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Fig. 4.12a. Spot-diagrams for the 3-lens corrector of Fig. 4.11. The circle represents 0.5 arcsec. (a) Basic focus

As we shall see, the quality is fully comparable with the 3-lens RC primary corrector (without an aspheric) of Wynne, calculated for a similar field and mirror geometry. Faulde and Wilson concluded that the aspheric surface in the paraboloid corrector can replace one lens in the all-spherical 4-lens solution. The aspheric is a moderate ellipse with a Schwarzschild constant from Eq. (3.11) of -0.0619, but the higher order terms show appreciable departures from the series corresponding to the above ellipse. Relative to a sphere with the same curvature at the pole, the asphericity at the edge of the lens is about 0.22 mm, but this would be much reduced by another reference sphere. The aspheric form emerges above all from the control of fifth order coma. Whether some exactly elliptical form would be adequate would require further investigation. In view of aspheric testing with null-systems, it does not follow that an exact ellipse is the best form for manufacture. For ease of testing, it was considered essential to apply the aspheric to a concave surface. This aspheric assumes a quite different role from that of the overcorrection of an RC primary because the relative pupil position (E-value) and significant incidence angle of the principal ray cause it to influence the field aberrations as well as the axial spherical aberration.

It should be noted that the front lens of the Faulde-Wilson 3-lens design is larger, relative to the primary, than that of the Wynne 4-lens system. For

362 b

Correct Lens Wynne Corrector
Fig. 4.12b, c. Spot-diagrams for the 3-lens corrector of Fig. 4.11. The circle represents 0.5 arcsec. (b) Focus shift +0.05 mm, (c) focus shift -0.05 mm

the 3.5 m MPIA primary and an unvignetted field diameter of 0.982°, the free diameter of the front lens was 471.2 mm. However, its thickness of about 50 mm gives it a robust form. The corrector has slight negative power, the case with most such correctors: it converts the relative aperture from f/3.0 to f/3.35.

For the same reasons given for the Wynne 4-lens corrector, this corrector is also limited by the higher order chromatic aberrations. The role of the aspheric, relative to the size of the corrector, is further discussed in the next section.

4.3.1.3 Modern correctors for RC primaries. The invention of Paul [4.4] to use a Ross-type afocal doublet and correct the spherical aberration with the RC primary is still valid today as a modern solution for fields up to the order of 30 arcmin diameter. Reference was made above to Rosin's design [4.26] with a hyperboloid, who used a doublet of two glasses to reduce the lateral chromatic aberration C2 in the sense discussed by Ross (see above). Small longitudinal chromatic aberration Ci had to be accepted. Rosin aimed to achieve 0.1 arcsec resolution over a field of 1° diameter (or more!). He claimed this quality was attained over the 1° diameter field for the wavelengths 589, 656 and 486 nm. Unfortunately, he did not give spot-diagrams in the conventional form. Extension to extreme wavelengths 1014 and 365 nm places much greater chromatic demands on the system. More significantly, he assumed a primary of 2 m at f/12.5 and with a Schwarzschild constant bs1 = —2.49! This has little relevance for modern RC telescopes and must be seen as a special telescope of exceptional length.

In general, doublet correctors for RC primaries will be of more interest if of the single glass type. An excellent example is the doublet corrector of two quartz lenses designed by Wynne [4.14] [4.5] for the 105-inch, f/4 RC primary of the McDonald telescope with m2 = —2.25. Spot-diagrams are given for a maximum field diameter of 28 arcmin. They are within 0.5 arcsec for the wavelength range 770 to 365 nm except for the lateral chromatic aberration C2. If the axial chromatism Cl is corrected, it is the lateral chromatism that limits these doublet systems. This is a consequence of the fact that no compensation is possible by a further element or elements such as can take place with triplet correctors.

For many purposes, the corrected field with the doublet will be satisfactory; but for classical photography or multi-fibre spectrographs wider fields are desirable. In the 1960's, when the fundamental work was done, PF photography with baked plates was still a significant demand.

The classical work on the application of 3-lens correctors to RC primaries was performed by Wynne [4.24] [4.14] [4.5]. In 1965, Wynne [4.24] investigated such a corrector for the Kitt Peak 3.8 m RC telescope with an f/2.8 primary. Five different designs were considered, the best consisting of a front doublet with leading negative lens and a rear cemented triplet which was

Image

Fig. 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)

monochromatically effectively a single lens, but whose centre lens was of a high dispersion glass. The front lenses were also in different glasses. The spot-diagrams are shown for a field diameter of 30 arcmin and are mainly within 0.5 arcsec. Secondary spectrum effects of the lateral chromatic aberration C2, due to the different glasses, are significant at the edge of the field.

In his 1968 paper [4.14], Wynne described a new design for the Kitt Peak telescope for a field of 50 arcmin diameter. The magnification had been slightly reduced compared with the original proposal (f/2.8 to f/8 instead of f/9). This increased the eccentricity of the primary and favoured the corrector design. It is a three-lens system of one glass as shown in Fig. 4.13 and has become the standard basic design for RC primaries. In view of the inevitable limitations of higher order chromatic aberrations, the spectral range was split over 2 interchangeable correctors in order to increase the field coverage at high quality. Spot-diagrams are within 1 arcsec for the field diameter of 50 arcmin and a spectral range of 486 to 405 nm.

Wynne applied the same basic design to a corrector for the ESO 3.6 m, f/3-f/8 quasi-RC telescope. Because the primary of this telescope had been optimized for use with a singlet corrector at the Cassegrain focus (see below), its eccentricity was higher than the strict RC form would give, favouring the corrector. Even the strict RC form would have been more favourable than the Kitt Peak telescope because m2 = —2.67 instead of —2.86 for Kitt Peak, apart from the advantage of the somewhat smaller relative aperture. Such a higher eccentricity would still not be sufficient to compensate the spherical aberration of a Ross-type doublet placed similarly to the front lens of these 3-lens designs; but it relaxes the amount of spherical aberration the front lenses have to compensate. The rear positive lens compensates the astigmatism of the front pair, as well as the lateral chromatic aberration generated by their separation. Data for this corrector are given in [4.14]. Wynne [4.5] compares this 3-lens corrector with the design using 3 aspheric plates and a field flattener given by Kohler [4.12] [4.13], discussed in §4.2.1.3 above. Wynne's comparative spot-diagrams show that his lens system gives superior performance over the entire field and spectral range specified. This is confirmed by the comparison of Cao and Wilson [4.16], where the plate and lens systems are compared without vignetting for the wide spectral range

Fig. 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)

from 334 - 1014 nm. This comparison is shown in Fig. 4.14. Above all, the 3-lens system is superior in the higher order chromatic aberrations, particularly chromatic difference of coma. This arises because the lenses have 2 parameters, power and bending, compared with essentially 1 parameter

Correct Lens Wynne Corrector
Fig. 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])

for the plates, the asphericity. Even the monochromatic performance of the plate system is slightly inferior, although this is not the limitation in either system. The only advantages of the plate system would be its ghost images, as discussed in § 4.1, and the insensitivity of plates to flexure through sag effects.

In connection with the ESO 3.6 m design, a 3-lens corrector with opposite power distribution to that of Wynne, namely negative-positive-negative, was investigated by Baranne [4.31] [4.32], using one aspheric surface. Its performance was less good than the Wynne form, which appears to be the definitive solution.

Wynne [4.5] [4.14] also gives results for a 4-lens corrector, similar to his system for a paraboloid shown in Fig. 4.9. He reported a modest reduction of the largest skew aberrations to about 75%. A similar level of improvement was obtained by adding an aspheric to each of the three lenses of the triplet corrector, with 2 powers of figuring. Little improvement was obtained with less than three aspherics. Wynne concluded that these gains were too modest to justify the additional complication. Wilson [4.15] approached this aspect from the opposite direction by introducing weak lens power with one plane surface into the 3-aspheric plate solution, discussed above. This produced considerable improvement, but the final system of 3 weak aspheric lenses was no better than the Wynne all-spherical 3-lens corrector.

This matter was investigated by Cao and Wilson [4.16]. Experience with the Wynne-type 3-lens corrector for the ESO 3.6 m quasi-RC telescope had revealed flexure problems for the first two lenses. The chromatic performance improves with reduced thicknesses, so these had been held as small as possible. The authors' aim was to thicken these elements and to attempt an improvement of the higher order chromatic aberrations by introducing as-pheric surfaces and, possibly, a fourth lens. This work essentially confirmed the trends reported by Wynne. The best system had 3 aspherics (one on each lens) with 2 powers of figuring plus a fourth lens to flatten the field. The front thicknesses were significantly increased: the second lens was certainly more resistant to flexure, the first lens probably so, but this required more investigation. The results confirmed conclusions drawn by Richardson, Harmer and Grundmann [4.33] [4.34], who also achieved improved performance through bigger correctors with an aspheric. Allowing a front lens some 62% larger relative to the primary compared with the Wynne-type corrector of the ESO 3.6 m telescope and using an aspheric on the second lens, Richardson et al.

[4.33] attained significantly improved performance in a corrector for the 3.9 m Anglo-Australian telescope (AAT). A similar system was designed by Henneberg at Carl Zeiss for the 3.5 m MPIA telescope [4.35]. Richardson et al.

[4.34] also showed that successful designs are possible with fast primaries of f/2.0. An example is given for the proposed 7.6 m telescope of the University of Texas - Fig. 4.15.

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