WO2013041847A1

WO2013041847A1 - Anastigmatic optical system

Info

Publication number: WO2013041847A1
Application number: PCT/GB2012/052299
Authority: WO
Inventors: Neil Edward BOWLES; Ranah IRSHAD
Original assignee: Isis Innovation Limited
Priority date: 2011-09-20
Filing date: 2012-09-18
Publication date: 2013-03-28
Also published as: GB201116247D0

Abstract

According to an embodiment of the invention there is provided an optical system for receiving and focusing optical signals, comprising a primary mirror for creating an accessible intermediate image at an intermediate plane,a tertiary mirror, quaternary mirror; and a final mirror,wherein the tertiary mirror, quaternary mirror and final mirror are arranged for forming a final image at a final image plane.

Description

Anastigmatic Optical System

The present invention relates to all reflective optical systems. Background

Reflective optical systems are preferred for many applications due to their lightweight construction and size, and also due to their broad spectral coverage. They also provide good thermal stability, resistance to radiation and reduced defects from chromatic aberration.

A particular type of all-reflective optical system is a three mirror anastigmat (TMA). The TMA permits correction of three types of geometric aberration (spherical aberration, coma and astigmatism). An application of the TMA is in the design of a radiometer. It is desirable with such instruments to use an uncooled detector array. However, since uncooled detectors have poor signal-to-noise performance compared with cooled detectors, a final focus of the instrument should be relatively fast e.g. f/#2. However, it has been noted that instruments having a fast beam at the detector may have compromised performance, particularly in certain frequency ranges. The compromise in performance may result in high a signal-to-noise level and spurious signals at the detector.

It is an aim of embodiments of the invention to address one or more problems of prior art optical systems.

Summary of the Invention

Embodiments of the invention include an accessible intermediate image plane in which one or more optical components may be located for modifying light beams passing there-through. By "accessible" it is understood that the image formed at the intermediate image plane is not overlapped or intersected by light beams reflecting from other components of the optical system or that individual image spots in the intermediate plane do not themselves overlap. The image spots are spaced apart in the intermediate plane. The image spots may be spaced apart horizontally and/or vertically. The optical components locatable at the intermediate image plane may comprise one or more filters. Some embodiments of the invention include magnification of the image at the intermediate image plane. The image is magnified at the intermediate image plane in comparison with a final image formed at a final plane. The final plane is also accessible to allow for one or more optical detectors to be arranged at the final plane.

Some embodiments of the invention include one or more mirrors for folding a beam path of the optical system to provide a compact footprint of the optical system. The mirror(s) extend focal lengths and fold a beam path of the optical system. Some embodiments of the invention include five mirrors. In some embodiments, four of the five mirrors are aspheric whilst the remaining mirror is spheric. The aspheric mirrors may be even aspherics. Brief Description of the Drawings

Embodiments of the invention will now be described by way of example only, with reference to the accompanying figures, in which: Figure 1 is a schematic illustration of an optical system according to an embodiment of the present invention;

Figure 2 is a perspective view of the optical system shown in Figure 1 ; Figure 3 is an illustration of image spots in intermediate and final planes of the optical system; and

Figure 4 is a perspective view of an optical bench according to an embodiment of the invention.

Detailed Description of Embodiments of the Invention

Figure 1 illustrates an optical system 100 according to an embodiment of the invention. The optical system 100 comprises a primary mirror 110, a secondary mirror 120, a tertiary mirror 130, a quaternary mirror 140 and a final mirror 150. The optical system is shown in perspective in Figure 2. An optical bench corresponding to the optical system of Figure 1 is shown in Figure 4 where like parts have identical reference numerals. The optical bench of Figure 4 also includes a scanning mirror 170.

In Figure 1, beam paths are illustrated from an aperture 105. Figure 2 illustrates the optical system in perspective. In Figure 2 the optical system is exposed to light from a circular aperture comprising four different off-axis fields at ±4° from a central axis. The central axis may be defined as an axis intersecting a centre of all the mirrors and the aperture, parallel to the optical bench.

Embodiments of the invention form an accessible in-focus image at an accessible intermediate plane 160 which is located between the secondary mirror 120 and the tertiary mirror 130. In embodiments of the invention, the image formed at the intermediate plane 160 may be modified by the inclusion of one or more optical components at the intermediate plane 160. The optical components may include one or more filters (not shown in the Figures). This is in contrast to prior art TMA designs where an intermediate is not expected to be projected onto an optical component and thus an intermediate image plane is overlapped by other light beams, such as those reflected by a mirror subsequent to the intermediate plane, making the intermediate image inaccessible. In particular, in embodiments of the invention, the image at the intermediate plane 160 is a magnified version of the image formed at a final plane 170, such as where a detector (not shown in Figure 1) may be located. The magnification ratio between the intermediate and final image planes may be in the range of 1.5: 1-2.5: 1 (intermediate: final); or around 1.7: 1-1.9: 1; in some embodiments the magnification ratio may be around 1.7: 1 or 1.8: 1. Other magnification ratios may be envisaged. Magnification of the intermediate image improves the accessibility of the intermediate image and allows a greater number of fields, or individual image spots, to be formed at the intermediate plane 160 since filter size is a significant limiting step in the design of a compact radiometer. In embodiments of the invention, the intermediate image is formed at the intermediate plane 160 by a combination of reflecting surfaces of the primary 110 and secondary mirrors 120. However, embodiments of the invention may be envisaged which only comprise the primary mirror prior 110 to the intermediate plane 160. Advantageously, the secondary mirror 120 extends a focal length of the optical system and folds the beam path of the optical system, thus enabling an optical system with a smaller footprint to be formed. The primary mirror 110 is an asphere, such as a parabolic or a higher-order aspheric mirror. The secondary mirror 120 is spheric mirror. The secondary mirror 120 is arranged to fold and to accelerate a beam path of the optical system 100 to bring the beam into focus at the intermediate plane 160. The intermediate plane 160 is easily accessible whilst remaining relatively close to the primary mirror 110 to ensure the compact size of the optical system 100. Advantageously, this allows a field offset to be increased thereby allowing the optical system 100 to operate further off-axis that a conventional TMA. The secondary mirror 120 also corrects for some field curvature caused by the primary mirror 110.

The image at the intermediate plane 160 formed by the corrector mirror 120 is arranged to allow one or more optical components, such as filters, to be located at the intermediate plane 160. In some embodiments, a plurality of filters may be arranged at the intermediate plane 160. In some embodiments, the secondary mirror 120 may form a plurality of image spots having a spot size of around 1 μιη at the intermediate plane 160 wherein each image spot may be differentiated from every other image spot i.e. without overlapping. In this way, one or a series of filters may be placed at corresponding image positions. A filter may be placed at a corresponding image spot position. The filters may be interference-type or wire-mesh filters. When each image spot position is provided with appropriate baffling for a corresponding filter mount there may be minimal interference from stray or scattered light. In contrast to mounting the one or more filters directly in front of a detector, the location of the filters at the intermediate plane 160 may improve the filter performance, especially in certain frequency ranges, such as far-infrared (0.7-1.4μιη). Stray radiation may be scattered from the filter(s) and result in high noise levels and/or spurious signals when the filters are located close to the detector. Figure 3(a) illustrates four image spot positions in the intermediate plane 160 as shown in Figure 2. It will be noted that the optical system may form fewer or more than four image spots. As can be appreciated, the first to fourth image spots 210-240 are spaced apart in both horizontal and vertical directions. However, embodiments of the invention may be envisaged in which the image spots are only separated in the horizontal or vertical directions.

The tertiary mirror 130 is an asphere having a positive power and includes convex and concave surface regions. In order to avoid beams reflected from the tertiary mirror 130 overlapping with those at the intermediate plane 160, the tertiary mirror 130 spreads light rays through a large angle. The shape of reflecting surface required to provide such a large angular distribution may introduce a geometric aberration. Consequentially, the quaternary mirror 140 may be required to correct for at least some of the geometric aberration. The quaternary mirror 140 is an asphere, such as a hyperboloid or higher order aspheric.

The final mirror 150 collects light in the optical system 100 and brings it to a fast focus at the final plane 170. The shape of the final mirror 150 may also correct for a large amount of field curvature which may be present due to earlier optical components. The tertiary 130 and final 150 mirrors are aspheres and may be conic, such as paraboloid, hyperboloid or ellipsoid, or a higher order aspheric.

Figure 3(b) illustrates image spots formed in the final plane 170. The arrangement of the image spots may correspond to the arrangement in the intermediate plane 150. However, it will be appreciated that the image spots in the final plane 170 are of smaller dimension resulting from the magnification between the intermediate and final planes 160, 170.

As will be appreciated, the mirrors 110, 120, 130, 140, 150 may be designed using a ray-tracing software program such as Zemax. The surface of each of the mirrors is determined by a sag equation, which may be in the form of:

Where z is the surface sag, R is the radius, K is the conic constant, r is the radial distance from the centre of the mirror and al, a2, a3 etc. are constants. From this equation, a prescription for each mirror of the optical system 100 can be generated. One such prescription is shown in Table 1.

Table 1: Optical prescription according to an embodiment of the invention The optical system 100 described in Table 2 has excellent image quality, and may include a spot size at the final plane of around 0.5μιη. The final plane 170 may have a speed of generally f/4. It must be stressed, however, that the prescription of the mirrors in the optical system 100 may be determined appropriate for the intended application. Therefore optical systems according to embodiments of the invention for different applications may have different prescriptions.

The mirrors 110, 120, 130, 140, 150 may be manufactured from glass, metal, plastic or advanced composite. The method of fabrication is dependent on the composition. Fabrication processes included conventional polishing, computer-controlled polishing, precision machining, replication and moulding. When being assembled the mirrors 110, 120, 130, 140, 150 may be aligned by being bolted together onto a common optical bench, as shown in Figure 4. Again, the method of alignment is dependent on the composition, method of fabrication and intended application. The optical bench and mounting components for the mirrors may be formed from aluminium, such as Al 6082, although the present invention is not limited in this respect.

Figure 4 illustrates an optical bench 200 according to an embodiment of the invention. The optical bench 200 includes the optical system 100 shown in earlier Figures arranged on a supporting substrate where like parts have the same reference numerals. The optical bench 200 further includes a scan mirror 170 which is electrically moveable as will be appreciated by those skilled in the art. One or more filters may also be mounted upon the optical bench 200 at the intermediate plane 160, as previously discussed. A detector such as a mid-IR (3-8μιη) and/or visible detector device such as a CCD may be located at the final plane 170, although not shown in Figure 4.

It will be realised that embodiments of the present invention provide a compact, re- imaging, magnifying, all-reflective optical system 100 that is especially suited to wide field of view applications, for example in scanning systems.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. The claims should not be construed to cover merely the foregoing embodiments, but also any embodiments which fall within the scope of the claims.

Claims

An optical system for receiving and focusing optical signals, comprising: a primary mirror for creating an accessible intermediate image at an intermediate plane; a tertiary mirror a quaternary mirror; and a final mirror; wherein the tertiary mirror, quaternary mirror and final mirror are arranged for forming a final image at a final image plane.

The optical system of claim 1 or 2, wherein the intermediate image has a greater magnification than the final image.

The optical system of claim 2, wherein a magnification ratio of the intermediate image to the final image is generally in the range 1.5: 1 -2.5:1.

The optical system of any of claims 1 to 3, wherein the intermediate plane is accessible for accommodating one or more optical components at the intermediate plane.

The optical system of claim 4, wherein the optical components are one or more filters.

The optical system of any preceding claim, comprising a secondary mirror for forming the intermediate image at the intermediate plane in cooperation with the primary mirror.

7. The optical system of claim 6, wherein the secondary mirror has a spherical reflecting surface.

8. The optical system of any preceding claim, wherein the tertiary mirror,

quaternary mirror and final mirror are aspheric.

9. The optical system of claim 8, wherein the tertiary mirror, quaternary mirror and final mirror are even aspheres.

10. The optical system of any preceding claim, wherein the tertiary and final

mirrors are each selected from paraboloid, hyperboloid or ellipsoid.

11. The optical system of any preceding claim, wherein the quaternary mirror is a hyperboloid.

12. The optical system of any preceding claim, wherein one or more image spots formed at the intermediate plane have a spot size of around 0.5 μιη.

13. The optical system of any preceding claim, wherein one or more image spots formed at the final plane have a spot size of around 1 μιη.

14. The optical system of any preceding claim, wherein a focus speed at the final plane is around f/4.

15. An optical bench comprising the optical system of any preceding claim.

16. The optical bench of claim 15, comprising a detector for detecting one or more light beams at the final plane.

17. The optical bench of claim 16, wherein the detector is an uncooled detector.

18. The optical bench of claim 15, 16 or 17, comprising one or more filters arranged at the intermediate plane.

19. A radiometer comprising the optical system of any of claims 1 to 14 or the optical bench of any of claims 15 to 18.