 100

3.5 The function f (m2) = (m2+1Hm2-1)2 for DK telescopes. The left-hand curve refers to Cassegrain solutions, the right-hand curve to Gregory solutions, if the image is real .. . 104

3.6 Spot-diagrams for a DK Cassegrain telescope with the geometry of the ESO 3.5 m NTT (f/11; m2 = -5) for a flat field.

Compare with Fig. 3.4 where the field is 10 times larger 106

3.7 Spot-diagrams for an SP Cassegrain telescope with the geometry of the ESO 3.5 m NTT (f/11; m2 = -5), for a flat field. Compare with Fig. 3.6 with field 4 2 times larger and Fig. 3.4

with field 45 times larger 110

3.8 Karl Schwarzschild's first impractical telescope solution fulfilling four Seidel conditions [3.1] 113

3.9 Karl Schwarzschild's original aplanatic telescope (1905)

3.10 Spot-diagrams for the Schwarzschild telescope 1905 [3.1]

for an aperture of 1 m with f/3.0 118

3.11 Geometrical construction from the sine condition of the form of an RC telescope compared with a classical Cassegrain

3.12 The Couder (aplanatic) anastigmatic telescope (1926) [3.25] 124

3.13 Spot-diagrams for the Couder telescope (1926) [3.25]

for an aperture of 1 m with f/3.0 125

3.14 Third order spherical aberration as wavefront aberration 129

3.15 Third order spherical aberration: longitudinal and lateral forms . 130

3.16 Third order coma as wavefront aberration 132

3.17 Third order coma: lateral aberration form 132

3.18 Third order coma: the "coma patch" 133

3.19 Third order astigmatism: wavefront aberration reversal in the t- and s-sections due to the cos 2^ term 136

3.20 Third order astigmatism: astigmatic surfaces and lines 136

3.21 Tangential and radial astigmatic lines at the t-focus and s-focus respectively 137

3.22 Distortion: (a) barrel, (b) pincushion 139

3.23 Stop-shift effect for a single third order aspheric plate shifted from the pupil 141

3.24 Heights of the paraxial aperture and paraxial principal rays as they pass through a Cassegrain telescope 143

3.25 Fundamental form of a wide-field telescope without correction of spherical aberration 149

3.26 The Bouwers concentric telescope (1941) 151

3.27 The Schmidt telescope (1931) 151

3.28 Profile function (p4pl — kpippl) for Schmidt corrector plates with various values of the form profile parameter kpi.

The glass plate is formed by considering the area under the curves to be filled with glass down to an abscissa tangential to the curve in question. To the resulting axial thickness, the constant thickness (dpi)0 is added to give the necessary minimum plate thickness. (After Bahner [3.5]) 154

3.29 Spot-diagrams for the ESO 1 m, f/3.0 Schmidt telescope with the original singlet corrector plate. Optimum curved field of radius 3050 mm and ±3.20° field for 24 cm x 24 cm plates .. . 162

3.30 The dispersion function for a typical optical glass ( — = . 163

3.31 The effect of achromatisation: the dispersion function is rotated to minimize its slope, giving desired zero points and A2 164

3.32 The optical glass diagram (from the Schott Catalogue, courtesy Hans F. Morian and the Schott Glaswerke, Mainz) 165

3.33 Spot-diagrams for the ESO 1 m, f/3.0 Schmidt telescope with the achromatic (doublet) corrector plate (glasses UBK7 and LLF6). Optimum curved field of radius 3050.5 mm and ±3.20° field for 24 cm x 24 cm plates (format identical with Fig. 3.29, but scale five times larger) 166

3.34 The basic form of the Maksutov telescope (1944) 167

3.35 Spot-diagrams for the "short" Maksutov telescope of Table 3.11 with aperture 400mm and f/3.0 171

3.36 Effect of stop shift on transverse chromatic aberration C2

in a Maksutov meniscus: the "short" version (b) causes refraction and dispersion of the principal ray at the first surface, but the effect is largely (though not entirely) compensated at the second surface 172

3.37 Spot-diagrams for a "short" Maksutov telescope with aperture 400mm and f/3.0 optimized with an achromatic field flattener (Table 3.12) 173

3.38 The solid Schmidt camera in the direct form (a)

and folded form (b), with effective focal length f '/n' 175

3.39 The semi-solid Schmidt camera with effective focal length f '/n'. . 176

3.40 The Wright-Vaisala telescope (1935) 177

3.41 Spot-diagrams for a Wright-Vaisala telescope of aperture 400 mm and f/4.0 179

3.42 Schmidt-Cassegrain systems proposed by Baker (1940) 181

3.43 Monocentric (concentric) Schmidt-Cassegrain proposed by Linfoot (1944) 183

3.44 Spot-diagrams for a Linfoot monocentric Schmidt-Cassegrain with spherical mirrors and a singlet (non-achromatic)

3.45 Typical modern aplanatic Schmidt-Cassegrain for advanced amateur use with aperture 400 mm, f/2 - f/10 188

3.46 Spot-diagrams for the aplanatic Schmidt-Cassegrain system of Fig. 3.45 with 400 mm, f/2 - f/10, and an achromatic corrector plate 192

3.47 Spot-diagrams for a Slevogt aplanatic Schmidt-Cassegrain with 400 mm, f/2.0 - f/3.25, and an achromatic corrector plate.

The field is flat 194

3.48 Two-glass concentric (monocentric)

Bouwers-Cassegrain telescope 195

3.49 Spot-diagrams for an achromatic, monocentric

Bouwers meniscus-Cassegrain telescope as in Fig. 3.48, but with a singlet field flattener added. The geometry is lightly modified to 400 mm, f/3.11 - f/6.0, and the stop is shifted to the meniscus 197

3.50 Classical Bouwers telescope with additional weak lens at the stop 198

3.51 Spot-diagrams of a classical Bouwers telescope (prime focus) with weak achromatising positive lens in the pupil (400 mm, f/3).

3.52 Spot-diagrams of a classical Bouwers-Cassegrain with weak achromatising lens (400 mm, f/3.0 - f/6.0), as shown in Fig. 3.50 200

3.53 Maksutov-Cassegrain in "long" and "short" versions with secondary separated from the meniscus. Example with aperture 400 mm, f/3.5 - f/10.71 202

3.54 Spot-diagrams for the "short" version of the Maksutov-Cassegrain of Fig. 3.53 and Table 3.14 for an optimum curved field 203

3.55 A Maksutov-Cassegrain with secondary combined with the meniscus. Aperture 400 mm, f/3.5 - f/15.20 204

3.56 Spot-diagrams of the Maksutov-Cassegrain of Fig. 3.55 205

3.57 Hawkins-Linfoot Schmidt-Bouwers telescope with f/1.2

in the prime focus 208

3.58 Spot-diagrams for the Hawkins-Linfoot monocentric Cassegrain of Fig. 3.57, aperture 400 mm, f/3.0 - f/6.0 210

3.59 Baker Super-Schmidt, with f' = 200 mm, effective aperture ratio f/0.82 and field ± 26° 211

3.60 Baker-Nunn camera, designed for satellite tracking, aperture 508 mm, f/1.0 211

3.61 Double-meniscus system due to Wynne [3.59] (schematic) 212

3.62 Double-meniscus system due to Wynne with strongly asymmetric meniscus thicknesses [3.59] (schematic) 213

3.63 Buchroeder design of a Houghton-type correctorin Schmidt geometry (200 mm, f/3) given by Ruttenand van Venrooij [3.12(g)]214

3.64 Lurie design of a Houghton-type corrector in Wright-Vâisaiâ camera geometry (200 mm, f/4)

given by Rutten and van Venrooij [3.12(g)] 215

3.65 Spot-diagrams for a modified Lurie-Houghton design with aperture 400 mm at f/3.5 and geometry like the

3.66 Original Mangin system for searchlight projection 217

3.67 Dialyte telescope due to Plossl (1850) 219

3.68 Brachymedial due to Hamilton (1814) 220

3.69 Brachymedial due to Schupmann (1899) 220

3.70 The Medial telescope due to Schupmann 221

3.71 Compact system using Mangin secondary and Brachymedial geometry due to Delabre 222

3.72 Two 3-mirror anastigmatic, flat-field solutions proposed by Korsch (1972): (a) single-axis system, (b) 2-axis system 225

3.73 3-mirror system due to Paul (1935), see also footnote on p. 324

concerning Mersenne 227

3.74 The Willstrop Mersenne-Schmidt telescope with f/1.6 and a 4° diameter field (1984) 230

3.75 Baker 3-mirror, 2-axis anastigmatic telescope (1945) 231

3.76 Dual-purpose Newton telescope due to Loveday (1981) 232

3.77 3-mirror system proposed by Robb (1978) 239

3.78 3-mirror system given by Laux (1993) for a fast, flat-field 2.5m wide-field survey telescope with f/2.18 primary and f/4.0 final image, with a field diameter of 2.0° to 2.5° 239

3.79 3-mirror, 4-reflection telescope proposed by Korsch (1991)

for a future large space telescope 240

3.80 First solution of a 2-axis system with 4 powered mirrors (spherical primary and secondary) and a folding flat (f/1.5

and f/7.29), proposed by Wilson and Delabre (1993, 1995) 243

3.81 Spot-diagrams of the first, 2-axis solution of Table 3.19

and Fig. 3.80: (a) axis to ± 9 arcmin with circle 0.20 arcsec;

(b) ± 12 arcmin to ± 18 arcmin with circle 1.00 arcsec 245

3.82 First solution, 2-axis system as in Fig. 3.80, but with two identical "Nasmyth-type" foci 246

3.83 Second, 2-axis solution with 4 powered mirrors (spherical primary and secondary) and a folding flat (f/1.5 and f/6.01), proposed by Wilson and Delabre (1993, 1995) 247

3.84 Spot-diagrams of the second, 2-axis solution of Fig. 3.83:

(a) axis to ±9 arcmin with circle 0.2 arcsec

(b) ± 12 arcmin to ± 18 arcmin with circle 1.00 arcsec 248

3.85 Single-axis, 4-mirror system with f/1.2 - f/2.657

giving a field diameter of 1.50°. The primary is spherical 249

3.86 Spot-diagrams for the fast, wide-field, 4-mirror design of Fig. 3.85. The circle diameter is 1 arcsec 249

3.87 Single-axis, 4-mirror concept for a fast, wide-field telescope with improved field curvature 250

3.88 Single-axis, 4-mirror system using an afocal feeder and a spherical primary 250

3.89 A 2-axis system with 5 powered mirrors capable of a fast output beam (faster than f/3.0) and a flat field. The primary and secondary mirrors are spherical as in Fig. 3.83 251

3.90 A 2-axis solution with 4 powered mirrors proposed by Sasian (1990). Either Mi or M2 is spherical, M3 is toroidal . . 252

3.91 2-axis form of the system of Fig. 3.85 proposed by Baranne and Lemaitre (1986), the mirror pair M3M4 forming a corrector and focal transfer system with a magnification of -1

in the TEMOS concept, giving f/2.0 - f/4.5 - f/4.5 253

3.92 Double-Cassegrain 4-mirror telescope with intermediate image after M2, proposed by Korsch (1986) 254

3.93 The Kutter Schiefspiegler [3.103] [3.104] showing 3 solutions

(after Rutten and van Venrooij [3.12(j)]) 256

3.94 The basis of coma compensation in a Czerny-Turner monochromator 257

3.95 Schiefspiegler achieved by off-axis sections of a centered,

2-mirror telescope 261

3.96 Schiefspiegler interpretation of lateral decentering in a Cassegrain telescope 261

3.97 Strict case of lateral decenter in a 2-mirror telescope 263

3.98 Schiefspiegler with spherical primary and insensitive to lateral decenter [3.114] 277

3.99 Exit pupil (x,y,z) and image plane Z) coordinate systems. . 294

3.100 Fraunhofer diffraction at a rectangular aperture showing the function I = []2 (after Born-Wolf [3.120(b)]) 296

3.102 Fraunhofer diffraction at a circular aperture showing the function I = [Jw]2 (after Born-Wolf [3.120(b)]) 299

3.103 Fraunhofer diffraction at a circular aperture 6 mm in diameter, magnification 50 x, A = 579 nm. The central maximum has been overexposed to reveal the weak subsidiary maxima. (After Born-Wolf [3.120(b)] and Lipson, Taylor and Thompson, courtesy Brian Thompson) 300

3.104 Energy encircled in the radius w of the pattern due to Fraunhofer diffraction at a circular aperture. The fraction of energy is given by Lw = 1 — J02(w) — Jl(w) with w = kpmw. (After Born-Wolf [3.120(b)]) 301

3.105 The diffraction PSF at an annular aperture showing the effect of increasing the central obscuration factor e

(after Born-Wolf [3.120(c)] and G.C. Steward [3.132]) 303

3.106 Idealized sinusoidal ripple showing 3 complete wavelengths of ripple in the pupil radius 309

3.107 Transfer of a sinusoidal wave with reduced contrast through an optical system

(a) without phase shift, (b) with phase shift p 313

3.108 Shearing of a circular pupil corresponding to the calculation of the OTF from the autocorrelation function for shears of ARs, ARt 316

3.109 One-dimensional pupil shear to demonstrate the low bandpass filter function of an optical system 317

3.110 The MTF for a circular pupil free from aberrations and obstruction, corresponding to the diffraction PSF

with incoherent illumination 318

3.111 MTF for a circular pupil free of aberration with central obstruction e (after Lloyd [3.140]) 319

3.112 The MTF for a non-obstructed circular aperture with incoherent illumination in the presence of pure defocus aberration (after Hopkins [3.142]) 321

3.113 Normal form of supporting spider for secondary mirrors shown here without central obstruction of the secondary 323

3.114 Typical astronomical photograph of a star field where diffraction spikes appear on the bright star images. The galaxy is NGC 253, photographed with the 2.2 m telescope at La Silla with 40 m exposure. (Courtesy ESO) 323

Chapter 4

4.1 Ghost images through 2 reflections at a plane parallel plate 326

4.2 Transformation of a real corrector plate to a virtual plate in object space 328

4.3 Spot-diagrams for the PF Gascoigne plate-field flattener corrector (with filter) of the ESO 3.6 m telescope on La Silla. The Schwarzschild constant of the primary is bsl = -1.1567

for a quasi-RC solution 332

4.4 Schematic appearance of aspherics on PF plate correctors: (a) Gascoigne plate (singlet) corrector for RC hyperbolic primaries; (b) 2-plate corrector for strongly hyperbolic primaries correcting Sj = Sjj = Sjjj = 0;

(c) 2-plate corrector for parabolic or RC primaries correcting

^2>Sj = Sjj = 0; (d) 3-plate corrector for parabolic or RC primaries correcting Sj = Sjj = Sjjj = 0 338

4.5 Conjugate virtual plate in object space for a real aspheric plate at distance g in front of the Cassegrain focus 341

4.6 Spot-diagrams for a quasi-RC telescope with Gascoigne plate corrector and field flattener based on the geometry of the ESO 3.6 m telescope (f/3 - f/8) . . . 346

4.7 The 3-lens Ross corrector for the Mt. Palomar 5 m, f/3.3 parabolic primary (schematic, after Wynne [4.5] [4.24]) 353

4.8 Reflector-corrector due to Baker [4.27] 357

4.9 Wynne design for a 4-lens corrector of the Palomar 5 m, f/3.34 paraboloid. The cross shows the focus of the naked primary

4.10 Spot-diagrams for the Wynne design of Fig. 4.9: (a) on axis, (b) 6 arcmin off-axis, (c) 9 arcmin off-axis, (d) 12.5 arcmin off-axis. The circle has a diameter of 0.5 arcsec.

4.11 3-lens corrector for paraboloids using one aspheric by Faulde and Wilson [4.30] 360

4.12 Spot-diagrams for the 3-lens corrector of Fig. 4.11.

The circle represents 0.5 arcsec. (a) Basic focus 361

4.12 Spot-diagrams for the 3-lens corrector of Fig. 4.11.

The circle represents 0.5 arcsec. (b) Focus shift +0.05 mm,

4.13 3-lens corrector by Wynne [4.14] [4.5] for the Kitt Peak 3.8 m, f/2.8 - f/8 RC telescope. All three lenses are of UBK7.

(After Wynne) 364

4.14 Spot-diagrams for two correctors for the ESO 3.6 m quasi-RC telescope: (a) the basic plate system of Kohler with field flattener, recalculated without vignetting;

(b) the Wynne-type lens corrector also recalculated without vignetting. (After Cao and Wilson [4.16]) 365

4.15 3-lens prime focus corrector designed by Richardson et al. [4.34] for the then proposed 7.6 m, f/2 primary of University of Texas. (After Richardson et al.) 367

4.16 3-lens PF corrector for the 10 m Keck primary

4.17 Spot-diagrams for the system of Fig. 4.16 for two of the four spectral bands given (after Epps et al. [4.36]) 369

4.18 Spot-diagrams for the new Delabre prime focus (f/3.0) corrector for the ESO 3.6 m telescope using two glasses, Schott FK5

and fused silica (1996) 371

4.19 Spot-diagrams for the singlet lens corrector of the ESO 3.6 m quasi-RC telescope (after Wilson [4.15]). Circle 0.18 arcsec 383

4.20 Three-element corrector for an f/2.00 to f/5.28 classical Cassegrain designed by Epps et al. [4.36] for a 300-inch telescope, giving f/6.00 with the corrector 385

4.21 Spot-diagrams for the design of Fig. 4.20 for a field of 1° diameter (Epps et al. [4.36]) 386

4.22 Spot-diagrams for the 2.2 m MPIA telescope as quasi-RC using a 2-lens corrector of one glass (quartz) (after Wilson [4.15]).

### Circle 0.47 arcsec 387

4.23 Spot-diagrams for the 2.2 m MPIA telescope as strict RC using a 2-lens corrector of 2 different glasses (PK50 and BaF3) (after Wilson [4.15]) Circle 0.48 arcsec 388

4.24 Spot-diagrams for a doublet corrector using a single glass designed by Su, Zhou and Yu [4.51] for the Chinese 2.16 m, strict RC telescope with f/3tof/9 389

4.25 Spot-diagrams for a three-element corrector designed by Epps et al. [4.36] for an f/1.80 - f/4.50 Cassegrain system using a significantly hyperbolic primary (bs1 = -1.152 310 5) 391

4.26 Dispersion variation by opposite rotation of two prism pairs 394

4.27 Performance of the ADC designed by Wynne and Worswick for the William Herschel 4.2 m telescope with Z = 70°.

Curve A shows the uncorrected atmospheric dispersion; curve B the correction achieved with the glasses used (UBK7 and LLF6); curve C what could be achieved with FK50 and Calcium Fluoride. (After Wynne and Worswick [4.60]) 395

4.28 Configurations of prisms for maximum dispersion setting, in cases 1 and 2 (angles exaggerated) for ADC

in the prime focus (PF). (After Wynne and Worswick [4.62]) 396

4.29 (a) Spot-diagrams, reproduced from Wynne and Worswick [4.62], for their PF ADC for the 4.2 m, f/2.5 William Herschel Telescope.

(a) Zero dispersion setting 397

4.29 (b) and (c) Spot-diagrams, reproduced from Wynne and Worswick [4.62], for their PF ADC for the 4.2 m, f/2.5 William Herschel Telescope. (b) At ± 45°

and (c) maximum dispersion setting ± 90° 398

4.30 Section through the complete ADC-corrector system for the

4.2 m, f/2.5 PF of the WHT, reproduced from Bingham [4.63]... 399

4.31 "Lensm" design of Su [4.64] for an ADC integrated into a doublet corrector for a 5 m, f/2 to f/4.5 strict RC (Cassegrain) focus.

The glasses are from the Chinese glass catalogue 399

4.32 Two types of lensm corrector designed by Wang and Su [4.65]

for the PF of a 7.5 m, f/2 paraboloid 400

4.33 Points in the field for the calculation of spot-diagrams, from Wang and Su [4.65] 400

4.34 Spot-diagrams for lensm corrector type I for a field diameter of 45 arcmin with a 7.5 m, f/2 paraboloid. Rotation angles of the lensms are 0°, 0°, i.e. zero dispersion. Circle diameter = 1 arcsec. Reproduced from Wang and Su [4.65] 401

4.35 Spot-diagrams for lensm corrector type II for a field diameter of 45 arcmin with a 7.5 m, f/2 paraboloid. Rotation angles of the lensms are 0°, 0°, i.e. zero dispersion.

Circle diameter = 1 arcsec. Reproduced from Wang and Su [4.65] 401

4.36 Optical design of the LADC - schematic

(after Avila, Rupprecht and Beckers [4.78]) 402

4.37 Basic design (schematic) for an FR without intermediate image for a field of 0.9° diameter at the Cassegrain (RC) focus of the 3.5 m MPIA, f/3 to f/8 telescope [4.15]. mFR = 1/2.67 ... 407

4.38 Spot-diagrams for the FR system of Fig. 4.37 [4.15].

The circle is 0.98 arcsec = 50gm 408

4.39 Basic design (schematic) for an FR with intermediate image for a field of 0.9° diameter at the Cassegrain (RC) focus of the 3.5 m MPIA f/3 to f/8 telescope [4.15]. mFR = 1/2.67 ... 409

4.40 Spot-diagrams for the FR system of Fig. 4.39 [4.15].

The circle is 0.98 arcsec 410

4.41 Basic focal reducer geometry using a Schmidt-based mirror system for the 3.5 m MPIA telescope.

Reduction is f/8 to f/1.7 (mFR = 1/4.71) [4.15] 410

4.42 Spot-diagrams for a focal reducer f/8 to f/1.7 designed for the 3.5 m MPIA telescope for a field diameter of 1°. Doublet corrector (corrected intermediate image), one field lens and a Hawkins-Linfoot camera. Circle = 0.50 arcsec = 14 ^m. Image radius 1031 mm [4.15] 412

4.43 Spot-diagrams for a focal reducer f/8 to f/1.7 designed for the 3.5 m MPIA telescope for a field diameter of 1°. Doublet corrector (uncorrected intermediate image), 2 field lenses and Baker-type camera with 2 menisci and one plate.

Circle = 0.50 arcsec = 14 ^m. Plot field for axial spot-diagrams = 10 ^m. Image radius = 1052 mm [4.15] .. . 413

4.44 Focal reducer designed by Meinel et al. [4.72]

for the Texas 7.6 m telescope project. The f/13.5 Nasmyth focus is converted to f/3.0 over a field of 8 arcmin diameter 415

4.45 INCA (Inverted Cassegrain) focal reducer proposed by MacFarlane [4.73] 415

4.46 FR concept using a field mirror proposed by MacFarlane [4.73] . . 416

Chapter 5

5.1 90 cm lightweighted, built-up blank made by Lord Rosse in 1839 (courtesy Rolf Riekher) 420

5.2 Whiffle-tree support system in 4 stages designed by Thomas Grubb for the Rosse 6-foot reflector completed in 1845 (courtesy Rolf Riekher) 421

5.3 Lord Rosse's 6-foot (1.82 m) telescope completed in 1845 (courtesy Deutsches Museum, Munich) 422

5.4 Original single astatic counterweight described by Lassell in 1842

[5.7] (reproduced from Danjon and Couder [5.1]) 423

5.5 Lassell's 1.22 m telescope set up in 1861 in Malta

(reproduced from the original plate of [5.8]) 424

5.6 James Nasmyth's 20-inch Cassegrain-Nasmyth telescope about 1845 (reproduced from King [5.2]) 425

5.7 The 4-foot (1.22 m) Melbourne reflector erected in 1869 (reproduced from King [5.2]) 427

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) 428

5.9 Foucault's largest (80 cm) silver-on-glass reflector, completed in 1862 (reproduced from King [5.2]) 430

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) 433

5.11 Ritchey's 60 cm telescope at Yerkes, 1901

(courtesy Deutsches Museum, Munich) 434

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) 436

5.13 Ritchey's 60-inch Mt. Wilson reflector: optical arrangement (courtesy Rolf Riekher) 437

5.14 Ritchey's 60-inch Mt. Wilson reflector, 1908 (courtesy Donald Osterbrock and the Observatories of the Carnegie Institute of Washington) 439

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) ... 441

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) 442

5.17 George Willis Ritchey in Paris, 1927, with a built-up cellular mirror disk (courtesy D. E. Osterbrock, photograph by James Stokley) 443

5.18 The primary of the 200-inch Mt. Palomar telescope in testing position with John A. Anderson (left) and Marcus H. Brown (courtesy Palomar/Caltech) 446

5.19 The Palomar 200-inch (5 m) telescope as drawn by R.W. Porter (courtesy Palomar/Caltech) 448

5.20 The Russian 6 m telescope at the Zelenchuk Observatory in the Caucasus (courtesy "Ciel et Espace", Paris, through Serge Brunier) 451

5.21 The 4.0 m Kitt Peak telescope

(courtesy National Optical Astronomy Observatories, Tucson) . . . 458

5.22 The 3.6 m ESO telescope (courtesy ESO) 459

5.23 The building of the 3.6 m ESO telescope with the smaller building of the 1.4 m Coude Auxiliary Telescope (CAT)

on La Silla (2400 m) (courtesy ESO) 460

5.24 The 4.2 m William Herschel Telescope (WHT) with its Alt-Az mounting on La Palma (Roque de los Muchachos 2400 m) (courtesy Royal Greenwich Observatory, through Richard Bingham and Peter Andrews) 461

5.25 The original optical layout of the "Universal Telescope"

of the Karl-Schwarzschild Observatory at Tautenburg, Germany (courtesy Rolf Riekher) 462