SPECTRUM

Technical Field

The present invention relates generally to co\or-corrected optical systems. More particularly, this invention relates to refractive objective lenses with reduced residual longitudinal chromatic aberration. Such lens systems are particularly suitable for use in telescopes.

Background Art

The optical properties of a lens depend on the refractive index of the optical material. The dispersion of optical material causes the properties of a lens vary with wavelength of the light transmitted by it. Effect of the dispersion is to produce various chromatic aberrations. Most refractive optical systems deal with white light and, therefore, suffer from chromatic aberrations. One of the most difficult aberration to control is secondary longitudinal chromatic aberration.

It is conventional to specify the dispersion properties of the optical material by its Abbe number and partial dispersions. Manufacturers usually provide refractive indexes, Abbe numbers and partial dispersions of optical materials. The crown glass has relatively low dispersion and high Abbe number. The flint glass has relatively high dispersion and low Abbe number. Optical glass catalogs also provide plots of a relative partial dispersion versus Abbe number. In such plot most glasses fall close to a nearly straight line known as Abbe normal line or normal glass line. These glasses are called normal glasses or glasses with normal partial dispersions. Many of them are inexpensive, easy to fabricate and readily available. The glasses that fall significantly away from the normal glass line are

the glasses with anomalous partial dispersion, also called anomalous dispersion glasses or abnormal glasses.

To correct longitudinal chromatic aberration in the refractive telescope it is usually necessary to have at least two lenses made of optical materials with different dispersion. By combining lenses made of different optical materials it is possible to correct the longitudinal chromatic aberration for two wavelength. Such kind of lens systems is called achromatic. The typical example is an achromatic doublet consisting of two lenses made of crown and flint glasses. However, an achromatic lens brings only two given colors to the same focus. The focal points of other wavelengths do not coincide with the common focal point of the two colors mentioned above. The residual color error at wavelengths other than these two is called secondary longitudinal chromatic aberration, secondary spectrum or secondary color. The secondary spectrum of a typical achromat is about 0.05% of the focal length over the spectral range from Fraunhofer spectral line F (486 nm) to spectral line C (656 nm). Secondary spectrum often is a limiting aberration in refractive telescopes, large photographic objectives, collimators and other refractive optical systems. There are different strategies in controlling of this aberration. Almost ail conventional refractive optical systems with corrected secondary spectrum require the use at least one optical material having anomalous partial dispersion. These anomalous dispersion optical materials are abnormal glasses, crystals or special optical liquids. In this way it is possible to obtain color correction for three or more wavelengths with very small residual chromatic aberrations. Such kind of color correction is called apochromatic. There are different levels of the secondary spectrum correction. Lens systems with a reduced but still significant secondary spectrum is sometimes called semiapochromatic. The secondary spectrum in semiapochromatic telescopes is reduced at least by half, when compared with a typical achromat. The term "corrected secondary spectrum" is used herein in the wide sense, including not only the elimination of secondary spectrum, but the significant reduction of secondary spectrum as well.

The correction of the secondary spectrum in conventional apochromatic doublets is obtained by appropriate choice of optical glasses. The basic idea is to choose optical materials with equal, or close to equal, relative partial dispersions. On the other hand, Abbe numbers of these optical materials should be as different as possible in order to

avoid steep surface curvatures. However, the normal glasses approximately satisfy the linear relationship between relative partial dispersions and Abbe numbers. For this reason, the pair of normal glasses can not be used in apochromatic doublet. At least one lens of apochromatic doublet should be made of optical material with anomalous partial dispersion. This rule is also applied to more complex conventional apochromatic lens systems.

Probably the first practical apochromatic telescope objective was suggested by H. D. Taylor in 1892 . His air spaced triplet was disclosed in British patent 17,994/1892. In the original design an anomalous dispersion flint glass was used. The main disadvantage of Taylor design is that the curvatures of interior refractive surfaces are very steep. Taylor apochromatic telescopes are very long since a typical relative aperture is about f/18.

In the middle of the 20th century dense flint glasses were developed. Dense flints have high refractive index, low Abbe number and many of them display slightly anomalous dispersion. By using these optical glasses, it became possible to build so-called dense flint apochromats with a typical f-number of F/15 (Reference. A. Kόnig and H. Kόhler, Die Fernrohre und Entfernungsmesser, published by Springer Verlag in 1959, p. 135, nr. 12).

Better performance can be obtained with the use of short flint glasses. These abnormal glasses are widely used in various apochromatic lenses. However, short flints are very expensive and sensitive.

Nowadays, most of apochromatic telescopes utilize so-called fluor-crowns. This is another kind of special optical glasses with highly anomalous dispersion like Schott FK51, FK54, FK56, Ohara FPL51, FPL52, FPL53 glasses or similar products of other manufacturers. Typical fluor-crown apochromatic telescopes have relative aperture about F/7. Alternatively, it is possible to obtain even better optical performance with use of fluorite (calcium fluoride). This crystalline material has large anomalous dispersion.

It is also possible to combine different kinds of optical glasses having anomalous dispersion in order to design apochromatic lens. The example of such apochromatic triplet is given by Michael J. Kidger in Intermediate Optical Design, SPIE Press (2004), pp.104-105.

However, there are a lot of problems with anomalous glasses and crystals. These optical materials are extremely expensive, unobtainable in large pieces, fragile and difficult

to work with. In addition, it is difficult to fabricate anomalous dispersion materials with high homogeneity. For these reasons, in general, the use of normal glasses is preferable. Thus, a need exists in the art for lens systems with corrected secondary spectrum without the use of anomalous dispersion glasses. For a long time it had generally been accepted that the elimination or considerable reduction of the secondary spectrum in refracting optical systems requires the use of optical materials having anomalous dispersion. However, in 1955 E. L. McCarthy in United States patent 2698555 disclosed optical system with corrected secondary spectrum by using only normal glasses. Years later, in 1977, CG. Wynne published the paper "Sec- ondary spectrum correction with normal glasses" in Optics Communication 21 (3). In the next 1978 year CG. Wynne published the paper "A comprehensive first-order theory of chromatic aberration secondary spectrum correction without special glasses" in Optica Acta 25 (8). In these papers CG. Wynne explained new extended theory of first-order chromatic aberration. According with this theory the correction of secondary spectrum is possible without abnormal glasses. CG. Wynne also described example of such optical system. These achievements of McCarthy and Wynne demonstrate the possibility of secondary spectrum correction by using only normal glasses.

McCarthy's patent depicts optical system with corrected secondary spectrum having a zero-power doublet with over-corrected longitudinal chromatic aberration is placed a long distance in front of positive doublet with under-corrected longitudinal chromatic aberration. The separation between the doublets is given approximately by the focal length of the second doublet.

The lens system disclosed by CG. Wynne comprises a multi-lens corrector placed in front of an achromatic doublet. The corrector comprises widely separated components and it has substantially zero optical power at the mean wavelength. The original Wynne design is relatively complicated.

Although McCarthy and Wynne designs look different, in fact they are very close. In both of these designs, substantially afocal correctors are placed in front of doublets and the power of the lens system is contributed by the last component. Moreover, in both of these designs, components are widely separated. Additional discussion of McCarthy and Wynne designs can be found in a number of publications, including: M. Rosete- Aguilar, "Correction of secondary spectrum using standard glasses", SPIE Proc, Vol.

2774 (1996), pp.378-386; M. Rosete-Aguilar, "Application of the extended first-order chromatic theory to the correction of secondary spectrum", Revista Mexicana de Fisica, 43, No. 6 (1997), pp.895-905; Michael J. Kidger, Intermediate Optical Design, SPIE Press (2004), pp.109-112. While in both McCarthy and Wynne designs secondary spectrum is indeed reduced without resort of abnormal glasses, many other harmful aberrations remain. These aberrations limit applicability of the lens systems to low apertures if diffraction limited performance is assumed. For this reason, telescopes designed according McCarthy patent or Wynne papers are generally not practical.

Disclosure of Invention

Technical Problem

The technical problem to be solved by the present invention consists in providing a lens system with corrected secondary spectrum by using inexpensive glasses. It is another object of the present invention to provide a color-corrected lens system with relatively high relative aperture.

Technical Solution

The lens system with corrected secondary spectrum of the present invention comprises three widely spaced lens components. The front lens component of positive refractive power comprises at least two lens elements. The middle lens component comprises at least two lens elements. The rear lens component of positive refractive power comprises at least one lens element. Both the middle and rear lens components have significantly smaller clear aperture than the front lens component. All lens elements of the lens system are made of inexpensive glasses.

Advantageous Effects

The lens system of the present invention deliver high optical performance without the use of expensive optical materials. The lens system of the present invention can be made with high relative aperture. This is important advantage for photography and CCD imaging. The lenses of the middle and rear components of the lens system have smaller size than lenses of the front component. This is a further advantage because the lens system can be manufactured in a more economical manner. In addition, in the present invention the secondary spectrum correction extends over a wider spectral range than in conventional apochromats.

Description of Drawings

Fig. 1 is a schematic illustration of the lens system according to Embodiment 1 of the present invention.

Fig. 2 is a schematic illustration of the lens system according to Embodiment 2 of the present invention.

Fig. 3 is a schematic illustration of the lens system according to Embodiment 3 of the present invention. Fig. 4 is a schematic illustration of the lens system according to Embodiment 4 of the present invention.

Fig. 5 is a schematic illustration of the lens system according to Embodiment 5 of the present invention.

Fig. 6 is a schematic illustration of the lens system according to Embodiment 6 of the present invention.

Fig. 7 is a schematic illustration of the lens system according to Embodiment 7 of the present invention.

Fig. 8 shows the on-axis OPD curves of the achromatic doublet with an aperture of 80 mm and an f-number of f/15. Fig. 9 shows the on-axis OPD curves of the lens system depicted in Fig. 1.

Fig. 10 shows the on-axis OPD curves of the lens system depicted in Fig. 2.

Fig. 11 shows the on-axis OPD curves of the lens system depicted in Fig. 3.

Fig. 12 shows the on-axis OPD curves of the achromatic doublet with an aperture of

100 mm and an f-number of f/12.

Fig. 13 shows the on-axis OPD curves of the lens system depicted in Fig. 4.

Fig. 14 shows the on-axis OPD curves of the lens system depicted in Fig. 5. Fig. 15 shows the on-axis OPD curves of the achromatic doublet with an aperture of

120 mm and an f-number of f/10.

Fig. 16 shows the on-axis OPD curves of the lens system depicted in Fig. 6.

Fig. 17 shows the on-axis OPD curves of the achromatic doublet with an aperture of

100 mm and an f-number of f/10. Fig. 18 shows the on-axis OPD curves of the lens system depicted in Fig. 7.

Best Mode for Carrying Out the Invention

In conventional apochromatic lens systems the correction of secondary spectrum is ob- tained owing to anomalous dispersion of optical glasses. This is a generic feature of conventional apochromats regardless what kind of anomalous material is used. In contrast, a generic feature of known normal glass apochromats is a presence of wide air spaces between lens components. However, in two-component optical systems a wide air space between components results in difficulties with the correction of the chromatic difference of magnification. In order to reduce this aberration the front component of McCarthy lens system has virtually zero optical power at the mean wavelength. In other words, the requirement of the correction of the chromatic difference of magnification causes constraints on shape of all lenses of McCarthy lens system. As a result of these constraints, McCarthy design is unfavorable for correction of other aberrations. Since Wynne design is very close to McCarthy one, it suffers from the same problem.

In the present invention the lens system with corrected secondary spectrum comprises three widely air spaced lens components. In contrast with McCarthy and Wynne designs, in the present invention the front lens component has large positive refractive power. The widely separated third lens component of small size mainly helps in elimination of the chromatic difference of magnification. As a result, the middle and rear components of the lens system of the present invention are of significantly smaller size than the front component. The effect of this is a reduction of manufacturing costs. Moreover, the

three-component design of the present invention has good correction of longitudinal and transverse chromatic aberrations, spherical aberration, spherochromatism and coma. In the present invention the correction of secondary spectrum can be obtained with the use of only two different optical materials with normal partial dispersion, for instance with most common crown and flint glasses.

Illustrative embodiments will now be described with reference to the accompanying drawings and tables. While the present invention is described herein with reference to the embodiments for particular applications, it should be understood that the invention is not limited thereto. All optical glasses listed in the embodiments are from Schott glass catalog, but other glass manufacturers make nearly equivalent glass types. Along with Schott glass type designation, the descriptions of the embodiments will show the internationally recognized six-digit glass code for each glass type. In the six-digit glass code the first three digits indicate the refractive index after the decimal point and last three digits indicate the Abbe number (Example: BK7 with a refractive index n _d — 1.51680 and Abbe number V _d = 64.17 has the six-digit glass code 517642). All refractive surfaces in the following embodiments are spherical and the dimensions are given in millimeters. The effective focal lengths are calculated at the design wavelength of 555 nm.

The specifications for all embodiments are summarized in Tables I through VII. These tables list, in order from the object side, the lens element identifiers, the surface identifiers, the radii of curvature of refractive surfaces, the on-axis lens thicknesses or separations, as well as the material of each lens element.

Embodiment 1 Reference should now be made to Fig. 1, which is a schematic illustration of the telescope objective with reduced secondary spectrum having an aperture of 80 mm, an effective focal length of 560 mm and an f-number of f/7.

As shown on Fig. 1, the objective comprises three widely separated lens components.

The front lens component has a positive refractive power and comprises lens elements

LIl and L12. The lens element LIl is a meniscus lens having negative refractive power with its concave surface on the object side. The lens element LIl is on the object end of the objective. The lens element L12 is a meniscus lens having positive refractive power with its convex surface on the object side.

The middle lens component has a negative refractive power and comprises lens elements L13 and L14. The lens element L13 is a biconvex lens. The lens element L14 is a biconcave lens. The lens elements L13 and L14 are cemented together.

The rear lens component consists of the lens element L15. The lens element L15 is a meniscus lens having positive refractive power with its convex surface on the object side.

The lens elements LIl and L13 are made of BK7 glass (six-digit glass code 517642). The lens elements L12, L14 and L15 are made of F2 glass (six-digit glass code 620364). As shown on Fig. 1, both the middle and rear lens components have significantly smaller clear apertures than the front lens component.

The specifications for this design are summarized in Table I.

Embodiment 2

Reference should now be made to Fig. 2, which is a schematic illustration of the telescope objective with reduced secondary spectrum having an aperture of 80 mm, an effective focal length of 640 mm and an f-number of f/8.

As shown on Fig. 2, the objective comprises three widely separated lens components.

The front lens component has a positive refractive power and comprises lens elements

L21 and L22. The lens element L21 is a meniscus lens having negative refractive power with its convex surface on the object side. The lens element L21 is on the object end of the objective. The lens element L22 is a meniscus lens having positive refractive power with its convex surface on the object side.

The middle lens component comprises lens elements L23 and L24. The lens element L23 is a biconvex lens. The lens element L24 is a biconcave lens. The lens elements L23 and L24 are cemented together. The rear lens component consists of the lens element L25. The lens element L25 is a meniscus lens having positive refractive power with its convex surface on the object side.

The lens elements L21 and L23 are made of BK7 glass (six-digit glass code 517642). The lens elements L22, L24 and L25 are made of F2 glass (six-digit glass code 620364). The specifications for this design are summarized in Table II.

Embodiment 3 Reference should now be made to Fig. 3, which is a schematic illustration of the

telescope objective with corrected secondary spectrum having an aperture of 80 mm, an effective focal length of 560 mm and an f-number of f/7.

As shown on Fig. 3, the objective comprises three widely separated lens components.

The front lens component has a positive refractive power and comprises lens elements L31 and L32. The lens element L31 is a meniscus lens having negative refractive power with its concave surface on the object side. The lens element L31 is on the object end of the objective. The lens element L32 is a biconvex lens.

The middle lens component has a negative refractive power and comprises lens elements L33 and L34. The lens element L33 is a biconvex lens. The lens element L34 is a biconcave lens.

The rear lens component consists of the lens element L35. The lens element L35 is a meniscus lens having positive refractive power with its convex surface on the object side.

The lens elements L31 and L33 are made of BK7 glass (six-digit glass code 517642). The lens elements L32, L34 and L35 are made of F2 glass (six-digit glass code 620364). The specifications for this design are summarized in Table III. Although in the above embodiments the middle lens components consist of crown-in- front doublets, the opposite order of the crown and flint lenses is possible. In such case, the optical performance is similar to the above presented embodiments. The lens systems with the middle lens components consisting of flint-in-front doublets will be described in the Embodiment 6 and Embodiment 7.

The lens systems of Embodiments 1 through 3 of the present invention demonstrate a significant reduction of the residual chromatic aberration by using of crown glass BK7 and flint glass F2, which are among the most inexpensive optical glasses available at the market. Nevertheless, many other optical glasses can be used in order to obtain better performance or other advantages. Moreover, different optical materials can be used in the present invention, for example plastics.

The performance of the above lens systems can be significantly improved by adding lens elements. For instance, the front lens component can be formed of three or more lens elements. Such modifications are obvious for those having ordinary skill in the art. To illustrate these possibilities, the slightly more complex design will be now described.

Embodiment 4

Reference should now be made to Fig. 4, which is a schematic illustration of the telescope objective with corrected secondary spectrum having an aperture of 100 mm, an effective focal length of 700 mm and an f-number of f/7.

As shown on Fig. 4, the objective comprises three widely separated lens components. The front lens component has a positive refractive power and comprises lens elements

L41 and L42. The lens element L41 is a meniscus lens having negative refractive power with its concave surface on the object side. The lens element L41 is on the object end of the objective. The lens element L42 is a meniscus lens having positive refractive power with its convex surface on the object side. The middle lens component has a negative refractive power and comprises lens elements L43, L44 and L45. The lens element L43 is a meniscus lens having positive refractive power with its concave surface on the object side. The lens element L44 is a biconcave lens. The lens element L45 is a biconvex lens. The lens elements L43, L44 and L45 are cemented together. The rear lens component comprises the lens elements L46 and L47. The lens element

L46 is a biconvex lens. The lens element L47 is a biconcave lens.

The lens elements L41, L43 and L47 are made of BK7 glass (six-digit glass code 517642). The lens elements L42, L44 and L46 are made of F2 glass (six-digit glass code 620364). The lens element L45 is made of inexpensive fluor-crown N-FK5 (six-digit glass code 487704).

The specifications for this design are summarized in Table IV. Sometimes it is advantageous to use in the present invention glasses with some kind of anomalous dispersion. In this case, extra cost can be compensated by better color correction or other advantages. There are different categories of optical materials with respect to their partial dispersions. Most glasses have nearly normal partial dispersions. Most fluor-crowns and short flint glasses have highly anomalous partial dispersions. However, there are optical glasses which are placed in an intermediate position between normal and highly abnormal glasses. These glasses are often relatively inexpensive when compared to highly abnormal fluor-crowns or short-flints. Usually, the partial dispersions of such glasses are not anomalous enough to produce competitive conventional apochromats. Though, these glasses with slightly anomalous partial dispersions can be successfully used in the present invention. This important feature of the present invention leaves

considerable freedom to designer in choosing the design with optimum cost- perform a nee combination. Therefore, by using the present invention it is possible to achieve optimum balance of performance and cost for various specific applications. In order to illustrate this feature three additional designs will be described. In the following embodiments some dense flint glasses and dense barium crown glasses are utilized. These dense flint glasses have slightly anomalous partial dispersions. Although said glasses are not so cheap as most common crowns and flints but they are inexpensive when compared to highly abnormal glasses. It is also possible to use in the present invention many other types of slightly anomalous glasses, for example some dense barium flints or lanthanum flints and crowns.

Embodiment 5

Reference should now be made to Fig. 5, which is a schematic illustration of the telescope objective with corrected secondary spectrum having an aperture of 100 mm, an effective focal length of 550 mm and an f-number of f/5.5. As shown on Fig. 5, the objective comprises three widely separated lens components.

The front lens component has a positive refractive power and comprises lens elements

L51 and L52. The lens element L51 is a meniscus lens having negative refractive power with its concave surface on the object side. The lens element L51 is on the object end of the objective. The lens element L52 is a meniscus lens having positive refractive power with its convex surface on the object side.

The middle lens component has a negative refractive power and comprises lens elements L53 and L54. The lens element L53 is a biconvex lens. The lens element L54 is a biconcave lens. The lens elements L53 and L54 are cemented together.

The rear lens component comprises the lens element L55. The lens element L55 is a meniscus lens having positive refractive power with its convex surface on the object side. The lens element L51 is made of BK7 glass (six-digit glass code 517642). The lens elements L52 and L54 are made of SFl glass (six-digit glass code 717295). The lens element L53 is made of SK16 glass (six-digit glass code 620603). The lens element L55 is made of SF4 glass (six-digit glass code 755276) of relatively high refractive index. The use of high-index glasses in the rear lens component is often advantageous. The specifications for this design are summarized in Table V.

Embodiment 6

Reference should now be made to Fig. 6, which is a schematic illustration of the telescope objective with corrected secondary spectrum having an aperture of 120 mm, an effective focal length of 540 mm and an f-number of f/4.5. The objective comprises three widely separated lens components. The front lens component has a positive refractive power and comprises lens elements

L61 and L62. The lens element L61 is a meniscus lens having negative refractive power with its concave surface on the object side. The lens element L61 is on the object end of the objective. The lens element L62 is a meniscus lens having positive refractive power with its convex surface on the object side. The middle lens component has a negative refractive power and comprises lens elements L63 and L64. The lens element L63 is a biconcave lens. The lens element L64 is a biconvex lens. The lens elements L63 and L64 are cemented together.

The rear lens component comprises the lens element L65. The lens element L65 is a meniscus lens having positive refractive power with its convex surface on the object side. The lens element L61 is made of BK7 glass (six-digit glass code 517642). The lens elements L62 and L63 are made of SFl glass (six-digit glass code 717295). The lens element L64 is made of SK16 glass (six-digit glass code 620603). The lens element L65 is made of SF2 glass (six-digit glass code 648339).

The specifications for this design are summarized in Table VI. Embodiment 7

Reference should now be made to Fig. 7, which is a schematic illustration of the telescope objective with corrected secondary spectrum having an aperture of 100 mm, an effective focal length of 400 mm and an f-number of f/4. The objective comprises three widely separated lens components. The front lens component has a positive refractive power and comprises lens elements

L71 and L72. The lens element L71 is a meniscus lens having negative refractive power with its concave surface on the object side. The lens element L71 is on the object end of the objective. The lens element L72 is a meniscus lens having positive refractive power with its convex surface on the object side. The middle lens component comprises lens elements L73 and L74. The lens element

L73 is a biconcave lens. The lens element L74 is a biconvex lens.

The rear lens component comprises the lens element L75. The lens element L75 is

a biconvex lens.

The lens element L71 is made of BK7 glass (six-digit glass code 517642). The lens elements L72 and L73 are made of SF4 glass (six-digit glass code 755276) . The lens element L74 is made of N-SSK5 glass (six-digit glass code 658509). The lens element L75 is made of SF2 glass (six-digit glass code 648339).

The specifications for this design are summarized in Table VII. The optical performance of the above embodiments will now be discussed in comparison with equivalent achromatic telescopes.

It is conventional to measure secondary spectrum as a longitudinal, a transverse or an wavefront aberration. However, the effective focal length of the lens system accordingly the present invention is usually significantly shorter than the effective focal length of the equivalent achromat. In this case it is reasonable to use wavefront aberrations. In order to illustrate the correction of the secondary spectrum in the present invention, optical path differences (OPD) plots are shown in Figs. 8 through 18. The term "equivalent achromat" as used herein means a typical achromatic doublet having the same aperture and overall length as the lens system according the present invention. The term "overall length" as used herein means the length as measured by the separation between the vertex of the front surface and the focus of the optical system.

In the presented OPD plots the vertical axis represents the normalized entrance pupil coordinate and the horizontal axis represents the on-axis optical path difference in waves. The OPD curves are presented for four wavelengths, namely Fraunhofer lines g (436 nm), F (486 nm), e (546 nm) and C (656 nm). The reduction of the residual longitudinal chromatic aberration is clearly visible in the OPD plots because the secondary spectrum is the dominant aberration in achromatic telescopes of this size. Although in the lens systems of Embodiment 1 and Embodiment 2 the paraxial longitudinal chromatic aberration is corrected for two wavelengths, the secondary spectrum is significantly reduced. In the lens systems of Embodiments 3 through 7 the longitudinal chromatic aberration is corrected for three wavelengths.

Fig. 8 shows OPD plot of the achromatic doublet which is the equivalent achromat for the lens systems of Embodiment 1, Embodiment 2 and Embodiment 3. This equivalent achromat has an aperture of 80 mm, an f-number of f/15 and an overall length of 1200 mm. Fig. 9 shows OPD plot of the lens system of Embodiment 1. The curves of

Fig. 9 demonstrate that the wavefront aberrations of the lens system of Embodiment 1 are considerably reduced, compared with the aberrations of the equivalent achromat. This is the effect of the reduction of secondary spectrum in the lens system of Embodiment 1. This telescope can be rated as semiapochromatic. Fig. 10 shows OPD plot of the lens system of Embodiment 2, which has somewhat better color correction than the lens system of Embodiment 1. Fig. 11 shows OPD plot of the lens system of Embodiment 3. This telescope has better performance than the previous designs because the lenses of the middle component are air spaced.

Fig. 12 shows OPD plot of the achromatic doublet which is the equivalent achromat for the lens systems of Embodiment 4 and Embodiment 5. This equivalent achromat has an aperture of 100 mm, an f-number of f/12 and an overall length of 1200 mm. Although the equivalent achromat is relatively long-focus telescope, it has a large secondary spectrum. This causes serious degradation of the image quality. In contrast, Fig. 13 demonstrates that the lens system of Embodiment 4 has a good correction of the secondary chromatic aberration, especially in the violet region. This telescope has apochromatic-type color correction. Fig. 14 shows OPD plot of the lens system of Embodiment 5. The curves of Fig. 14 demonstrate that the aberrations of the lens system of Embodiment 5 are much better corrected, compared with the aberrations of the equivalent achromat. Fig. 15 shows OPD plot of the achromatic doublet which is the equivalent achromat for the lens system of Embodiment 6. This equivalent achromat has an aperture of 120 mm, an f-number of f/10 and an overall length of 1200 mm. Fig. 16 shows OPD plot of the lens system of Embodiment 6. The curves of Fig. 16 show a great improvement in color correction of the lens system of Embodiment 6 over the equivalent achromat. With an f-number of f/4.5, the lens system of Embodiment 6 has a high relative aperture for a refractive telescope.

Fig. 17 shows OPD plot of the achromatic doublet which is the equivalent achromat for the lens system of Embodiment 7. This equivalent achromat has an aperture of 100 mm, an f-number of f/10 and an overall length of 1000 mm. Fig. 18 demonstrates that the lens system of Embodiment 7 has a good correction of the secondary chromatic aberration, compared with the equivalent achromat. With an f-number of f/4, the lens system of Embodiment 7 is a very fast refractor.

The presented illustrative embodiments of the present invention are intended for use in the visible range. However, the present invention can be applied at a different waveband from the visible. If appropriate optical materials are used the present invention can be applied in the infrared (IR) and ultraviolet (UV) parts of spectrum. The ability to correct secondary spectrum even with only two optical materials of different relative partial dispersions is a very important feature of the present invention. This feature is especially important for IR and UV wavebands because in these spectral regions there are a lot less optical material to choose from.

The presented OPD plots show that the correction of the secondary spectrum in the present invention is especially good when the wide wavelength range is considered. This is another important feature because it is uncommon for conventional apochromats. Due to the ability to correct secondary spectrum for a wide spectral range the present invention is useful in lens systems for use over extended wavebands.

The present invention is not limited to the aforementioned embodiments, as it will be obvious that various alternative implementations are possible. For example, values such as the number of lens elements, the radii of curvature of each of the lens, the air spaces between the lenses are not limited to the examples indicated in aforementioned embodiments, as other values can be adopted. Other embodiments of the presented invention could be designed using other optical materials, as well as their order in the construction. The present invention has been described herein with reference to particular embodiments for a particular applications but the lens system with corrected secondary spectrum of the present invention may be used in various image forming devices other than the telescopes for amateur astronomers discussed above.

Table 1

Lens Element Surface Radius of Curvature Axial Thickness or Material

[mm] Separation [mm]

Rl -160.574844

LIl 8.0 BK7

R2 -219.970251

0.0 Air

R3 186.491866 L L1122 8.0 F2

R4 363.704856

508.0 Air

R5 93.474949

L13 12.0 BK7

R6 -100.895511

L14 6.0 F2

R7 116.035487

552.0 Air

R8 76.145743 L L1155 6.0 F2

R9 306.731135

Table Il

Lens Element Surface Radius of Curvature Axial Thickness or Material

[mm] Separation [mm]

Rl 239.525901

L21 8.0 BK7

R2 97.320119

6.0 Air

R3 100.623083 L L2222 11.0 F2

R4 250.223223

265.0 Air

R5 116.980731

L23 14.0 BK7

R6 -113.232474

L24 6.0 F2

R7 168.400990

784.0 Air

R8 98.463859 L L2255 6.0 F2

R9 440.206389

Table 111

Lens Element Surface Radius of Curvature Axial Thickness or Material

[mm] Separation [mm]

Rl -192.666950

L31 8.0 BK7

R2 -548.714866

0.0 Air

R3 253.856058 L L3322 8.0 F2

R4 -6041.312075

428.0 Air

R5 104.765524

L33 11.0 BK7

R6 -91.128065

0.0 Air

R7 -92.428494

L34 8.0 F2

R8 136.346584 641.0 Air

R9 83.236079

L35 6.0 F2

RlO 725.044549

Table IV

Lens Element Surface Radius of Curvature Axial Thickness or Material

[mm] Separation [mm]

Rl -223.756186

L41 10.0 BK7

R2 -384.483551

0.0 Air

R3 282.148910 L L4422 10.0 F2

R4 1789.474870

484.0 Air

R5 -315.045519

L43 8.0 BK7

R6 -97.487083

L44 5.0 F2

R7 125.408973

L45 10.0 N-FK5

R8 -152.631047 513.0 Air

R9 100.642667

L46 5.0 F2

RlO -214.494838

L47 5.0 BK7

RIl 150.711452

Table V

Lens Element Surface Radius of Curvature Axial Thickness or Material

[mm] Separation [mm] Rl -214.397621

L51 10.0 BK7

R2 -275.710819

0.0 Air

R3 246.204550 L52 10.0 SFl

R4 439.301959

555.0 Air

R5 141.172343

L53 10.0 SK16 R6 -131.699095

L54 6.0 SFl

R7 170.008970

503.0 Air

R8 81.234625 L55 6.0 SF4

R9 228.667280

Table Vl

Lens Element Surface Radius of Curvature Axial Thickness or Material

[mm] Separation [mm]

Rl -228.965183

L61 10.0 BK7

R2 -312.338420

0.0 Air

R3 269.106060 L L6622 10.0 SFl

R4 606.960178

470.0 Air

R5 -338.215090

L63 6.0 SFl

R6 105.318391

L64 14.0 SK16

R7 -251.247215

584.0 Air

R8 82.372040 L L6655 ^' 6.0 SF2

R9 391.722212

Table VII

Lens Element Surface Radius of Curvature Axial Thickness or Material

[mm] Separation [mm] Rl -220.531921

L71 10.0 BK7

R2 -411.603804

0.0 Air

R3 266.174867 L72 10.0 SF4

R4 873.360907

383.0 Air R5 -413.983997

L73 6.0 SF4 R6 90.825373

0.43 Air R7 91.395641 L74 13.0 N-SSK5

R8 -243.507952 472.0 Air

R9 101.842955

L75 6.0 SF2

RlO -8251.459018