AU686393C - High speed optical system - Google Patents

Description

"HIGH SPEED OPTICAL SYSTEM"

TECHNICAL FIELD

This invention relates to an optical lens system and, optionally, an associated optical relay. BACKGROUND ART

Solid state imaging arrays (CCDs, CTDs, etc) have now become the sensors of choice in many applications. Being planar, geometrically accurate (to the limit of microlithography technology) and with a high quantum efficiency in the visible and near-infrared spectral domains, such devices have the potential to be virtually perfect image detectors.

For the purpose of low-light-level imaging or astrography with CCD devices, the instrument designer's problem is to find an optical system with a matching performance, not only in exceptional resolution and distortion characteristics, but also in speed so as to achieve the highest possible information acquisition rate. However, when aperture diameters exceed 150mm, the homogeneity of optical glass becomes an intrusive problem and design solutions usually reduce to catoptric or catadioptric systems which generally require only one refractive component of the full aperture diameter. Few such systems exist which combine the characteristics of high speed

(eg. faster than f74) and high - and uniform - resolution to the dimensional limit required by CCD pixel structures. If, to these notional constraints, there are added such pragmatic aspects as ease of fabrication and moderate tolerances, the list of suitable designs tends toward zero length. The top of the list is occupied by the Schmidt camera and its variations; however, as design parameters tend towards higher speed and uniform flattened- field resolution, the limitations of the full-aperture aspheric corrector become evident in the form of more difficult and expensive fabrication, significant residual sphero-chromatic aberration and obliquity effects. Maksutov camera designs also suffer from problems associated with their massive full-aperture thick meniscus corrector component; to such an extent that the advantage of smaller obliquity effects is overridden by high-order sphero- chromatism as the design speed is increased.

An additional obstacle which some low-light-level designs must surmount is the need to accommodate a cryostat for the CCD. Ideally this requires that the focus be accessible externally, which in turn implies a Cassegrain system, or at least a folded format.

We acknowledge commonly owned New Zealand Patent Application No.

236307/236308 and Japanese Patent Application No. JAP 4-185640 published as

6-82699. This invention relates to improvements to the system. This invention is an optical design which is novel in its assembly of techniques into a format that fits a previously unoccupied area of the speed/diameter relationship and which overcomes the problems previously mentioned. Furthermore, preferred forms of the present invention provide a high speed optical system of economic construction or which, at least, provides the public with a useful choice.

DISCLOSURE OF THE INVENTION

Accordingly the present invention may broadly be said to consist in a lens system suitable for focusing substantially parallel incident light onto a detector, said system comprising (A) a concentric spherical Cassegrain-like system of two mirrors,

(B) a concentric spherical focal reducer,

(C) a transfer lens which combines the concentricity of the Cassegrain- like system of two mirrors and of the concentric spherical focal reducer by imaging the first centre of concentricity (that of the system of two mirrors) on the second centre of concentricity (that of the focal reducer) to thereby provide a single optically concentric system which combines their advantages,

(D) means to correct the sum of the spherical aberration of all of the spherical mirrors in the entire system, and (E) an aperture stop. Preferably said lens system further includes:

(F) image detection means (hereafter "detector") at the focus of the focal reducer. Preferably said concentric spherical Cassegrain-like system of two mirrors does not include any aperture stop.

Preferably said concentric spherical focal reducer includes at least one spherical mirror element.

Preferably said concentric spherical focal reducer includes at least one refractor element. Preferably said transfer lens system is a refractive single lens.

Preferably said concentric spherical focal reducer is selected from the group comprising: i. Modified forms of Baker camera; ii. Modified form of Hawkins and Linfoot camera; iii. Derivation of Maksutov or Bouwers camera.

Preferably said concentric spherical focal reducer is a modified form of the Hawkins and Linfoot camera system and said means to correct the sum of the spherical aberration of all of the spherical mirrors in the entire system and said aperture stop forms part thereof. Preferably said means to correct the sum of this spherical aberration of all of the spherical mirrors in the entire system is a concentric meniscus concentric with the concentric focal reducer.

Preferably the chromatic aberration introduced by the said concentric meniscus is compensated by a refractive component located at the aperture stop. Preferably said refractive component is a zero-power chromatic lens or lens combination, for example, a doublet lens or lens combination.

Alternatively said refractive component is a weakly positive power singlet lens.

Preferably said zero power refractive component includes an aspheric zonal corrector surface sufficiently weak not to introduce any substantial degree of focal difficulties when instant light is angled into the overall lens system other than axially.

Preferably said lens system is substantially faster than ill.

Preferably said lens system is about f/0.8. Preferably said detector is included.

Preferably said detector is a solid state detector.

Preferably said detector has a substantially planar detection surface. BRIEF DESCRIPTION OF THE DRAWINGS:

Figure 1 shows a speed size relationship for astrocameras including an embodiment of the invention;

Figure 2 is a drawing of prior art concentric Cassegrain Schmidt or Maksutov cameras;

Figure 3 is a drawing of one preferred embodiment of the present invention;

Figure 4 is a perspective sectional drawing of an optical relay optionally forming part of the invention;

Figure 5 is a perspective sectional drawing of one preferred embodiment of the present invention;

Figure 6 is a profile of the asphere of a preferred embodiment of the present invention in accordance with Example 1; Figure 7 is a cross-sectional, side elevation view of the system of Example

1;

Figure 8 is a graphical illustration of the performance of the system of

Example 1;

Figure 9 is a cross-sectional, side elevation view of the system of Example 2;

Figure 10 is a graphical illustration of the performance of the system of Example 2;

Figure 11 is a cross-sectional, side elevation view of the system of Example

3; Figure 12 is a perspective sectioned view of an alternative form of the relay which includes a concentric window for a cryostat; Figure 13 is a graphical illustration of the performance of the system of Example 3; and

Figures 14a, 14b and 14c are illustrations of prior art Maksutov, Baker and Hawkins & Linfoot cameras which can be modified to provide concentric spherical focal reducers in a preferred form of the invention. DETAILED DESCRIPTION OF THE INVENTION:

Preferred forms of the present invention is a concentric Cassegrain like system with a focal reducing relay, all critical surfaces being spherical. The relay described herein is also a concentric system and provides the f/1 speed characteristic at an external focus, but it should be noted that other relays can be used to give different speedVimage-scale parameters.

Figure 1 shows the ranges of apertures and speeds for which the named design types are appropriate. This invention is appropriate for the area named "new zone". The starting point for the concept description is the concentric Cassegrain

Schmidt or Maksutov camera designs shown as alternatives in Figure 2. These include an aperture stop 23, a primary mirror 24, a secondary mirror 25 and a focal surface 26. Apart from obliquity effects in the Schmidt aspheric corrector 21 or high-order spherochromatism in the Maksutov corrector 22, the image quality of these designs is uniform over the whole field. The Schmidt corrector, located at the common centre of curvature of the mirrors and which fills the aperture stop, has an axis of symmetry, as does the Maksutov meniscus in its achromatic forms. The fabrication penalties of these designs are the need for a full-aperture aspheric or for a full-aperture thick meniscus corrector and the length of the structure or tube required to support the corrector.

Referring now to Figure 3, if the corrector is omitted from these designs but the aperture stop left in position, at the common centre of curvature of the mirrors, the obvious result is the introduction of severe spherical aberration at all field angles. If, now, a field lens is introduced near the Cassegrain focal position, an image of the aperture stop and of the common centre of curvature of the mirrors is created further behind the primary mirror 31. If a real aperture stop 32 is located coincident with this image, then it is axiomatic that the function of the classical Schmidt aperture stop (delineation of the marginal rays at all field angles), is duplicated, so that the classical stop can be eliminated. The common centre of curvature of the mirrors, and the classical aperture stop have been optically transferred to the new location. An immediate advantage is the reduction of the camera length to about the same dimension as the primary/secondary separation. The spherical aberration can then be corrected by insertion of a meniscus component 33 concentric with the centre of the new aperture stop 32, as this is now a new centre of concentricity 34 optically transferred from the classical Schmidt location. This transfer of the centre of concentricity is the prime function of the field lens 35, so is termed the transfer lens in the remainder of this specification.

Note that the Cassegrain focus is relocated to a position between the two mirrors 31 and 37; the optical train is shortened overall by the small forward shift of the secondary mirror 37. The corrected image is virtual and is located at 36, between the relocated "cassegrain" focus and the correcting meniscus 33, because of the net negative power of the latter. To reestablish a real image requires a relay lens which should, of course, be placed with its entrance pupil coincident with the aperture stop. Clearly, numerous specifications could be derived for relay lenses with differing conjugate ratios; the relay to be described here can reduce the relatively large virtual image to the dimensions typical of CCD devices, and shares the concentricity philosophy of the preceding optics, thus retaining the essential independence from off-axis aberrations.

Fast focal reducers are well known adducts for "slow" telescopes and small detector devices, but in this invention an unusually cooperative melding is possible between the subsystem described previously and the type of focal reducing relay shown in Fig. 4. The concentric meniscus 44 provides correction of the spherical aberration of the concave spherical mirror independently of field angle in the same manner as described previously for the basic inventions subsystem. The doublet 45 is afocal and introduces a chromatic error equal and opposite to the chromatic error of the meniscus 44. Being located at the aperture stop, doublet 45 acts equally on all ray bundles so does not disturb the overall concentricity of the system. It should be noted that there are two physical centres of curvature in Fig. 4.

The centre of the aperture stop 45 is the centre of curvature of the meniscus 44, but this centre is reflected to the position 40 by the folding flat 46. This arrangement makes it possible to achieve an external focus for greater accessibility. Field curvature is inherent in concentric designs, as is well known in

Schmidt cameras especially, and can be corrected by the insertion of a field flattening lens 48 close to the focal surface, but, at least in the usual Schmidt geometries, only at the expense of introducing significant off-axis aberrations. However, as the numerical aperture (the speed) is increased, this problem is at least partially offset by the smaller scale of the focal surface geometry. In the embodiments described in this specification, the field flattening lenses are so weak as to add no significant degradation to the residual sphero-chromatic blur. By merging the aperture stop of the relay and the transferred aperture stop of the new subsystem, the fast imaging system is assembled. Figure 5 shows the layout resulting from the merge with meridional rays shown at a typical off-axis angle in this example at 1.83 degrees off axis. The system has a primary mirror 51 and a secondary mirror 52. The system also includes a corrector group 54 and 55 (the equivalent of meniscus 44 and doublet 45 in Fig 4) and a folding flat 56. Also visible in Figure 5 is the small weak field flattener 58 which delineates the final flat focal surface. In this particular design, the field flattener lens 58 is intended to be optically cemented to the otherwise uncovered silicon structure of the CCD detector. This minimises additional aberrations contributed by the field flattener 58 and serves to protect the CCD surface from contaminants. Separation of this lens from the focal surface would cause it to intrude too far into the f/1 ray cone with implicit degradation of the image sharpness.

As shown in the system in Figure 5, the characteristics of this system can be summarised as: (a) the focal power resides in the mirrors, and so is non-chromatic.

(b) the spherical mirrors are optically concentric, thus eliminating coma and astigmatism when the aperture stop is located at the centre of curvature (or at the optical equivalent).

(c) spherical aberration correction is the only remaining necessary adjunct to the reflective optical elements. This is the function of the corrector group.

(d) the residual aberrations are low-amplitude combinations of secondary colour with weak high-order coma and astigmatism, generated primarily at the non-concentric surfaces of the transfer lens. (e) vignetting is minimal. The central obscuration is determined by the perforations in the folding and relay mirrors, provided that the design is adjusted so as to image the secondary mirror into the space between them. In Example 1 below, the central obscuration is about 31.2% on axis, increasing to 33% at the circumference of the 11mm diameter image. (f) distortion is rninimal, with an amplitude generally less than that of the blur spot dimensions.

To minimise future fabrication costs, the example designs make use of "Smith's List" of workshop tool radii for the radii of curvature of the optical components. Although non-optimum for residual aberration correction, the difference in performance is negligible.

The three following examples demonstrate very different variants of the basic design, a 200mm aperture f/0.9 visible/NIR, a 1000mm aperture f/0.8 visible/NIR and a 200mm aperture f/0.93 thermal infrared version.

The specification tables are based on a coordinate system in which the z- axis is the optical axis and the x and y-axes are mutually orthogonal to it. In these design examples the origin is the centre of curvature of the primary mirror. "Diam" and "diam" are the outside and inside diameters of annuli. The shape of the aspheric trimmer profile is defined by the polynomial equation: z = c.x²/2 + aj.x⁴ + a^.x⁶ + a_g.x⁸ where c is the curvature, x is the x-coordinate and a„ are the coefficients. The bandpass for the first two of the examples given for this class of the system is intended to match the spectral sensitivity of generic silicon CCD devices, for which the highest response lies between 450 and 1 lOOnm. The corresponding refractive index data for optical glass is published for the spectral lines g, e, d, C, r, s and n₁₀₆₀₀. amongst others, providing a good coverage for analytic purposes.

The initial ray-tracing process always shows some coma as the dominant residual aberration, for off-axis rays, emanating from the non-concentric components. This is largely corrected by introducing equal and opposite coma within the concentric Cassegrain subsystem, the technique chosen here being that of increasing the focal length of the transfer lens so as to make the transfer imperfectly concentric. The effect of this procedure is to displace the centre of the entrance pupil away from the classical Schmidt location (at the centre of curvature of the primary mirror), and laterally proportionate to off-axis angle, thus effecting the required compensation. Ray tracing is performed thereafter by ensuring that the aperture stop, located at the aspheric surface, accurately delineates the marginal rays for each spectral line.

The median ray aberration graphs which follow, have as their vertical axis the height of the ray in the entrance pupil and the horizontal axis gives the lateral position of the intercept with the focal plane. The 2D histograms are normalised in amplitude; any significant fine structure is derived from the line-spectrum ray trace. The most relevant feature is the maximum extent of the "footprint" on the 32 X 32μm focal patch. EXAMPLE 1

Surface Glass Z(vertex) Curvature Radius Surface Type diam Diam

0 618 Obstruction 95

1 1016 -0.00098425 - 1016 Mirror 85 255

2 633.5 -0.00157853 -633.5 Mirror 92

3 830 0.0046904 213.2 Lens 55

4 SK 11 848 -0.0052549 - 190.3 Lens 55

5 1009.45 0.01619433 61.75 Lens 60

6 SK 4 1029.27 0.02384927 41.93 Lens 60

7 1065 0 flat Lens 56.5

8 F 4 1071 -0.0072275 - 138.4 Lens 56.5

9 SK4 1075 0 flat+asph Lens(Stop) 56.5

10 1143.6 0 flat Mirror 47 105

11 1065 0.00679348 147.2 Mirror 60 150

12 1163.67 0.03298915 30.31 Lens 12

13 SF2 1164.67 0 flat focus 12

Aspheric Coefficients < )f Surface 9 Entrance Pupil Diam. = 200m

A2 -5.50E-05 Focal Length = 173.2mm

A4 1.281E-07 Geometrical Focal Ratio = 0.87

A6 -5.878E- 11 Central obscuration = 31.2%

A8 -2.184E- 14 Bandpass = 436nm to 1060nm

The profile of the aspheric zonal corrector surface is illustrated in Fig. 6. Note that the z-axis is expanded by a factor of 2000 relative to the (vertical) x- axis. Fig. 7 provides a side elevation of this example design, Fig. 8 gives a graphical illustration of the computed performance.

The meridional ray aberration curves in Figure 8, demonstrate moderate

stabihty, with no tendency to chaotic extremes as the field angle is increased. As

the 2D histograms show, all the energy from 436 to 1060nm is focused into only

part of the 32 X 32μm focal area even at 1.455° off axis (the angular radius

corresponding to the side of the 7 X 9mm - or "²/3 inch" video-standard image). EXAMPLE 2

Surface Glass Z(vertex) Curvature Radius Surface Type diam Diam

0 3550 Obstruction 250

1 6000 -0.0001667 -6000 Mirror 1045

2 3575 -0.0002797 -3575 Mirror 250

3 4401 0.0069004 144.9 Lens 62

4 F 4 4407 0.0187758 53.26 Lens 62

5 SK 4 4419 -0.0004764 -2099 Lens 62

6 4580.5 0.01619433 61.75 Lens 58

7 SK 4 4602.25 0.025 40 Lens 58

8 4636 0 flat Lens 55

9 F 4 4642.25 -0.0085063 -117.6 Lens 55

10 SK 4 4646 0 flat+asph Lens(Stop) 54.6

11 4793.2 -0.0067935 -147.2 Mirror 155

12 4693.241 -0.03365497 -27.36 Lens 14

13 SF 2 4692.241 0 flat focus 14

Aspheric Coefficients of Surface 10 Entrance Pupil Diam. = 1000mm

A2 = -9.6E-05 Focal Length = 814.8mm A4 = 2.206E-07 Geometrical Focal Ratio = 0.82 A6 = -9.976E-11 Entral Obscuration = 6.3% A8 = -3.159E-14 Bandpass = 436nm to 1060nm

At EPDs significantly greater than lm, constraints on the new design system are imposed by the greater scale of spherical aberration at the concentric Cassegrain focus, which generates significant errors of mapping of the ideal entrance pupil onto the system aperture stop by the transfer lens. The resulting high-order aberrations tend to exceed acceptable levels relative to the pixel dimensions of the appropriate CCD detectors.

Example 2 describes a 1000mm aperture version. There is an extra chromatic correction element used in the transfer lens of the lm variant. This helps to trim back the outer parts of the blur spot which are caused by the extremes of the spectral bandpass.

Figure 9 shows the side view of the optical layout and Figure 10 gives a graphic illustration of the computer median ray and blur spot performance. THERMAL INFRARED VARIANT

It is clear that other regions of the spectrum can be utilised, given the appropriate detectors and refractive media to which this design principle can be adapted. In recent years, arrays of thermal infrared detectors have been fabricated, the most useful in the context of the new imaging system being the Pt:Pt-Si CCD arrays that are now commercially available. With useful spectral sensitivity in the spectral domain 3.5 - 5.5μm, these detectors have overall and pixel dimensions similar to those of the normal visible/NIR silicon imagers. Moreover, in the 3.5 - 5.5μm spectral domain, Germanium is a low-cost, easily worked optical medium suited to the refractive components of the fast relay, with the benefit that the high refractive index allows large reductions in the spherical curvatures of the field/transfer lens giving a corresponding significant reduction of the high-order aberrations which limit the off-axis performance of the visible/NIR version of this design.

EXAMPLE 3

Surface Glass Z(vertex) Curvature Radius Surface Type diam Diam

0 620 Obstruction 88

1 1000 -0.002 -1000 Mirror 80 260

2 625 -0.0016 -625 Mirror 88

3 836.31 0.001523 656.6 Lens 54

4 Ge 846.31 0 flat Lens 54

5 1082.11 0.01727414 57.89 Lens 70

6 Ge 1089.35 0.01974334 50.65 Lens 70

7 1135 0.00021 4761.9 Lens 70

8 ALA 1140 0 flat+asph Lens (Stop) 70

9 1232.5 0 flat Mirror 53 125

10 1125 0.005 200 Mirror 75 180

11 1244.13 0.01236553 80.87 Lens 33

12 Ge 1248.41 0.01305654 76.59 Lens 33

13 1260.374 0.02457032 46.36 Lens 12

14 ALA 1261.374 0 flat focus 12

Aspheric Coefficients of Surface 9 Entrance Pupil Diam. = 196mm A2 -1.14E-05 Focal Lerigth = 181.2mm A4 5.19E-08 Geometrical Focal Ratio = 0.93 A6 -1.03E-11 Central obscuration = 29% A8 -9.00E-15 Bandpass = 3.7 - 5.5«m

The table of Example 3 lists the optical design of a thermal version of the new system, comparable in most characteristics to those of the example given in Table 1. Figure 11 shows the side view of the optical layout.

The significant differences in detail include the use at the aperture stop of a synthetic sapphire spectral dispersion corrector 115 which has only a singlet format, but which has a weak positive power exactly sufficient for the associated positive longitudinal chromatic aberration to correct the negative chromatic aberration of the Germanium concentric meniscus corrector over the spectral band 3.7 - 5.5μm. An essential component of a thermal camera of this type is the cryostat sub¬ system. The cryostat window is usually made as an optical flat, but with the fast optics in this design, it is more appropriate to fabricate the window as a concentric memscus 119 with its centre of curvature coincident with the reflection of the common centre of curvature created by the folding flat. A perspective view is shown in Fig. 12. This window then contributes to the corrective negative spherical aberration of the system and introduces no off-axis aberrations.

Figure 13 illustrates the computed performance of the median ray bundles and of the blur spot.

A possible advantage of at least preferred forms of the present invention over prior art designs such as provided by the Hawkins and Linfoot camera as the basis for the design of the focal reducing relay, is that the corrector meniscus is truly concentric, and the chromatic doublet has zero power, thus giving more degrees of freedom to the designer. The classical Maksutov corrector is designed specifically non-concentric, so as to intrinsically compensate for its chromatic aberration for small angles off axis; however, it is believed that at least preferred forms of the present invention exceed allowable off-axis angles in this case. The extra degrees of freedom mentioned above can be used to apochromatise the system, in that optical glasses can be selected for the meniscus and the chromatic doublet which permit a sharp focus for more than the usual two wavelengths implicit in the usual achromatic correction.

Specifically, by choosing Schott FK 51 for the meniscus, the chromatic aberration to be corrected by the doublet is minimised to the extent that the glasses KzF N2 and PK 5 la can be utilised to provide apochromatisation over the bandpass 400 to HOOnm (the entire sensitive bandpass of typical silicon-based CCD imaging devices), while retaining the zero-power specification of the doublet. A triplet form allows even further gains in aberration control.

The result of this is that the resolution of the system is improved by a factor of approximately 3, with blur-spot dimensions potentially as low as 5 micrometres over the entire image area, at speeds of the order of f/0.8 and for the bandpass mentioned above. This, in turn, implies the ability to scale up the design so as to accommodate the newer very large silicon CCD imaging devices now coming into use in astronomical and other scientific research. SCALING All three examples given here are based on the use of a "²/3inch" video standard CCD detector, which effectively deteπnines the linear dimensions of the fast relay sub-system for a specified Numerical Aperture. Other detector dimensions may require modification of the relay to provide the appropriate combination of speed, linear field at the field/transfer lens, and residual aberration blur.

From the three examples it can be seen how, once the detector/relay combination has been initially determined, the Cassegrain components can then be established which will match the required object field angle to the linear field of the relay. Melding of the two sub-systems is then achieved by detailed adaptation of the field transfer and corrector components.

Claims (21)

CLAIMS:

1. A lens system suitable for focusing substantially parallel incident tight onto a detector, said system comprising

(A) a concentric spherical Cassegrain-like system of two mirrors, (B) a concentric spherical focal reducer,

(C) a transfer lens system which combines the concentricity of the Cassegrain-like system of two mirrors and of the concentric spherical focal reducer by imaging the first centre of concentricity (that of the system of two mirrors) on the second centre of concentricity (that of the focal reducer) to thereby provide a single optically concentric system which combines their advantages,

(D) means to correct the sum of the spherical aberration of all of the spherical mirrors in the entire system, and

(E) an aperture stop.

2. A lens system as claimed in claim 1 further including:

(F) image detection means (hereafter "detector") at the focus of the focal reducer.

3. A lens system of claim 1 or 2 wherein said concentric spherical Cassegrain- like system of two mirrors does not include any aperture stop.

4. A lens system of claim 1, 2 or 3 wherein said concentric spherical focal reducer includes at least one spherical mirror element.

5. A lens system as claimed in claim 1, 2 or 3 wherein said concentric spherical focal reducer includes at least one refractive element.

6. A lens system as claimed in an one of claims 1 to 4 wherein said transfer lens system is a refractive single lens.

7. A lens system as claimed in any one of claims 1 to 5 wherein said concentric spherical focal reducer is selected from the group comprising: i. Modified forms of Baker camera; ϋ. Modified form of Hawkins and Linfoot camera; iii. Derivation of Maksutov or Bouwers camera.

8. A lens system as claimed in any one of claims 1 to 6 wherein said concentric spherical focal reducer is a modified form of the Hawkins and Linfoot camera system and said means to correct the sum of the spherical aberration of all of the spherical mirrors in the entire system and said aperture stop forms part thereof.

9. A lens system as claimed in any one of claims 1 to 7 wherein said means to correct the sum of this spherical aberration of all of the spherical mirrors in the entire system is a concentric memscus concentric with the concentric focal reducer.

10. A lens system as claimed in any one of claims 1 to 7 wherein the chromatic aberration introduced by the said concentric meniscus is compensated by a refractive component located at the aperture stop.

11. A lens system as claimed in claim 9 wherein said refractive component is a zero-power chromatic doublet lens.

12. A lens system as claimed in claim 9 wherein said refractive component is a weakly positive power singlet lens.

13. A lens system as claimed in claim 9 wherein said refractive component includes an aspheric zonal corrector surface sufficiently weak not to introduce any substantial degree of focal difficulties when instant light is angled into the overall lens system other than axially.

14. A lens system as claimed in any one of claims 1 to 12 faster than 171.

15. A lens system as claimed in any one of claims 1 to 13 wherein said system is about f/0.8.

16. A lens system as claimed in any one of claims 1 to 14 wherein said detector is included.

17. A lens system as claimed in any one of claims 1 to 15 wherein said detector is a solid state detector.

18. A lens system as claimed in any one of claims 1 to 16 wherein said detector has a substantially planar detection surface.

19. A method of imaging onto an imaging detector substantially parallel incident light, said method comprising the steps of (i) creating an intermediate image with a concentric spherical Cassegrain-like system of two mirrors with an entrance pupil located at the centre of curvature of the mirror, and

(ϋ) relaying the intermediate image to the imaging detector by a concentric spherical fast focal reducer the method being characterised in that

(A) a refractive transfer lens or lens system located at or near the intermediate image serves the following functions

(a) as a field lens for the intermediate image,

(b) as a means to link the independent concentricities of the concentric spherical Cassegrain-like system and the concentric spherical fast focal reducer, and

(c) as a means to define an aperture stop being the image of the entrance pupil, and

(B) a concentric meniscus corrector is provided in the fast focal reducer to correct the spherical aberration of the whole system.

20. A method of claim 19 wherein the relative sizes of the entrance pupil and the aperture stop match the detector characteristics to the task of the system.

21. A method of claim 19 wherein the size of the aperture stop can be chosen to be much smaller than that of the entrance pupil, enabling high values of Numerical Aperture and an exceptional degree of aberration correction, thus permitting an optimum matching of the detector characteristics to the task of the system.