Compact high na all-reflective microscope objectives

Abstract

A reflective microscope objective includes 6 reflective mirrors including first, second and third mirrors, a fold mirror, a convex penultimate mirror, and a concave final mirror arranged in an asymmetrical optical geometry providing a non-obscured optical path, wherein the first mirror is configured for receiving input light from an object plane and the final concave mirror is configured for providing output light to a final image plane. The object and final image planes are perpendicular to and coaxially centered about a common mechanical axis, the first mirror in combination with the second mirror and the third mirror are arranged in a front group of tilted off-axis mirrors that receive the input light and form an intermediate image, the penultimate convex mirror and the final concave mirror are arranged in a rear group of mirrors, and the fold mirror is positioned between the front group of mirrors and the rear group of mirrors to re-direct the input light from the front group of mirrors into the rear group of mirrors and the rear group of mirrors relays the intermediate image to the final image plane. Freeform mirrors are implemented in each of the front group and the rear group of mirrors to correct aberrations induced by the asymmetrical optical geometry. The result is an all-reflective, unobscured, freeform microscope objective that can provide diffraction-limited performance for high NA values, as well as relatively compact architecture with low distortion and a distinct back focal plane.

Description

Attorney Docket: 6615.000996 UR: 2-24031-PCT COMPACT HIGH NA ALL-REFLECTIVE MICROSCOPE OBJECTIVES STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0001] This invention was made with government support under EB034084 awarded by the National Institutes of Health. The government has certain rights in the invention. BACKGROUND

[0002] Broadband microscopes are desirable for biological imaging applications as they enable the combination of multiple imaging modalities using a single objective. However, current solutions are typically refractive lens-based and, thus, suffer from chromatic aberrations that limit their optimal performance to a relatively narrow wavelength band. Further, broadband refractive objectives can suffer low throughput due to Fresnel losses at each lens surface, even if appropriately coated. Also, refractive objectives introduce group delay dispersion into short pulses, limiting their effectiveness for various nonlinear optical microscopy modalities.

[0003] A solution to a refractive objective’s shortcomings is to use an all-reflective objective. However, the only all-reflective solution on the market is the rotationally symmetric Schwarzschild reflective objective. While this objective solves the problems associated with dispersion, it suffers from other issues. Being rotationally symmetric, it requires a central obscuration that limits its throughput and causes diffraction effects at the image plane. They are also lacking in image quality for high NAs or large fields of view.

[0004] Reflective off-axis tilted mirror systems including four- and five-mirror optical designs have been proposed such as in US Pat. Pub. No.2021 / 0255446. No actual systems, however, appear to have been previously described that meet further desirable combined design requirements of being compact, low distortion, and having accessible and distinct back focal plane for use in systems such as phase contrast microscopy and other modalities that require an optical component at the back focal plane of the objective.

[0005] Thus, there is a need for an all-reflecting microscope objective that can achieve diffraction-limited performance with a high NA without the loss of light from a central obscuration, as well as being relatively compact, low distortion, and having an accessible and distinct back focal plane. This device would be enabling for many biological imaging modalities.Attorney Docket: 6615.000996 UR: 2-24031-PCT SUMMARY

[0006] The solution presented here is to use an optical architecture that breaks rotational symmetry, thus eliminating the need for a central obscuration. To enable combined design features of being relatively compact, low distortion, and having an accessible and distinct back focal plane, reflective mirrors are arranged in a front group of tilted off-axis mirrors that receive the input light and form an intermediate image, and a rear group of mirrors including a penultimate convex mirror and a final concave mirror, and an additional fold mirror is positioned between the front group of mirrors and the rear group of mirrors to re- direct the input light from the front group of mirrors into the rear group of mirrors. Then, to correct the aberrations induced by the asymmetrical optical geometry, freeform mirrors are implemented in each of the front group and the rear group of mirrors. The result is an all-reflective, unobscured, freeform microscope objective that can provide diffraction- limited performance for high NA values, as well as relatively compact architecture with low distortion and a distinct back focal plane.

[0007] More particularly, in specific embodiments a reflective microscope objective is described comprising 6 reflective mirrors including first, second and third mirrors, a fold mirror, a convex penultimate mirror, and a concave final mirror arranged in an asymmetrical optical geometry providing a non-obscured optical path; wherein the first mirror is configured for receiving input light from an object plane and the final concave mirror is configured for providing output light to a final image plane; wherein the object and final image planes are perpendicular to and coaxially centered about a common mechanical axis; wherein the first mirror in combination with the second mirror and the third mirror are arranged in a front group of tilted off-axis mirrors that receives the input light and forms an intermediate image; wherein the penultimate convex mirror and the final concave mirror are arranged in a rear group of mirrors; and wherein the fold mirror is positioned between the front group of mirrors and the rear group of mirrors to re-direct the input light from the front group of mirrors into the rear group of mirrors and the rear group of mirrors relays the intermediate image to the final image plane.

[0008] In specific embodiments, at least one of the reflective mirrors in the front group of mirrors and both the penultimate and final mirrors in the rear group of mirrors include freeform reflective surfaces that correct optical aberrations induced by the asymmetrical optical geometry to provide diffraction-limited performance for NA values of at least 0.5.Attorney Docket: 6615.000996 UR: 2-24031-PCT

[0009] In further specific embodiments, the reflective microscope objective is configured to provide an accessible and distinct back focal plane.

[0010] In further specific embodiments, the reflective microscope objective has less than 2% distortion.

[0011] In further specific embodiments, the microscope objective fits within either i) a cylindrical volume of less than or equal to 50 mm diameter (25 mm radius) and 70 mm length, with the central axis of the cylinder coincident with the common mechanical axis to which the object and image planes are perpendicular to and coaxially centered about, or ii) a combined volume of a cylinder of less than or equal to 40 mm diameter (20 mm radius) with 70 mm length and with a single radial bulge of less than or equal to 12 mm extending from one side of the cylinder’s surface over at most 180 degrees of the cylinder’s circumference, with the central axis of the cylinder coincident with the common mechanical axis to which the object and image planes are perpendicular to and coaxially centered about. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG.1 illustrates a designed four-mirror system with a front group of two mirrors forming an intermediate image, and a rear group of two mirrors relaying the intermediate image to the final image plane of the objective, wherein all four mirrors include freeform surfaces.

[0013] FIGs.2.1-2.3 illustrate three designed five-mirror systems with a front group of three mirrors forming an intermediate image, and a rear group of two mirrors relaying the intermediate image to the final image plane of the objective, wherein at least three mirrors include freeform surfaces.

[0014] FIGs.3.1-3.3 illustrate three designed six-mirror systems with a front group of three mirrors forming an intermediate image, a rear group of two mirrors relaying the intermediate image to the final image plane of the objective, and a flat-fold mirror positioned between the front group of mirrors and the rear group of mirrors to re-direct the input light from the front group of mirrors into the rear group of mirrors, wherein at least three mirrors include freeform surfaces.

[0015] FIG.4 illustrates a designed six-mirror systems with a front group of three mirrors forming an intermediate image, a rear group of two mirrors relaying the intermediate image to the final image plane of the objective, and a flat-fold mirrorAttorney Docket: 6615.000996 UR: 2-24031-PCT positioned between the front group of mirrors and the rear group of mirrors to re-direct the input light from the front group of mirrors into the rear group of mirrors, wherein three mirrors include freeform surfaces and the system provides increased working distance relative to the embodiments of FIGs 3.1-3.3. DETAILED DESCRIPTION

[0016] In particular embodiments, optical designs for high numerical aperture microscope objectives are described comprising all reflective surfaces making the entire system achromatic.

[0017] For the reflective objective to seamlessly work with existing microscope systems, the center of the input light and output light from the microscope objective must lie along the same mechanical axis as the rest of the microscope system. The object and image planes of the microscope objective are centered about a common mechanical axis and are perpendicular to that axis. An intermediate image plane may be further employed to extend the optical path given typical short focal lengths (e.g., 5.38 mm) associated with desired entrance pupil diameters (e.g., 6-8 mm) and relatively high NA (e.g., at least 0.5, or at least 0.65, or from 0.5 to 0.7, or from 0.65 to 0.7), which also enables the object and image to be coaxial. An intermediate image plane is also useful to put a field stop to help suppress stray light.

[0018] To be small enough to work with commercial microscope systems, the microscope objective should fit within either i) a cylindrical volume of less than or equal to 50 mm diameter (25 mm radius) and 70 mm length, with the central axis of the cylinder coincident with the common mechanical axis to which the object and image planes are perpendicular to and coaxially centered about, or ii) a combined volume of a cylinder of less than or equal to 40 mm diameter (20 mm radius) with 70 mm length and with a single radial bulge of less than or equal to 12 mm extending from one side of the cylinder’s surface over at most 180 degrees of the cylinder’s circumference, with the central axis of the cylinder coincident with the common mechanical axis to which the object and image planes are perpendicular to and coaxially centered about. Designs described herein may be significantly smaller than these maximums. Nominal optical design lengths from 50 to 70 mm are preferred, although shorter optical design lengths may have a mechanical spacer added to extend the length to a preferred range.Attorney Docket: 6615.000996 UR: 2-24031-PCT

[0019] Freeform surfaces are used to provide diffraction-limited imaging in an asymmetric package that has no light-blocking obscurations and within a compact format. The designs benefit from an in-line light input and output, making them suitable for commercial microscope systems. These achromatic objectives can find use in many biological imaging modalities and in the combination of modalities at different wavelengths.

[0020] In various embodiments, less than 2% distortion is preferably achieved.

[0021] In various embodiments, the reflective microscope objective is further specifically configured to provide an accessible and distinct back focal plane. Due to the system asymmetry, it is further important to constrain the X and Y back focal planes to be coincident in objective optical design. To be accessible, it is preferred that there is at least 3 mm of clearance with the back focal plane and the nearest optic of the objective. Further preferred is that there is no more than 15 mm clearance between the back focal plane and the nearest optic to keep the system compact in size. Providing an accessible and distinct focal plane is particularly useful in phase contrast microscopy and other modalities that employ an optical component at the back focal plane of the objective.

[0022] In particular embodiments, the reflective microscope objective desirably meets the following combined specifications: Parameter Units Specification CommentsAttorney Docket: 6615.000996 UR: 2-24031-PCT Distortion % < 2ive mirrors and a back group of two reflective mirrors, with a sixth reflective mirror added and positioned between the front group of mirrors and the rear group of mirrors to re-direct the input light from the front group of mirrors into the rear group of mirrors. Such added sixth mirror can be as simple as, but is not limited to, a flat-fold mirror, and allows for new system geometries that had been previously unexplored.

[0024] The front group of mirrors that form the intermediate image is preferably selected to be in a well-corrected geometry for freeform optics. A well-corrected geometry is one that has the correct combination of surface power distribution and surface tilt directions that produce aberrations with magnitudes and orientations that are more easily correctable with freeform optics. To enable the desired combination of performance in a relatively compact design, three-mirror front groups which enable well-corrected imaging form for freeform optics may be based on , e.g., a Reflective Triplet system employing a concave first mirror, a convex second mirror, and a concave third mirror without an intermediate image (as described, e.g., in Bauer et al., Nat. Comm.9:1756 (2018), the disclosure of which is incorporated by reference herein in its entirety), and a ball geometry system employing a convex first mirror, a concave second mirror, and a concave (or flat) third mirror (as employed, e.g., in Fuerschbach et al., Opt. Express 19(22), 21919-21928 (2011), the disclosure of which is incorporated by reference herein in its entirety). Additional aspects for selection of well corrected geometries may be further explained in Bauer et al., Nat. Comm.9:1756 (2018).

[0025] The rear group of mirrors that relay the intermediate image to the final image plane preferably includes a penultimate convex mirror and a final concave mirror. Such last two mirrors of the system may be close in shape to off-axis sections of conics, where the final concave mirror is a relatively large mirror close to an off-axis ellipse and the penultimate convex mirror is close to an off-axis hyperbola. This is advantageous inAttorney Docket: 6615.000996 UR: 2-24031-PCT design as such off-axis conic sections may have perfect imaging from focus to focus for a point source and where the final image is relatively small.

[0026] While the described three-mirror front groups and two-mirror rear group of mirrors provide well-corrected geometries, a three-mirror front group is difficult to manage in such combination because the light is propagating in a wrong direction after reflecting from the third mirror in order to be incident on the rear group of conic-like freeform surfaces. To re-direct the light in a compact manner, a fold mirror located near the final image of the front group is employed in the present described systems. In particular embodiments, a front group of mirrors comprising a first convex mirror, a second concave mirror, and a third concave or flat mirror is specifically employed, as use of such a front group for forming the intermediate image advantageously enables a long back focal distance, and allows for sufficient room to place the fold mirror after the tertiary mirror.

[0027] Examples 1-4 provided herein present various 4-, 5- and 6-mirror optical designs for microscope objectives optimized in attempts to meet desired characteristics. In such examples, various mirrors employed include freeform surfaces. In all examples, the first mirror and last mirror in the optical path of the objective are arranged to provide coaxial centered object and image planes.

[0028] EXAMPLE 1: A first Option A considered as shown in FIG.1 illustrates a designed four-mirror system with a front group of two mirrors (M1, M2) forming an intermediate image, and a rear group of two mirrors (M3, M4) relaying the intermediate image to the final image plane of the objective, wherein all four mirrors include freeform surfaces. Replacing any of the mirrors with non-freeform mirrors results in performance or packaging trade-offs that are undesirable. The system has an accessible back focal plane and distortion is less than 2%, but the RMS WFE performance of the system (0.071 waves RMS at λ=500 nm) is slightly worse than the diffraction-limit (0.07 waves RMS at λ=500 nm). The length of the optics (30 mm) is shorter than desired, so it would have to be extended using some mechanical spacers when mounted in a microscope system. Such a four mirror system thus struggles to meet the desired combined system requirements.

[0029] EXAMPLE 2: Second Option B mirror arrangements considered as shown in FIGs.2.1-2.3 illustrate three designed five-mirror systems with a front group of three mirrors (M1, M2, M3) forming an intermediate image, and a rear group of two mirrors (M4, M5) relaying the intermediate image to the final image plane of the objective. TheAttorney Docket: 6615.000996 UR: 2-24031-PCT back focal plane is located at the stop surface for all three iterations illustrated in FIGs 2.1- 2.3. When using all freeform surfaces as shown in the design in FIG.2.1, this layout can achieve a bulge-free layout (fitting within a cylindrical volume of 20 mm radius) with good optical performance (RMS WFE of less than 0.03 waves RMS at λ=500 nm, and distortion of less than 2%). If the design is shorter than ideal, it could be extended using a mechanical spacer. To lower the expected cost to make such a system, reducing the number of freeform elements was explored. The solution illustrated in FIG.2.2 replaced the highly tilted freeform first mirror with a spherical surface. In this case, there is a requirement to have a bulge and the achievable performance decreases (RMS WFE of less than 0.057 waves RMS at λ=500 nm, and distortion of less than 2%). The length is still less than ideal. If the sphere first mirror is further simplified to a flat as illustrated in the design in FIG.2.3, the required bulge increases to its maximum value of the desired design parameters and the performance is slightly better (RMS WFE of less than 0.065 waves RMS at λ=500 nm, and distortion of less than 2%) than the diffraction-limit requirement leaving little room for manufacturing errors to yield an as-built diffraction-limited system. The length is still less than ideal.

[0030] The first mirror in the five-mirror Option B systems of FIGs.2.1-2.3 further requires an extreme tilt which can be detrimental to any potential broadband coatings required on that surface as coating performance varies greatly with angle of incidence. Also, the extreme tilt results in a more sensitive surface to manufacturing errors.

[0031] Adding another mirror to the Option B embodiments layout does not necessarily result in successful designs. The only apparent location to add another mirror in FIGs. 2.1-2.3 Option B embodiments due to space constraints is at / near the first, highly tilted mirror. In this case, adding a folding mirror at / near the first mirror can reduce the tilt angle required to direct the light to the bulge mirrors. However, it extends the optical path significantly because the bulge mirrors still need to be located above the final two mirrors so the light can propagate unobstructed to the image. Since the light beams are diverging in the space prior to the bulge mirrors, the bulge necessarily increases in size if the optical path is extended, resulting in suboptimal packaging, and decreases the achievable performance.

[0032] EXAMPLE 3: Option C mirror arrangements as Shown in FIGs.3.1-3.3 illustrate three designed six-mirror systems with a front group of three mirrors (M1, M2,Attorney Docket: 6615.000996 UR: 2-24031-PCT M3) forming an intermediate image, a rear group of two mirrors (M5, M6) relaying the intermediate image to the final image plane of the objective, and a flat-fold mirror (M4) positioned between the front group of mirrors and the rear group of mirrors to re-direct the input light from the front group of mirrors into the rear group of mirrors. By adding a sixth mirror to the system at such a position, new layout options are enabled. In this case, using a ball geometry-based three-mirror front group to create the intermediate image, then using a fold mirror to re-direct that light into the two-mirror back group enables a geometry that has the desired package size, best performance, and most cost-effective options for manufacturing. The back focal plane is located at approx.1-4 mm top the left of the stop surface for all three iterations illustrated in FIGs 3.1-3.3. A non-obvious aspect of this layout is that adding another mirror can result in smaller / better packages and more cost- effective solutions. Without using a sixth mirror, this geometry would not be possible because the front three-mirror group forming the intermediate image, which is well- corrected and a main differentiator between the designs of Options A-C, cannot direct the light in the proper direction to get it to an image plane coaxially located with the object plane.

[0033] In the solution illustrated in FIG.3.1, there are five freeform mirrors and one fold mirror. The optical performance achievable is excellent (RMS WFE of less than 0.025 waves RMS at λ=500 nm, and distortion of less than 2%), there is no bulge, and the packaging cylinder radius is quite small. The expected manufacturing cost can be reduced by changing one of the freeform mirrors to a flat as demonstrated in the illustrated solution in FIG.3.2. Excellent performance is achieved (RMS WFE of less than 0.048 waves RMS at λ=500 nm, and distortion of less than 2%) still in a package with no bulge and a cylinder radius that is small. Finally, as illustrated in the solution in FIG.3.3, we can further decrease the expected manufacturing cost by changing another freeform mirror into a spherical mirror. The resulting performance again is stellar (RMS WFE of less than 0.036 waves RMS at λ=500 nm, and distortion of less than 2%), there is no bulge, and the cylinder radius is just 1 mm larger than preferred. The sensitivity to manufacturing errors of the layouts of Option C FIGs.3.1-3.3 is less compared to the five-mirror solution because the front mirror group has three mirrors over which to distribute the power and none are used at extreme angles of incidence. Furthermore, the ability to replace freeform mirrors with spheres or flats also contributes to this layout having lower sensitivity to manufacturing errors.Attorney Docket: 6615.000996 UR: 2-24031-PCT

[0034] EXAMPLE 4: In order to further improve the manufacturability of the reflective microscope mirror arrangements shown in FIGs.3.1-3.3, the system package size was increased to aid in reducing the sensitivity of the optical surfaces to manufacturing errors, working distance was added to facilitate the mounting of M5, and a CaF2 coverslip was added to cover the specimen under observation. The front group of mirrors can be packaged entirely within a 20 mm radius cylinder, while a bulge extending approximately 7 mm beyond the 20 mm radius cylinder further accommodates the fold mirror and top of M6. With these alterations, the ideal combination of freeform and spherical surfaces was found, where M1 and M3 are spherical mirrors and M2, M5, and M6 are freeform mirrors. Additional higher-order freeform terms were added to M2, M5, and M6 using the Extended Fringe Zernike polynomial description as defined in the CODE V software. The resulting design as shown in FIG.4 has excellent nominal performance with a maximum RMS wavefront error of 0.025 waves for wavelength of 500 nm, distortion of less than 2%, a working distance of 5 mm, and has decreased sensitivity to manufacturing errors.

[0035] Generally, the freeform surfaces are used to correct optical aberrations induced by tilting and decentering of the reflective surfaces in the illustrated folded geometries. As used herein, a nonsymmetric surface with bi-axial symmetry is referred to as an anamorphic surface, while a freeform optical surface is any rotationally nonsymmetric surface whose asymmetry goes beyond bi-axial symmetry or toroidal shape. A freeform surface may be parameterized by normalized basis functions such as the phi-polynomials (e.g. Zernike sets, Q-polynomials, other sets of orthogonal polynomials, XY polynomials, Radial Basis Functions, Splines, or Non-Uniform Rational Basis Spline (NURBS).

[0036] As described above, representative geometric tilts of the mirror surfaces are shown in FIGS.1-4, while the diffraction limited performance achieved in terms of RMS wavefront error of less than 0.07 waves for 500 nm wavelength light for each design, as is noted in each Figure.

[0037] Design parameters for the optical surfaces of the components of the representative illustrated system embodiments of FIGs.3.1, 3.3 and 4 are further set forth in Tables 1.1-1.2, 2.1-2.2, and 3.1-3.2 below, where the optical surfaces for each embodiment are numbered increasingly from the entrance pupil to the image plane. For the embodiment in FIG.3.1 (and corresponding Tables 1.1 and 1.2), mirrors M1 - M6 in the optical path from the object plane to the image plane correspond to Surface Nos.2 - 7,Attorney Docket: 6615.000996 UR: 2-24031-PCT respectively, in Tables 1.1 and 1.2. For the embodiment in FIG.3.3 (and corresponding Tables 2.1 and 2.2), mirrors M1-M6 in the optical path from the object plane to the image plane correspond to Surface Nos.2 - 7, respectively, in Tables 2.1 and 2.2. For the embodiment in FIG.4 (and corresponding Tables 3.1 and 3.2), mirrors M1 - M6 in the optical path from the object plane to the image plane correspond to Surface Nos.2 - 7, respectively, in Tables 3.1 and 3.2.

[0038] Table 1.1 (FIG.3.1 embodiment) Surface # Surface Type Label Y Radius (mm) Thickness Glass Refract Mode Semi Aperture (mm) Alpha Tilt (deg) Normalization Radius (mm) k Object Sphere 1.00E+18 1.00E+18 AirAttorney Docket: 6615.000996 UR: 2-24031-PCT

[0039] Table 1.2 (FIG.3.1 embodiment) Fringe Zernike Coefficient # S2 S3 S4 S6 S7 1 -0.0018685421 -0.0007554560 0.0005364552 0.0378882175 0.0358873771 2 0.0000000000 0.0000000000 0.0000000000 0.0000000000 0.0000000000Attorney Docket: 6615.000996 UR: 2-24031-PCT

[0040] Table 2.1 (FIG.3.3 embodiment) Surface # Surface Type Label Y Radius (mm) Thickness Glass Refract Mode Semi Aperture (mm) Alpha Tilt (deg) Normalization Radius (mm) k Object Sphere 1.00E+18 1.00E+18 Air Stop Sphere 1.00E+18 31.25732 Air 3.5Attorney Docket: 6615.000996 UR: 2-24031-PCT

[0041] Table 2.2 (FIG.3.3 Embodiment) Fringe Zernike Coefficient # S2 S6 S7 1 0.0000716824 0.0635870026 0.0403081938 2 0.0000000000 0.0000000000 0.0000000000Attorney Docket: 6615.000996 UR: 2-24031-PCT

[0042] Table 3.1 (FIG.4 embodiment) Surface # Surface Type Label Y Radius (mm) Thickness Glass Refract Mode Semi Aperture (mm) Alpha Tilt (deg) Normalization Radius (mm) k Object Sphere 1.00E+18 1.00E+18 Air Stop Sphere 1.00E+18 30.0496848076 Air 3.5Attorney Docket: 6615.000996 UR: 2-24031-PCT

[0043] Table 3.2 (FIG.4 Embodiment) Extended Fringe Zernike Coefficient #S3 S6 S7 1 0.0002311 0.0840327 0.0724656 2 0.0000000 0.0000000 0.0000000 3 0.0055902 0.2990078 -0.1231417Attorney Docket: 6615.000996 UR: 2-24031-PCT

[0044] It will be appreciated that variants of the above-disclosed embodiments and other disclosed features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.

Claims

Attorney Docket: 6615.000996 UR: 2-24031-PCT CLAIMS 1. A reflective microscope objective comprising 6 reflective mirrors including first, second and third mirrors, a fold mirror, a convex penultimate mirror, and a concave final mirror arranged in an asymmetrical optical geometry providing a non-obscured optical path, wherein the first mirror is configured for receiving input light from an object plane and the final concave mirror is configured for providing output light to a final image plane, wherein the object and final image planes are perpendicular to and coaxially centered about a common mechanical axis, wherein the first mirror in combination with the second mirror and the third mirror are arranged in a front group of tilted off-axis mirrors that receive the input light and form an intermediate image, wherein the penultimate convex mirror and the final concave mirror are arranged in a rear group of mirrors, and wherein the fold mirror is positioned between the front group of mirrors and the rear group of mirrors to re-direct the input light from the front group of mirrors into the rear group of mirrors and the rear group of mirrors relays the intermediate image to the final image plane.

2. A reflective microscope objective according to claim 1, wherein at least one of the reflective mirrors in the front group of mirrors and both the penultimate and final mirrors in the rear group of mirrors include freeform reflective surfaces that correct optical aberrations induced by the asymmetrical optical geometry to provide diffraction-limited performance for NA values of at least 0.

5.

3. A reflective microscope objective according to claim 1 or 2, wherein the reflective microscope objective is configured to provide an accessible and distinct back focal plane.

4. A reflective microscope objective according to any one of claims 1-3, wherein the reflective microscope objective has less than 2% distortion.

5. A reflective microscope objective according to any one of claims 1-4, having an entrance pupil diameter of from 6 to 8 mm, and an NA of from 0.5 to 0.

7.

6. A reflective microscope objective according to any one of claims 1-5, having an entrance pupil diameter of from 6 to 8 mm, and an NA of from 0.65 to 0.7.Attorney Docket: 6615.000996 UR: 2-24031-PCT 7. A reflective microscope objective according to any one of claims 1-6, wherein the microscope objective fits within either i) a cylindrical volume of less than or equal to 50 mm diameter (25 mm radius) and 70 mm length, with the central axis of the cylinder coincident with the common mechanical axis to which the object and image planes are perpendicular to and coaxially centered about, or ii) a combined volume of a cylinder of less than or equal to 40 mm diameter (20 mm radius) with 70 mm length and with a single radial bulge of less than or equal to 12 mm extending from one side of the cylinder’s surface over at most 180 degrees of the cylinder’s circumference, with the central axis of the cylinder coincident with the common mechanical axis to which the object and image planes are perpendicular to and coaxially centered about.

8. A reflective microscope objective according to any one of claims 1-6, wherein the microscope objective fits within a cylindrical volume of less than or equal to 50 mm diameter (25 mm radius) and 70 mm length, with the central axis of the cylinder coincident with the common mechanical axis to which the object and image planes are perpendicular to and coaxially centered about.

9. A reflective microscope objective according to any one of claims 1-6, wherein the microscope objective fits within a combined volume of a cylinder of less than or equal to 40 mm diameter (20 mm radius) with 70 mm length and with a single radial bulge of less than or equal to 12 mm extending from one side of the cylinder’s surface over at most 180 degrees of the cylinder’s circumference, with the central axis of the cylinder coincident with the common mechanical axis to which the object and image planes are perpendicular to and coaxially centered about.

10. A reflective microscope objective according to any one of claims 7-9, wherein a nominal optical design lengths of the objective is from 50 to 70 mm.

11. A reflective microscope objective according to any one of claims 1-10, wherein the mirror between the reflective mirrors in the front group of mirrors and the rear group of mirrors is flat, spherical, or freeform.

12. A reflective microscope objective according to any of the preceding claims, wherein the objective provides diffraction limited performance for light wavelength of 500 nm.

Images