Field of the Invention

The invention relates to refractor optics for use in optical devices such as telescopes, binoculars and cameras. The invention has particular application in the design of optics for astronomical telescopes.

Background and Prior Art Known to the Applicant

The basic dialyte design as conceived in the 19 ^th century had a single positive objective lens, and a single negative lens of the same material as the objective lens interposed between the objective lens and the focus. Although this design gives good images on-axis, off-axis rays are strongly chromatic.

Recent work (Blakley, R., "Dialyte-refractor design for self-correcting lateral colour" Opt. Eng. 42(2) 400-404, February 2003) has improved the off-axis performance by the use of a field lens at the focus of the objective lens, and by employing a set of weak positive relay lenses to correct for lateral colour - "The Blakley Design". The difficulty with this design is the length of the instrument. A 203mm telescope has a length of nearly 8 metres, which is impractical, and would be difficult to mount and use.

Furthermore, the inventors have found that it is not possible to correct for the aberrations present with the use of weak positive lenses in the Blakley Design if the overall focal ratio of the instrument is lower than approximately 15 to 1. The focal ratio of an astronomical

instrument is preferably between 5 to 1 and 10 to 1 so that a full range of magnifications for both wide-angle use and for high powers such as are employed for close double star work and for planetary viewing.

Summary of the Invention

The inventors have found that, if a negative lens is interposed between the field lens and the corrective positive lenses, it becomes possible to greatly shorten the optical train,

Accordingly, the invention provides an optical train comprising: a positive objective lens; a field lens, located at or close to the focus of said objective lens; one or more lenses, together forming a positive lens array, located beyond the focus of the said objective lens; and characterised by the provision of at least one negative lens, interposed between said field lens and said positive lens array.

The positive lens array is preferably arranged to correct for chromatic aberration, as described below.

The field lens - to gather off-axis rays - may be mounted at the focal plane of the objective lens. However, in this case, any imperfections in the field lens, or dust and the like located on the surface of the field lens, will come into focus, and adversely affect the image quality. Therefore, it is preferable that the field lens is not located precisely at the focal point, but wherein the said field lens and the focal point of the said objective lens is less than 5% of the focal length of the said objective lens.

More preferably, the distance between the said field lens and the focal point of the said objective lens is less than 1% of the focal length of the said objective lens.

Included within the scope of the invention is a telescope comprising an optical train as described above.

Preferably, the telescope is a refractor telescope, and more preferably, a dialyte refractor telescope.

Also included within the scope of the invention is a camera lens comprising an optical train as described above.

Also included within the scope of the invention are binoculars comprising an optical train as described above.6, Binoculars comprising an optical train according to any preceding claim.

The design allows optical instruments, such as telescopes, cameras and binoculars to have a shortened optical train, and a lower than normal focal ratio.

The invention provides a design that is very compact, has a wide field of view, and is free from longitudinal chromatic aberration across the field of view. This design is also free from lateral chromatic aberration, distortion and astigmatism. In preferred embodiments, all surfaces may be spherical, and the instrument is lightweight and simple to manufacture.

The objective lens may have a shorter focal length than in the Blakley Design, and (in the embodiment described below) the inventors have used from a focal ratio (ratio of focal length to objective diameter) of 8 to 1 to a focal ratio of 12 to 1. Blakley employs a focal ratio of 16.5 in his example.

A short focal ratio results in a more compact instrument, but makes it more difficult to achieve perfect achromatism.

Brief Description of the Drawings

The invention is described with reference to the accompanying drawings, in which:

Figure 1 illustrates the components of an embodiment of a telescope according to the present invention;

Figure 2 is an illustration of the objective lens of the embodiment of Figure 1 ; Figure 3 is an illustration of the field lens of the embodiment of Figure 1;

Figure 4 is and illustration of other lens elements of the embodiment of Figure 1 ; Figure 5 is an on-axis spot diagram of the embodiment of Figure 1; and Figure 6 is an off-axis spot diagram of the embodiment of Figure 1.

Description of Preferred Embodiments

Figure 1 is a schematic diagram of a telescope according to then present invention; this embodiment is particularly suitable for astronomical observation. Illustrated in Figure 1 are the objective lens, A, the field lens, B, and a train of corrective lenses, C. Also illustrated in this embodiment are two fold mirrors, Ml and M2, to fold the light path and thus reduce the overall length of the instrument.

Three ray paths are illustrated for on-axis rays (the dashed rays) and three for off-axis rays (the solid rays). Details of the lenses are given in Table 1, below, and illustrated more closely in Figures 2-4.

Figure 2 illustrates the objective lens, A, having its two surfaces Sl and S2. It can seen from Table 1 that the material beyond the surface Sl - i.e. in the direction of the light path - is glass type BK7 (defined in Table 2), and that beyond surface S2 is air.

Figure 3 illustrates the field lens, B, having three surfaces S5, S6 and S7. In this instance, therefore, the filed lens is a doublet. It can be seen from Table 1 that the material beyond S5 is glass type LAM7; the material beyond S6 is FTLlO, and beyond S7 is air. Again, the glass types are as defined in Table 2. The same nomenclature is used for the other lenses in the lens train.

Figure 4 illustrates the collection of lenses marked "C" in figure 1. For clarity, the lenses are illustrated in a more spaced-apart configuration to allow the surfaces of the lenses (S8- S22) to be identified. Lens Cl (a doublet, comprised of glass types PBH3 and FPL51) is the negative lens, interposed between the field lens, B, and the collection of corrective lenses C2-C5.

The collection of lenses, C2-C5, correct for chromatic aberration. Overall, this collection acts as a positive lens (and so contains at least one positive lens element), although the collection might include one or more negative lenses, such as C4 in this embodiment.

The resultant axial and off-axis spot sizes for this embodiment are given in Figures 5 and 6, respectively. The focal ratio of the objective lens is 12 to 1, and the effective focal ratio of the instrument is 6 to 1.

As may be seen from the axial spot diagram (Figure 5), all of the rays are contained within the Airy disk (for red and blue light) and virtually all for green light. The spot is also still diffraction-limited at 0.4 degrees semi-field off-axis (see below, and Figure 6).

Figure 5 is the axial spot diagram for on-axis rays. The Airey disc, 1, is illustrated by the circle in each of Figures 5(a)-5(c). The spot diagram metrics are:

Colour Diffraction Limit Geometrical Spot Geometrical Geometrical

(μm) Size (μm) RMS Y Size RMS X Size

(μm) (μm)

Overall 4.224 2.773 1. 961 1 .961

Green 4.224 3.273 2. 315 2 .315

Red 4.44 3.41 2. 411 2 .411

Blue 3.582 0.5795 0. 4098 0 .4098

Figure 6 is the axial spot diagram for 0.8 degrees field off-axis spot size (0.4 degrees semi-field off-axis). For the off-axis rays, the overall diffraction limit (the Airey disc) is 4.212 microns whilst the mean geometrical spot size is only 3.737 microns. The Airey disc, 1, is indicated by the circle in Figures 6(a) and 6(c). - it is unlabelled in Figure 6(b), for the sake of clarity. The geometrical RMS Y size is 3.163 microns, and the corresponding X size is 1.99 microns.

In this embodiment, the focal ratio of the objective lens is 12 to 1, and the effective focal ratio of the instrument is 6 to 1. In order to greatly shorten the instrument, two fold mirrors, Ml and M2, are employed. If the focal ratio of the objective lens is greater than 8

to 1, there is little or no astigmatism present. The overall length of the complete instrument is approximately 1300mm, which is convenient to mount and to use.

The remaining small amount of colour separation off-axis may be removed or reduced by careful choice of glass types. The example given is by no means exhaustive.

Although the example shows the use of 5 sets of doublets to correct the axial and off-axis rays (lenses C1-C5), single lens or triplets may be employed instead. Similarly, from 2 to 6 or more single elements or doublets or triplets may be employed, depending upon the final requirements of the system.

Table 1 - Specification of Lenses

Lens Surface Radius Thickness Aperture Glass ¹ Special

(mm) (mm)

A Sl 1566 8.834 203.2 BK7

A S2 1082.0 203.2 AIR

S3 -1082 145.4 Fold mirror 1

S4 882.6 88.0 Fold mirror 2

B S5 71.21 2.65 44.6 LAM7

B S6 46.76 7.95 44.0 FTLlO

B S7 -865.03 176.68 43.2 ABR

Cl S8 -24.43 1.77 13.0 PBH3

Cl S9 26.65 2.65 13.7 FPL51

Cl SlO 62.72 1.77 15.1 AIR

C2 SI l -108.11 7.95 16.8 FPL51

C2 S12 -13.77 2.65 22.1 BPH8

C2 S13 -26.95 0.88 24.2 AIR

C3 S14 1877.6 5.30 24.9 FPL51

C3 S15 -25.30 2.65 27.4 NPH2

C3 S16 -24.42 3.09 29.0 AIR

C4 S17 -22.21 5.30 29.3 PHM52

C4 S18 -18.13 3.53 32.2 BALI l

C4 S19 -29.78 8.83 34.0 AIR

C5 S20 521.45 2.65 35.4 PBM5

C5 S21 58.80 7.95 35.6 FPL51

C5 S22 -59.53 159.01 36.5 AIR

IMS 0.053 16.7

'The Glass type is the material beyond the specified surface in the optical train. A designation of "AIR" indicates a free surface. Glass designated as "BK7" is available from Schott AG (Hattenbergstrasse 10, 55122 Mainz, Germany); all others are available

from Ohara (Ohara Corporation, 23141 Arroyo Vista, Suite 200, Rancho Santa Margarita, CA 92688, USA).

Details of the various glass types are given in Table 2, below.

Table 2 - Specification Glass Type

Glass V number Refractive Index Refractive Index Refractive Index

Type (Partial at 500nm at 590nm at 620nm dispersion)

LAM7 35.28 1.761835 1.749237 1.746292

FTLlO 56.42 1.506488 1.501261 1.499999

FPL51 81.54 1.500501 1.496922 1.496049

NPH2 18.90 1.951494 1.922276 1.915740

PHM52 63.33 1.623614 1.617878 1.616486

BALIl 57.74 1.578204 1.572377 1.570963

PBM5 38.01 1.612630 1.603225 1.60105

PBH3 28.28 1.755249 1.739679 1.736091

BPH8 34.72 1.732506 1.720212 1.717325

BK7 64.17 1.521414 1.516699 1.515539