Catoptric Optical System Including Concave and Convex Reflectors

The present invention relates generally to the field of optical systems and more particularly to an all-reflective, unobscured optical system.

An all-reflective (or catoptric) optical system avoids chromatic aberration because there is no dispersion from a reflective surface as there is through a lens (which also produces transmission losses). A catoptric system thus allows a broad spectrum of electromagnetic radiation, from infrared to ultraviolet, to be focused identically within the limits of geometric optics.

Spherical aberration is produced by optical systems having different focal distances for different input ray heights, causing a rotationally symmetric blurred image of a point object. Comatic aberration, or coma, is produced when rays have different image heights depending on the distance of the ray from the center of the entrance pupil. The resulting image of a point object resembles a cone. Astigmatism is produced by optical systems having different focal distances in orthogonal directions, resulting in an image of a point ranging from a line to a circular blur, depending on the observation point.

An afocal optical system produces collimated exit beams from collimated entrance beams. A focal system produces focused exit beams from collimated entrance beams. The image of the aperture stop in object space is called the entrance pupil and is typically located at the primary mirror. The image of the aperture stop in image space is called the exit pupil.

Conventional afocal two-mirror and three-mirror systems, while providing aberration minimization, have the disadvantage of having a limited field of view when providing desirable diffraction-limited wavefront quality. Further, these conventional systems are typically overly long (having a length, or envelope, greater than three times the maximum pupil diameter) for many applications.

An example of a prior art two-mirror system is the Mersenne telescope which utilizes confocal paraboloidal mirrors producing an internal image. The field of view is quite narrow, typically 0.2.times.0.3.degree. for diffraction-limited wavefront quality at 1000 nm wavelength. Further, Mersenne telescopes have a low exit pupil clearance (less than one exit pupil diameter from the nearest beam) making coupling to subsequent image-forming systems difficult. An example of a three-mirror system is U.S. Pat. No. 3,674,334 to Offner. This is a concave-convex-concave mirror system with the convex mirror providing field curvature correction allowing a greater field of view in one dimension (for example, greater than 4.degree. total field) and greater exit pupil clearance. However, this three-mirror system has a limited field of view in the other dimension (for example, 0.1.degree..times.4.00.degree. for diffraction-limited wavefront quality at 1000 nm wavelength). This results in an extreme rectangular field of view. Further, the envelope length is approximately 2.5 to 4 times the maximum pupil diameter, making the system too long for many space-limited applications.

An example of a four-mirror paraboloidal system for avoiding aberration is disclosed in U.S. Pat. No. 3,062,101 to McCarthy. Incident light is reflected by a concave mirror to a fold-flat mirror which redirects the beam 90.degree. to a second concave mirror which is concentric with a third convex mirror and a fourth concave mirror, producing an image beyond the fourth mirror. Because of the concentric configuration, the fold-flat mirror causes obscuration which then necessarily limits the size of the fold-flat mirror and causes throughput limitations and diffraction effects on the image.

In summary, for many multiple mirror systems, there is the problem of possible obscuration caused by mirrors blocking rays and thereby decreasing both the amount of radiation gathered and the field of view while introducing adverse diffraction effects in the image. Further, multiple mirror systems, because of geometric requirements for achieving true images, may be overly large or bulky for many applications (such as airborne and outer space applications with their attendant packaging limitations). Still further, the entrance and exit pupils are typically not easily accessible, thereby causing other problems such as vignetting.

Accordingly, a principal object of the present invention is to provide an improved catoptric optical system. Further objects of this invention are to provide a compact, all-reflective optical system producing significant beam reduction while avoiding aberration and distortion and achieving a wide and symmetric field of view with high image quality. Still further objects of the invention are to provide a system having little or no obscuration, easily accessible exit and entrance pupils, and the capability of turning the line of sight on the order of 90 degrees.

These and other objects are realized by an optical system comprising substantially aspheric first, second, third, and fourth reflection means. In an afocal embodiment, the third reflection means is proximate the internal image produced by the first and second reflection means and provides aberration control and a compact beam folding function. In a focal embodiment, the second reflection means is proximate the internal image produced by the first reflection means.

A further understanding of the nature and advantages of the present invention may be realized by reference to the Detailed Description of the Invention and the attached drawings and tables.

FIG. 1 is a schematic diagram of an afocal four-mirror aspheric optical system according to the present invention.

FIG. 2 is a schematic diagram of a focusing four-mirror aspheric optical system according to the present invention.

FIG. 3 shows wavefront aberration plots along the horizontal and vertical pupil meridians for a 9:1 four-mirror afocal system according to the present invention.

FIG. 4 shows a wavefront aberration map over the full pupil at a wavelength of 1000 nm for a 9:1 four-mirror afocal system according to the present invention.

FIG. 1 is a schematic diagram of an optical system 100 according to a preferred embodiment of the invention. Incident beams 101 from an object (not shown) enter system 100. Beams 10 are reflected by a primary aspheric concave mirror 110 which causes the beams 101 to converge to beams 102. Beams 102 are incident on a secondary aspheric convex mirror 120 which reflects the convergent beams 103. Beams 103 are incident on a tertiary aspheric mirror 130. In one embodiment of the invention, mirror 130 is convex and in another embodiment, mirror 130 is concave, but in both embodiments, mirror 130 is preferably aspheric to correct aberration, but not precisely flat. Mirror 130 is also preferably disposed proximate the internal image formed by mirror 120. This allows mirror 130 to control field aberrations such as coma and astigmatism, thereby providing a large two-dimensional field of view which is substantially symmetric as either a square or a circle. A beam 104 is reflected from mirror 130 to an aspheric concave fourth mirror 140 which substantially collimates beam 105 to an exit pupil 150 Exit pupil 150 has a significant axial clearance below the nearest beam (103). As an example, the axial clearance can be two exit pupil diameters, thereby making beam 103 accessible to the entrance pupil of any following optical systems. Mirror 130 is disposed at a large tilt angle (for example 30.degree.) with the incident beam 101, thereby providing a beam folding configuration wherein reflected beams 104 and 105 pass in front of mirror 110 twice. This provides a beam folding function between entering and exiting beams that allows an extremely compact system configuration. In a preferred embodiment, system 100 is in a ball-like configuration advantageously suitable for gimballed and/or pod-mounted system packages. Further, the folding function provided by mirror 130 produces an exit beam 105 that is substantially normal to the incident beam 101. An entrance pupil may be disposed anywhere along beam 101 and is preferably disposed at mirror 110. Therefore, entrance and exit pupils are conveniently accessible. Exit pupil 150 is sufficiently clear of the last mirror 140 of system 100 to provide convenient coupling for image access instruments such as scan mirrors, beamsplitters, and focusing subsystems.

FIG. 1 further shows the axes of each of the mirrors 110, 120, 130, and 140. Short dashed lines 161, 162, 163, and 164 show the imagined curvature extensions of the surfaces of mirrors 110, 120, 130, and 140 respectively. Long dashed lines 171, 172, 173, and 174 denote the axes of mirrors 110, 120, 130, and 140 respectively. In the preferred embodiment, axis 173, is not coaxial. This non-coaxiality together with an optimized optical power distribution and placement of mirrors 110, 120, 130, and 140 provide image non-obscuration and achieve optimum image quality. However, primary mirror 110 and secondary mirror 120 in particular may be coaxial depending upon specific design requirements, and still be within the scope of the invention, and fourth mirror 140 can be coaxial with the reflected axes of mirrors 110 and 120 (reflected through mirror 130) in alternative embodiments of this invention.

Magnifying power is produced by the ratio of the radii of curvature of the mirrors and their relative locations. In a 9:1 magnifying power embodiment, the shapes of mirrors 110, 120, 130, and 140 are concave, convex, convex, and concave respectively. In the preferred embodiment, all mirrors 110, 120, 130, and 140 are aspheric. The relative powers in one compact 9:1 embodiment are 1:2.94:0.50:2.17. In an alternate embodiment having a 4:1 magnifying power, the shapes of mirrors 110, 120, 130, and 140 are concave, convex, concave, and concave respectively. The relative mirror powers in a compact 4:1 embodiment are 1:2.69:0.56:1.57. Various alternate embodiments in different mirror shape orders and different powers are within the contemplation of this invention. For example, an intermediate 6.5:1 image reduction embodiment utilizes a tertiary mirror 130 having almost no base curvature. The relative mirror powers for a compact 6.5:1 embodiment are 1:2.90:0.006:1.94.

The afocal, all-reflective (catoptric) system of this invention therefore provides significant magnifying power while minimizing aberration and providing full spectrum coverage at high wavefront quality. The nearly flat aspheric tertiary mirror 130 disposed proximate the internal image provides a large, even, symmetric (for example, a 1:1 aspect ratio) field of view which can be either square or circular. An example of this embodiment has a 100 mm diameter entrance pupil and a 3.times.3.degree. object space field of view that is diffraction-limited at 1000 nm. The folding function provided by mirror 130 and the configuration of mirrors 110, 120, 130, and 140 provide a compact design. In the embodiment prescribed below, the maximum envelope diameter is less than 2.3 times the maximum pupil diameter. Exit pupil 150 in this embodiment is 17 exit pupil diameters away from mirror 140 and approximately two exit pupil diameters below beam 103, thereby providing excellent pupil clearance. In fact, in the preferred embodiment of the invention, exit pupil 150 is external even when the aperture stop is external to, or at, primary mirror 110.

Table I presents surface prescription data for a 9:1 image reduction embodiment. In Table I, a "decenter" (D(j)) defines a new coordinate system (displaced and/or rotated) whereby subsequent surfaces are defined. Surfaces following a decenter are aligned on the local mechanical axis (z-axis) of the new coordinate system. The new mechanical axis remains in use until changed by another decenter. The order in which displacements and tilts are applied on a given surface is specified using different decenter types which generate different new coordinate systems. Positive radii of curvature means that the center of curvature is to the right and negative radii of curvature means that the center of curvature is to the left. All dimensions are given in millimeters. Thickness means axial distance to the next surface. As explanation of Table I, A(i) indicates the aspheric mirror i defined by the equation ##EQU1## where the base curvature (CURV) and aspheric constants K, A, B, C, and D are given in Table II. Aperture diameter is denoted by C-1 . . . C-4 and aperture data in terms of diameter, decenter system and rotation are given in Table III. Decenter system data are given in Table IV wherein tilt configurations are defined by angles alpha, beta, and gamma (degrees) following standard cartesian coordinate system nomenclature. The trailing code RETU means return to the coordinate system preceding the decentration and BEND means tilting the coordinate system following the reflection by an amount equal to the tilt of the surface in question. The reference wavelength is 1000.0 nm.

An example of the image quality achieved by the 9:1 four-mirror afocal embodiment of the present invention is shown in FIGS. 3 and 4. FIG. 3 shows wavefront aberration plots along the horizontal and vertical pupil meridians and FIG. 4 shows a wavefront aberration map over the full pupil at a wavelength of 1000 nm. RMS wavefront error at 1000 nm is less than 0.055 across the 3.times.3.degree. field for a 100 mm entrance pupil design.

FIG. 2 shows a focusing four-mirror optical system 200 according to an alternate embodiment of the present invention. Incident beams 201 from an object (not shown) enter system 200. Beams 201 are reflected by a primary aspheric concave mirror 210 which causes the beams 201 to converge to beams 202. Beams 202 are incident on a secondary aspheric convex mirror 220 which reflects the divergent beams 203. Beams 203 are incident on a tertiary aspheric mirror 230. The placement of secondary mirror 220 is such that the intermediate image is near secondary mirror 220 (and not near the tertiary mirror as in the afocal system of FIG. 1). This produces focused exit beams 205 at 250 from collimated entrance beams 201. In one embodiment of the invention, mirror 230 is convex and in another embodiment, mirror 230 is concave, but in both embodiments, mirror 230 is preferably aspheric to correct aberration, but not precisely flat. A beam 204 is reflected from mirror 230 to an aspheric concave fourth mirror 240 which substantially focuses beam 205 to an exit pupil 250. Exit pupil 250 has a significant axial clearance below the nearest beam (203). For example, the axial clearance is almost two exit pupil diameters, thereby making beam 203 accessible to the entrance pupil of any following optical systems. Mirror 230 is disposed at a large tilt angle (for example 35.degree.) with the incident beam 201, thereby providing a beam folding configuration wherein reflected beams 204 and 205 pass in front of mirror 210 twice. This provides a beam folding function between entering and exiting beams that allows an extremely compact system configuration. In a preferred embodiment, system 200 is in a ball-like configuration advantageously suitable for gimballed and/or pod-mounted system packages. Further, the folding function provided by mirror 230 produces an exit beam 205 that is substantially normal to the incident beam 201. An entrance pupil may be disposed anywhere along beam 201 and is preferably disposed at mirror 210. Therefore, entrance and exit pupils are conveniently accessible. Exit pupil 250 is sufficiently clear of the last mirror 240 of system 200 to provide convenient coupling for image access instruments such as scan mirrors or beamsplitters.

While the above description provides a full and complete description of preferred embodiments of the present invention, various modifications, alternate constructions, and equivalents may be employed while still remaining within the scope of the invention. As a first example, while some of the above descriptions refer primarily to afocal configurations, focal variations are within the scope of the invention. As a further example, embodiments other than the power ratio examples given and other applications not cited are within the scope of the invention. Therefore, the above description and illustrations should not be construed as limiting the scope of the invention which is defined by the following claims.