NZ236307A

NZ236307A - Reflector telescope lens system

Info

Publication number: NZ236307A
Application number: NZ236307A
Authority: NZ
Inventors: Allan David Beach
Original assignee: Ind Res Ltd
Priority date: 1990-12-03
Filing date: 1991-06-03
Publication date: 1996-05-28

Description

<div class="application article clearfix" id="description"> 236307 / 236308 NEW ZEALAND "Improvements in or Relating to Lens Systems" We, HER MAJESTY THE QUEEN IN RIGHT OF NEW ZEALAND, having an address at DSIR, Industrial Development, 24 Balfour Road, Pamell, Auckland, New Zealand hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- 236307 / ~ / 236308 FIELD OF THE INVENTION: This invention relates to an optical lens system and, optionally, an associated optical relay. BACKGROUND TO THE INVENTION: Some CCD devices have been successfully fabricated with over 10* operating pixels, each having dimensions <lOvm square. A good example is the Kodak Megapixel device, having 1035 X 1320 pixels of 6.8jum square in a 7 X 9mm array. 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. When aperture diameters exceed 150mm, the homogeneity of optical glass becomes an intrusive problem and design solutions usually reduce to catoptric or catadioptric systems. Few such systems exist which combine the characteristics of high speed (eg. faster than V4) 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. 23630 7/23 6308 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 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. This invention is an optical design which is novel in its assembly of known techniques into a format that fits a previously unoccupied area of the speed/diameter relationship. Furthermore, it is a low light high speed optical system of economic construction which, at least,provides the public with a useful choice. Accordingly, the invention consists in: a lens system comprising: a primary mirror having a spherical reflective surface; a secondary mirror having a spherical reflective surface and being arranged to SUMMARY OF THE INVENTION: . patent office -3- 2 - FEB 1996 RECEIVED 23630 7/ 236308 receive light from the said primary mirror; said primary and secondary mirrors having the same centre of curvature and so constructed and arranged that the focal surface of the combined system lies between said primary and secondary mirrors; and image receiving means situated on or adjacent to the said focal surface, said image receiving means comprising a lens operable to project an image to an image receiving station. Preferably the secondary image is subjected to correction prior to receipt at said image receiving station. Preferably said correction is provided by a meniscus corrector. Preferably the iris of said camera is located at said image receiving station. Preferably the curvature of said meniscus corrector is concentric with the geometrical centre of the iris of said camera. Preferably said lens system further including an optical relay means said optical relay including a spherical mirror operable to perform a focusing function; a folding flat reflector constructed and arranged to receive and reflect light from said spherical mirror, and a plurality of corrector elements located on the non-incident side of said spherical mirror, each of said corrector elements having surfaces concentric with the curvature of said spherical mirror as reflected in said folding flat, said corrector elements being operable to correct for spherical aberration and chromatic aberration induced by one or both of said corrector elements and said spherical mirror. "l.Z. patent office 2 - FEB 1996 RECEIVED -3A- 236307/23630$ Preferably said corrector elements comprise refractive elements. Preferably said refractive elements each comprise achromatic doublets. Preferably said lens system further includes an aperture stop. Preferably said relay means further includes a very weak aspheric correcting surface at said aperture stop to remove residual high-order spherical aberration. Preferably said relay means further includes a field flattening lens at the focal point. Preferably said spherical mirror includes a central aperture and is located at the aperture stop, a plane mirror being provided to reflect incident light on to the spherical reflective surface. nz. patent office 2 - FEB 1998 -SB- received 23 6 3 23 6 3 0 To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims. The disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limiting. 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 Smidt or Maksutov cameras; Figure 3 is a drawing of one embodiment of the invention; Figure 4 is a drawings of an optical relay optionally forming part of the invention; Figure 5 is a perspective drawing of one embodiment of the invention; Figure 6 is a profile of the asphere of an embodiment of the invention in accordance with Example 1; Figure 7 is an illustration of the path differences through the trimmer in Example l; Figure 8 is a cross-sectional, side elevation view of the system of Example 1; Figure 9 is a graphical illustration of the performance of the system of Example 1; Figure 10 is a cross-sectional, side elevation view of the system of Example 2; Figure 11 is a graphical illustration of the performance of the system of Example n.z. patent office 2 - FEB 1998 received 236307 / 236308 2; Figure 12 is a cross-sectional, side elevation view of the system of Example 3; Figure 13 is a graphical illustration of the performance of the system of Example Figure 14 is an illustration of an embodiment of the invention including a cryostat window; Figure 15 is an elevation of an alternative embodiment of the invention; Figure 16 is an elevation of a further alternative embodiment of the invention. DETAILED DESCRIPTION OF THE INVENTION: This invention is a concentric Cassegrain 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 speed/image-scale parameters. Figure 1 shows the ranges of apertures and speeds for which design types are appropriate. This invention is appropriate for the area 1. The starting point for the concept description is the concentric Cassegrain Schmidt or Maksutov camera designs shown 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 and shperochromatism is the Maksutov corrector 22, the image quality of this design is uniform over the whole field. The Schmidt corrector, located at the common centre of curvature of the mirrors, has an axis of symmetry, as does the Maksutov meniscus in its achromatic forms. The -8- 236307/ 236308 fabrication penalties of these designs are the full-aperture aspheric or 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, 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 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. 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 field/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 236307 236308 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 menisci 41 and 42 provide 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 use of two menisci 41 and 42 is dictated by the chromatic aberration introduced by these refracting elements. The stronger meniscus 41 provides the main spherical aberration correcting function, but as a singlet would introduce longitudinal . and lateral chromatic aberration. By making this a doublet, the longitudinal aberration can be controlled, and by introducing the second meniscus 42 as a correcting doublet and positioned closer to the object location, the residual lateral chromatic aberration can be virtually eliminated. It should be noted that there are two centres of curvature in Fig. 4. The centre of the aperture stop 43 is the centre of curvature of the two menisci 41 and 42, but this centre is reflected to the position 44 by the foldirg flat 45. This arrangement makes it possible to achieve an external focus at 46 for greater accessibility. In Fig. 4 the folding 48 and relay 47 mirrors are positioned so as to make the central perforation of the relay mirror act 47 as the aperture stop 43 of the relay, but this is only cosmetic. 236307 / 236308 The embodiment described hereinafter separates these two apertures to achieve minimum vignetting. 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 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 very first embodiment described in this specification, the field flattening lens is 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. Egure 8 shows the layout resulting from the merge with meridional rays shown at 1.24 degrees off axis. The system has a primary mirror 51 and a secondary mirror 52. The system also includes a corrector group 53 and a folding flap 54. Two main differences are evident from the relay described above; the lateral chromatic aberration correction task of the weaker meniscus 42 has been included in one of the field/transfer lenses 55, which form a singlet and doublet group, and, as previously mentioned, a small displacement of the relay mirror 56 central perforation from the aperture stop 57 has been introduced so as to minimise vignetting variations. 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 236307 / / 236308 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/l ray cone with implicit degradation of the image sharpness. As shown in the system in Figure 8, 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, necessarily associated with the field/transfer group for net chromatic correction of the refractive components. (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 field/transfer group. (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 the examples below, the central obscuration is about 30% on axis, increasing only to about 33% at the circumference of the 11mm diameter image. There is no vignetting of the marginal rays when all components are dimensioned as specified. 236 30 7/236 30 8 (f) distortion is minimal, caused primarily by some variants of the field/transfer design, with an amplitude generally no greater than that of the blur spot dimensions, but in examples 1 and 3 following, it is negligible. To minimise future fabrication costs, the example designs make use of common radii of curvature on some of the refractive components. Although non-optimum for residual aberration correction, the difference in performance is marginal and probably would be lost in workshop tolerance variations. All refractive surface radii are included in "Smith's List". The choice of optical glasses was set by empirical constraints: ready availability and relatively low cost; "ghost surface" achromatisation at the'd' line of helium (notionally the wavelength at the centre of the index range for 850 - 1080mm bandpass) so as to achieve minimum curvatures; moderately high refractive indices for minimum curvatures and also for the simplest anti-reflection coating. The aspheric trimmer is easier to fabricate if the polynomial curve is superimposed on a very-long-radius convex spherical surface so that no part of the surface is concave. The sagitta of the combined curve is 10^m in the examples following, and the desired interferometer test pattern is shown. The deviation from the long radius sphere is <lym. It was mentioned above that high index glasses were chosen for minimum curvatures and also for the simplest antireflection coatings. With an index of 1.6, even a single layer of MgF provides useful antireflection properties, and two-layer systems rather better over the SOOnm bandwidth. There are at least eight air-glass surfaces in this system, so a good antireflection coating is important. n.2. patent office 2 - FEB 1996 RlCLWED -10- 236307 /236308 Consideration also must be given to the need for high reflectance at the four mirrors if a high total transmission is to be achieved. JUuminising may not be the most appropriate coating system, when it is considered that the total reflective loss may be over 45%. Silvering is inherently superior, especially in the 500 - llOOnm spectral band, as the reflective loss of four surfaces should not exceed 10%. The three following examples have identical reflective components, varying only in the design of the field/transfer group (plus the necessary corresponding variations in the corrector group). The specification table is 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. "D" and "d" are the outside and inside diameters of annuli. Blanks are default O's. The shape of the aspheric trimmer profile is defined by the polynomial equation: z = cjt2/2 + a2-x4 + a«.x* + ^.x8 where c is the curvature, x is the x-coordinate and a„ are the coefficients. The bandpass 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 500 and llOOnm. The corresponding refractive index data for optical glass is published for the spectral lines e, d, C, r, s and nIOeo.o' amongst others, providing a good coverage for analytic purposes. The nI06o.o Neodymium glass laser line is labelled as Nd. The ci&sign process starts with the basic system assembly, using the Helium d-line to ray-trace the geometry and specify the aspheric trimmer polynomial coefficients. The concentric reflective components are located in their correct positions, and the 236307 / „ / 236308 basic refractive components inserted. At this stage chromatic correction is not necessary, provided that the glasses are chosen with nearly identical refractive indices at the d-line. The initial ray-tracing process always shows some coma as the dominant residual aberration, emanating from the off-axis functioning of the field/transfer group. This is largely corrected by introducing equal and opposite coma within the reflective components, the technique chosen here being that of increasing the focal length of the field/transfer group 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. In the following examples, this increase in focal length places the transferred centre of concentricity at a z-axis location of 1623.34, 1629 and 163L6 for example? 1, 2 and 3 respectively. This compares with the nominal location of 1600 for all three. Because of this modification, the median ray aberration graphs which follow, have as their vertical axis the height of the ray in the aperture stop, rather than that in the entrance pupil. For compactness, these vertical axes are not labelled. 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 18 X 15pm focal patch. 236307 / / 236308 EXAMPLE 1 Class Zvertex Curvature Radius M/& d D Notes laoo -.00066667 -1500 Mir 100 320 : Primary 950 -.00105263 -J>50 Mir 108 ' Secondary 1260 0.00265796 34J.V Len 60 1st field/ F 4 1270 -.00265796 Len 60 transfer 1315 0.00285796 349.9 Len 60 2nd field/tran F 4 1327.5 0.00497760 JOftJL Len 60 s lac chroma SK 4 1340 0.00285796 349.9 Len 60 corrector 1531.95 ; 0.01469508 68.05 Len 60 Sphsr. aberr. F 4 1540 0.00815727 "rarer Len 60 corrector SX 4 1546.74 0.01877582 53.26 Len 60 doublet 1592 0.00 flat Len 57.5 Asph. trimmer SF 2 1600 -.0000225 long+asph Len 57.5 9 aperture stop 1695 0.0 flat Mir 47 110 Folding mirror 1590 0.005 200 Mir 65 163 . Relay mirror 1722.67ft 0.01942125 51.49 Len 12 Field SF 2 1723.675 0.0 flat 2/3" CCD foe 12 flattener bonded te CCD The aspheric surface polynomial coefficients are: = -1.5 E-5 a« = +3.63 E-8 a« = -1.97 E-ll a^ = -5.20 E-15 The profile is illustrated in Fig. 6. Note that the z-axis is expanded by a factor of 1000 relative to the (vertical) x-axis. Fig 7 illustrates the path difference through the trimmer at 0.5/im intervals. Fig. 8 provides a side elevation of this example design, and Fig. 9 gives a graphical illustration of the computed performance. In this design the field/transfer group 81 has the simple form of a singlet and doublet with common external curvatures but with a different doublet interface curvature. The two components are well spaced and the "Cassegrain" focus is located between them to avoid imaging a surface onto the CCD array. The meridional ray aberration curves in Figure 9, demonstrate moderate stability, with no tendency to chaotic extremes as the field angle is increased. As the 2D histograms show, almost all the energy is focused into the 15 X 15pm focal patch at 1.232* off axis (the angular radius corresponding to the corner of the [<2/3 inch" image), -13- 236307 / 236308 and entirely so at 1' and below. Image distortion is essentially zero, being considerably smaller than the blur spot diameter. EXAMPLE 2 Glass Zvertex Curvature Radius M/L d D Notes 1S00 -.00066667 -1500 Mir 100 330 Primary 950 -.00105263 -950 Mir 108 Secondary 1270 0.00 £lat Len 60 1st field/ F 4 1280 -.00530223 -188.6 Len 60 transfer 1300 0.00530223 188.6 Len 60 2nd field/tran SX 4 1315 -.00530223 -188.6 Len 60 & lat chroma r 4 1330 0.00530223 188.6 Len 60 corrector 1531.95 0.01469508 68.05 Len 60 Spher. aberr. F 4 1540 0.00789951 126.59 Len 60 corrector SK 4 1546.74 0.01877582 53.26 Len 60 doublet 1592 0.00 flat Len 58 Asph. trimmer SF 2 1600 -.0000225 long+asph Len 58 0 aperture stop 1695 0.0 flat Mir 47 110 Folding mirror 1590 0.005 200 Mir 65 163 Relay mirror 1722.871 0.018392S 54.37 Len 12 Field SF 2 1723.871 0.0 flat £oc 12 flattener 2/3" CCD bonded to CCD The aspheric surface polynomial coefficients are: aj, = -1.81 E-8 a, = +3.93 E-8 a« = -1.88 E-ll a, = -8.6 E-16 The difference between this surface and that of the preceding example is minimal. Figures 6 and 7 illustrate the general form. This example illustrates the dependence of the residual aberrations upon the optical characteristics of the field/transfer group. Refer to Fig. 10. When the first component of this group is made as a plano-convex lens and the group design adjusted so that its convex surface and all surfaces of the following doublet component have common radii of curvature (for ease of fabrication), the resulting residual aberrations are significantly worse than those of example 1, as can be seen in the diagrams in Figure 11. -14- 236307 f 236308 The reason is, of course, that the curvatures of the air/glass surfaces have been increased to almost twice those of the corresponding surfaces in example 1. Note that the closer spacing of the field/transfer components has resulted in a smaller spread of the off-axis aberration curves, ie. the lateral chromatic aberration correction is improved even though residual coma is worse than that in example 1. The focal spot histograms in Figure 11 show that astigmatism is also significantly increased off-axis because of the greater curvatures of the field/transfer components' surfaces. Analysis of the focal surface intercept values and the corresponding off-axis angles in Figure 11 shows that a small amount (0.46%) of barrel distortion has been generated by this field/transfer design. EXAMPLE 3 Glass Zvertex Curvature Radius M/L d 0 Noees 1500 -.00066667 -1500.0 Mir 100 320 Primary 950 -.00105263 -950.0 Mir 108 Secondary 1280 0.00305064 327.8 Len 56 lse field/ r 4 130S -.00305064 -327.8 Len 56 transfer 1315 0.00305064 327.8 Len 56 2nd field/ r 4 1320 0.01263584 92.42 Len 56 transfer • SK 4 1335 -.01263584 -92.42 Len 56 6 lat. chroma F 4 1340 0.00305064 327.8 Len 56 corrector 1528.IS 0.01391982 71.84 Len 60 Spher. aberr. r 4 1540 0.0049776 200.9 Len 60 corrector SK 4 154S.74 0.01377582 53.26 Len 60 doublet 1592 0.00 flat Len 57. S Asph. trimmer SF 2 1600 -.00002 long+asph Len 57.5 9 aperture stop 1695 0.0 flat Mir 47 110 Folding mirror 1590 0.005 200 Mir 65 163 Relay mirror 1724.446 0.02353626 43.79 Len 12 Field sr 2 1725.446 0.0 flac foe 12 flattener 2/3- CCD bonded to CCD The aspheric surface polynomial coefficients are: a, = -1.12 E-8 a« = +3.72 E-8 a« = -2.07 E-ll a, = -8.4 E-13 The difference between this surface and that of example 1 is minimal and Figures 6 and 7 illustrate the general form. -18- 23630 if236308 This example is intended to illustrate the benefits of placing the strongest refractive surfaces of the field/transfer group as near as possible to the "cassegrain" focus (but not so close as to reimage the surfaces onto the CCD). The first component of the group is therefore made as a thick lens straddling the focus; the penalty being that the second component has to be a triplet in order to provide lateral colour correction with moderate curvatures of the element surfaces. In practice an inside-glass focus is undesirable as it is liable to introduce image defects due to inclusions, or other glass-melt features, but glass selection should eliminate all such problems. The advantage of this design is immediately obvious from inspection of the aberration curves and spot histograms in Fig. 13, where a more stable regime of residual aberrations is evident Unfortunately, this is offset by the more difficult fabrication. A characteristic of this example design is evident in the virtual absence of image distortion, the residual non-linearity between field-angle and image point being negligible compared to the diameter of the blur spot. The designs given are, of course, scalable, but are based around the readily available primary mirror blank of the nominal '12Ya inch' size, allowing a '/4m entrance pupil diameter. The relay mirror specification, with a strong curvature of 200mm radius, was adopted because such a mirror was available, recovered from an old Schmidt television projector. The combination gives a final speed close to f/l but obviously this can be varied by a different choice of relay minor curvature and adjustment of the corrector group and field/transfer group specifications. Any such change should be application-specific, and especially detector-specific, because of the 236307 interaction of design paranietezs. For example, using the same size primary mirror and detector format as in the previous examples, but with 1 150mm radius of curvature relay mirror, the principle can be adapted to give a 3.3*, ^0.83 system specification with the same blur spot dimensions as demonstrated previously. Scaling down is appropriate only while an advantage exists over refractive optics in current production. These are limited to about 150mm diameter by the volume homogeneity of present optical glass melts. Scaling up is appropriate until this same limit obtrudes into the design of the refractive components of the system itself. With the ratio of entrance pupil diameter to transfer and corrector lens diameters of about 4:1, it is probable that, at least for quantity production, the practical upper limit of scaling would give entrance pupil diameters of about 600mm. Such upward scaling would be a suitable match to the large-area scientific CCD arrays now available. Considerable scope exists to extend the simple refractive sub-system designs given in the three examples presented in this paper. Spectral bandpass specifications are easily changed, but more complex designs such as apochromatic vruiants are also possible, as are wholly refractive or catadioptric relay modules for other detector systems (including photographic emulsions). A further extension of importance when a cryostat is to be used, is that the cryostat window 141 must share in the concentricity of the relay optics. This is easily done in the design stage by partitioning the corrector so that some of the spherical aberration correction of the system is carried out by the cryostat window 141. An illustration of this is shown in Figure 14. The design concept described at the beginning of this specification implied that a 17- 236307 /236308 concentric spherical "cassegrain" starting point was necessary for the system's principle. This is not strictly true, because the spherical secondary can be replaced by a flat and still permit the requisite transfer of the classical Schmidt centre of curvature and aperture stop to the relocated position appropriate for the relay subsystem. Unfortunately, if the system is to remain uniaxial, a penalty is exacted - the central obscuration is increased. However, if a flat is employed, there is no reason why a quasi-Newtonian geometry can not be derived, as in Fig. 18 below. The optical parameters are different enough from those of the previous examples' common reflective optics, that some interesting corollaries become evident for example, some existing "fast" Newtonian optical designs could be converted into this system's type of "wide-angle" camera, although this may require that the paraboloid primary has to be reworked into a spherical figure. Similarly, if a flat is used, it can, in principle, be inserted anywhere in the optical train; the optimum location would probably be at that of the minimum cross-section. In the generic geometry of this invention, this would be between the field/transfer and . corrector groups, so by accepting a somewhat increased system length and an unfolded relay, the central obstruction can be made smaller and to have constant obscuration at all field angles. See Figure 16. The improvement in image quality achieved by the invention over previous designs is evidenced by a greatly increased (and virtually unvignetted) image brightness combined with a uniformity high sharpness over the whole image. Obviously the most appropriate applications are where existing designs do not fully resolve the conflict between low light levels and the requirement for high data acquisition rates and high 236307 / 236308 quality image parameters. Four areas of interest are: (a) Aatrography, including astrometry and fast multiple-point photometry, (b) Remote sensing from aircraft and orbiting satellites, (c) Security surveillance, especially nocturnal, (d) High speed frame captuze. Eg. fast electronic shuttering of moving objects. -19- </div>

Claims

<div class="application article clearfix printTableText" id="claims"> 23 6 30J7 23 6 3 00 WHAT WE CLAIM IS:

1. A lens system comprising: a primary mirror having a spherical reflective surface; a secondary mirror having a spherical reflective surface and being arranged to receive light from the said primary mirror; said primary and secondary mirrors having the same centre of curvature and so constructed and arranged that the focal surface of the combined system lies between said primary and secondary mirrors; and image receiving means situated on or adjacent to the said focal surface, said image receiving means comprising a lens operable to project an image to an image receiving station.

2. A lens system as claimed in Claim 1 wherein the secondary image is subjected to correction prior to receipt at said image receiving station.

3. A lens system as claimed in Claim 2 wherein said correction is provided by a meniscus corrector.

4. A camera including the lens system as claimed in Claim 3 and where the camera iris is located at said image receiving station.

8. A lens system as claimed in Claim 4 wherein the curvature of said meniscus corrector is concentric with the geometrical centre of the iris of said camera.

6. A lens system as claimed in Claim 1 further including an optical relay means said optical relay including a spherical mirror operable to perform a focusing function; a folding flat reflector constructed and arranged to receive and reflect light from N.2. patent office J 2 - FEB 19SS i -20- 23 6 3t) > 363 03" said spherical mirror, and a plurality of corrector elements located on the non-incident side of said spherical mirror, each of said corrector elements having surfaces concentric with the curvature of said spherical mirror as reflected in said folding flat, said corrector elements being operable to correct for spherical aberration and chromatic aberration induced by one or both of said corrector elements and said spherical mirror.

7. A lens system as claimed in Claim 6 wherein said corrector elements comprise refractive elements.

8. A lens system as claimed in Claim 7 wherein said refractive elements each comprise achromatic doublets.

9. A lens system as claimed in any one of claims 1 to 8 further including an aperture stop.

10. A lens system as claimed in Claim 9 wherein said relay means further includes a very weak aspheric correcting surface at said aperture stop to remove residual high-order spherical aberration.

11. A lens system as claimed in Claim 10 wherein said relay means further includes a field flattening lens at the focal point

12. A lens system as claimed in Claim 6 wherein said spherical minor includes a central aperture and is located at the aperture stop, a plane mirror being provided to reflect incident light on to the spherical reflective surface. n.z. patent office: 2 - FEB 1996 received -21- </div>