Multiconfiguration afocal reflective telescope

Abstract

Optical designs for an afocal telescope system that is switchable between a relatively low magnification, large field of view mode (e.g., for object searching) and a relatively high-magnification, low field of view mode (e.g., for object inspection) are described. An approach to designing such multiconfiguration afocal telescopes is developed and demonstrated. Freeform surfaces are used to maximize the achievable diffraction-limited zoom ratio while staying in a compact volume for a two-position multiconfiguration afocal telescope. In some embodiments, a 5x zoom ratio is achieved with a compensated exit pupil and diffraction-limited performance. A significant benefit in optical performance when using freeform reflective surfaces in such a multiconfiguration afocal telescope is shown compared to more conventional surface types.

Description

MULTICONFIGURATION AFOCAL REFLECTIVE TELESCOPESTATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0001] This invention was made with government support under IIP- 1822049 awarded by the National Science Foundation. The government has certain rights in the invention.BACKGROUND

[0002] As applications for earth-viewing satellites become more diverse, the required optical specifications of the systems can become increasingly complex and challenging to achieve with a single fixed-focus imager. Using all-rcflcctivc optics, which arc intrinsically achromatic, enables a single system to be used for multiple spectral bands with the additional benefit that mirrors are more amenable to large aperture sizes than lenses. All-reflective systems can be unobscured to maximize the light throughput for a given aperture size. When freeform optics [1] are applied to unobscured reflective telescopes, they may provide substantial reductions in the root-mean-square (RMS) wavefront error (WFE) and system volume [2-4], as well provide additional flexibility for other more system specific requirements [5-8],

[0003] Additional system flexibility can be achieved by utilizing an afocal system. Afocal foreoptics allow for the largest optics of a system to be untethered to a specific format sensor, providing the system application and spectral flexibility [9-13], By combing the technology of freeform optics with an afocal architecture, a highly adaptable and highly performant system can be designed. Bauer et al. found significant benefit is provided to unobscured, reflective afocal systems by adding freeform optics, including improving the quality of the exit pupil [4], which is necessary for a seamless integration with other imaging optics.

[0004] Operational flexibility for an optical system can be taken one step further through the use of a zoom system offering continuous imaging over the zoom range or by a multiconfiguration system, where a finite number of optical configurations are utilized. The most common type of zoom or multiconfiguration system is one where the focal length or magnification can be varied between a high and a low value. This feature allows a scene to be surveyed (low-magnification) or inspected (high-magnification), thus providing flexibility to be used in various situations. Using reflective optics in a zoom system offers similar benefits as in fixed focus optics and, though not common, is presentin the literature as focal imaging systems using off-axis conic surfaces

[0014] or freeform surfaces [15, 16] and as an afocal imaging system using off-axis conic surfaces

[0012] ,

[0005] It would be desirable to provide improved designs for an afocal telescope system that is switchable between a relatively low magnification, large field of view mode (e.g., for object searching) and a relatively high-magnification, low field of view mode (e.g., for object inspection).SUMMARY

[0006] Optical designs for a multiconfiguration afocal telescope system that is switchable between a relatively low magnification, large field of view mode (e.g., for object searching) and a relatively high-magnification, low field of view mode (e.g., for object inspection) are described.

[0007] In accordance with one embodiment of the disclosure, a multiconfiguration afocal reflective telescope system is described which more specifically is switchable between (i) a first configuration providing a first relatively high afocal magnification for incoming light along a first beam path between a first configuration system entrance pupil and a first configuration system exit pupil and (ii) a second configuration providing a second relatively low afocal magnification for incoming light along a second beam path between a second configuration system entrance pupil and a second configuration system exit pupil; wherein each of the first and the second configurations employs a common primary mirror (Ml), a different one of two separate secondary mirrors (M2a, M2b), and either a common tertiary mirror (M3) or a different one of two separate tertiary mirrors (M3 a, M3b) arranged in asymmetrical optical geometries providing non-obscured optical paths, for directing and magnifying incoming light along each of the first and second beam paths in the first and second configurations; wherein the primary mirror (Ml) is tiltable when switching between the first and second configurations to direct incoming light to be incident on either of the two separate secondary mirrors (M2a, M2b), and if employed the common tertiary mirror (M3) is translatable and rotatable in space when switching between the first and second configurations; and wherein each of the primary mirror (Ml), the two separate secondary mirrors (M2a, M2b), and either the common tertiary mirror (M3) or each of the two separate tertiary mirrors (M3a, M3b) comprise freeform reflective surfaces that correct optical aberrations induced by the asymmetrical optical geometries of the mirrors in each of the first and second beam paths.

[0008] In accordance with further embodiments of the present disclosure, each of the first and the second configurations of the multiconfiguration afocal reflective telescope system further employs a quaternary mirror (M4) arranged in the asymmetrical optical geometries providing non-obscured optical paths for directing and magnifying incoming light along each of the first and second beam paths in the first and second configurations, wherein the quaternary mirror (M4) is translatable and rotatable in space when switching between the first and second configurations; and wherein the quaternary mirror (M4) comprises a freeform reflective surface that further corrects optical aberrations induced by the asymmetrical optical geometries of the mirrors in each of the first and second beam paths.

[0009] An approach to designing such multiconfiguration afocal telescopes is developed and demonstrated. Freeform surfaces are used to maximize the achievable diffractionlimited zoom ratio while staying in a compact volume for a two-position multiconfiguration afocal telescope. In some embodiments, a goal of a 5x zoom ratio is achieved with a compensated exit pupil and diffraction-limited performance. A significant benefit in optical performance when using freeform reflective surfaces in such a multiconfiguration afocal telescope is shown compared to more conventional surface types.BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIG. 1 is an illustration of a natural disaster observing application for a multiconfiguration afocal telescope that motivated design specifications in accordance with one embodiment of the disclosure.

[0011] FIG. 2 is an illustration of a sample layout of a three-mirror beam path optical system using a novel approach to a multiconfiguration system in accordance with an embodiment of the disclosure.

[0012] FIG. 3 is an illustration of the free variables that impact the first-order optical properties between configurations of a multiconfiguration system in accordance with an embodiment of the disclosure.

[0013] FIG. 4 is an illustration of a starting design for a three-mirror beam path design multiconfiguration system in accordance with an embodiment of the disclosure.

[0014] FIGS. 5A-5C are illustrations of 2D layouts of uncompensated diffractionlimited 3-mirror beam path designs for zoom ratios of 2x (5 A) and 3x (5B and 5C) in accordance with embodiments of the disclosure.

[0015] FIGs. 6A and 6B are corresponding illustrations of 2D layouts of uncompensated 3-mirror beam path designs with a zoom ratio of 2x for (6A) freeform surface shapes and (6B) off-axis conic surface shapes.

[0016] FIGs 7A-7C are illustrations of 2D layouts for the systems in accordance with embodiments of the disclosure using both a separate secondary and a separate tertiary mirror.

[0017] FIG. 8 is an illustration of a starting design for a four-mirror beam path design multi configuration system in accordance with an embodiment of the disclosure.

[0018] FIGs. 9A-9D are illustrations of 2D layouts design results for four-mirror beam path systems in accordance with embodiments of the disclosure.

[0019] Figs. 10A and 10B are illustrations of 4x and 5x zoom ratio designs using a four-mirror beam path variation in accordance with embodiments of the disclosure.

[0020] FIGs. HA and 1 IB are corresponding illustrations of 2D layouts of 4-mirror beam path designs with a volume of 50 L for (1 IB) freeform surface shapes and (HA) off-axis conic surface shapes.

[0021] FIGs. 11C and 1 ID are corresponding illustrations of 2D layouts of 4-mirror beam path designs with a volume of 100 L for (1 ID) freeform surface shapes and (11C) off-axis conic surface shapes.DETAILED DESCRIPTION

[0022] The present disclosure provides detailed descriptions regarding design of first and second configurations for multiconfiguration afocal reflective telescopes employing freeform reflective surfaces. In such described systems, freeform optics are combined with an afocal two-position multiconfiguration system to create a novel optical system with the ultimate flexibility. Compared to a continuous zoom system, a two-position multiconfiguration system maintains the full benefit imaging at the extrema configurations (high- and low-magnification) but does not need to spend design degrees- of-freedom on the in-between magnifications, thus decreasing the system complexity for a given zoom ratio. Design possibilities are described within the space of three-mirror and four-mirror systems, including the achievable optical performance, zoom ratio, and system volume, using a novel method to create the afocal magnification changes. Also demonstrated is the significant advantage that freeform optics have in this space compared to designs using more conventional off-axis conic surfaces.

[0023] Generally as employed in the present disclosure, 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, Chebyshev polynomials, other sets of orthogonal polynomials, XY polynomials, Radial Basis Functions, Splines, or Non-Uniform Rational Basis Spline (NURBS)). Additionally, a freeform surface may exist within a circular, rectangular, or any other general shaped aperture.

[0024] To make the disclosed optical design specifications realistic and relevant, a specific representative application for the optical systems is described. As illustrated in Fig. 1, to assist in the surveyance and examination of the impact of natural disasters, e.g., a satellite 101 in low-earth orbit height 102 (e.g., 700 km) will house the described two- position multiconfiguration afocal telescope. It boasts a wide-field, low-magnification configuration for general scene observation 110, and a narrow-field, high-magnification configuration for targeted search activities 120. In the high-magnification configuration, a 250 mm entrance pupil diameter and a 1° x 1° full field-of-view (FOV) give a 1.9 m resolution and a 12 km x 12 km imaging area on the ground. To match the afocal optics to an imager with a 25 mm entrance pupil, the high-magnification configuration should have an afocal magnification of l Ox. In one embodiment, a 2x afocal magnification in the low- magnification configuration is targeted for the described system, which gives a 5x zoom ratio. The full FOV of the low-magnification configuration is 5° x 5° with an entrance pupil diameter of 50 mm. On the ground, this translates to imaging a 60 km x 60 km imaging area with a 9.4 m resolution. A maximum mirror size of 250 mm may be desired, which corresponds to a typical maximum fabrication capacity for some manufactures. With a 250 mm entrance pupil diameter specification in the high-magnification configuration, the low-magnification configuration may be constrained to use a portion of the mirror within the same 250 mm.

[0025] When designing the described multiconfiguration optical system, the mechanisms for changing the first-order optical properties of the system are less constricted compared to a continuous zoom system. A continuous zoom system must have some variable(s) that are smoothly and continuously varied to provide continuous first- order property changes (i.e., the translation of optical elements). A multiconfigurationsystem is not restricted to only continuous variable changes and can even have different beam paths for each configuration. For this disclosure, a novel design method is described enabled by a multiconfiguration system.

[0026] For the two-position multiconfiguration designs in this disclosure, a method is implemented that results in two separate (yet, similar) beam paths. To describe this method, in accordance with one embodiment assume that each configuration has a three- mirror beam path, meaning that the light in each configuration interacts with three mirrors, as illustrated, e.g., in Fig. 2. The three mirrors in each path may or may not be the same three mirrors. In the two configurations, the primary mirror (Ml) is shared and has two tilt positions about its local X-axis, allowing the reflected light to be incident on one of two separate secondary mirrors (M2a, M2b). These separated secondary mirrors can have completely different surface shapes and locations in space. Following a reflection from each respective secondary mirror, the light from both paths is incident on a shared tertiary mirror (M3) that can translate and rotate in space when switching between the two configurations. Additionally, the object-space aperture stop is allowed to translate in the Y-direction. Design results are presented herein for this two-position multiconfiguration method using a three-mirror beam path as illustrated in Fig. 2 (with an uncompensated exit pupil location). A three-mirror beam path variation is also described herein where the tertiary mirror is also separated to provide an additional degree of freedom. The same general method can be applied to systems with more than three mirror beam paths, where each additional mirror could be separated or shared. To demonstrate the advantages of an additional mirror, the space of a four-mirror beam path is also explored with a shared primary mirror, a separated secondary mirror, a shared tertiary mirror, and a shared quaternary mirror. Finally, the tertiary mirror in the four-mirror beam path designs is separated and the improvements to optical performance evaluated.

[0027] The concept motivating using an afocal system as foreoptics is to mate it to an existing imaging system that focuses the collimated light exiting the afocal forcoptics to a sensor. To combine the two subsystems most optimally, the entrance pupil of the existing image system should be matched in size, location, and angle to the exit pupil of the afocal system. Thus, it is useful to maintain a solid understanding of the exit pupil properties of the afocal system to facilitate its use with an existing imaging system. Beyond the first- order properties of the exit pupil (location, size, and tilt), there are higher-order effects known as pupil aberrations that impact the overall quality of the exit pupil and can have detrimental effects on the pupil-matching condition

[0017] . The present descriptionquantifies the general pupil quality and improves it through an optimization processes introduced by Bauer et al. [4], where there was a demonstrated trade-off in optical performance when implementing exit pupil quality constraints. Two types of pupil error are described - RMS pupil size error and RMS pupil offset error. The pupil size error looks at the size and shape of the real exit pupil for each field point and compares it to the perfect exit pupil size and shape. The pupil offset error quantifies the difference in the axial location of the real exit pupil relative to the defined exit pupil plane. For the designs explored herein, “good” exit pupil quality is considered to mean < 5% RMS pupil size emor and < 5% RMS pupil offset error.

[0028] The ideas of a compensated and uncompensated multiconfiguration system are relevant for the systems designed herein, so it is leveraged to explain the concepts. In a focal zoom or multiconfiguration system where the light is focused on a detector, a change of one group of powered optical elements (such as a translation) changes the focal length of the system. However, it also has the effect of changing the back focal distance of the system. So, while a single change in the configuration can yield the required focal length variation, the image will not be in focus for a stationary sensor plane. This scenario is referred to as an uncompensated zoom system. The sensor must be moved throughout the zoom motion to ensure an in-focus image. While doable, it is not ideal from a systemlevel standpoint. A preferred option is to move another group of powered optics coordinated with the first group's motion to give the desired focal length change and keep the image in focus at a stationary sensor plane. This situation is referred to as a compensated system. In general, N first-order optical properties that change through zoom (e.g., focal length and back focal distance) requires N first-order changes in the optical layout to satisfy.

[0029] While the description of compensated and uncompensated multiconfiguration systems was given for focal systems, the same concepts exist for afocal systems. For an afocal system, the focal length is replaced by afocal magnification and the back focal distance is replaced by the location of the exit pupil. In the case of an afocal system, having a compensated exit pupil may be even more attractive since an uncompensated zoom would require coordinating the motion of an existing imaging system through zoom to ensure that it correctly matches up to the moving exit pupil of the afocal system.

[0030] During the design of an afocal system, the location of the exit pupil can be calculated and constrained. A compensated afocal system can be achieved if there areenough first-order variables between the various optical configurations. Though, as demonstrated herein, for some systems compensated designs may not be feasible.

[0031] Before designing a two-position multiconfiguration system with three -mirror beam paths, it is useful to first understand what is feasible. The most critical aspect to study is the number of required first-order constraints compared to the number of changeable first-order variables between the two configurations. The afocal magnification change is the first and most apparent first-order constraint between the two configurations. The second first-order constraint is the location of the two exit pupils in space, where a compensated system would require the two exit pupils to be coincident. The third constraint affected by the choice of the first-order mirror powers is the Petzval curvature.

[0032] The variables that affect the first-order optical properties between zooms, illustrated in Fig. 3, are the curvatures of the separate M2s, the Ml -M2 airspace, the M2- M3 airspace, and the location of the aperture stop in object space. However, both Ml -M2 airspace and aperture stop location are heavily constrained due to packaging and ray clearance requirements. Thus, these two variables can help refine the first-order specifications but are not completely free variables.

[0033] In summary, for a three-mirror beam path system, there are three first-order constraints to hold between the configurations but only two free variables with which to hold those constraints. In the present disclosure, holding the afocal magnification constraint for the designed system was considered non-negotiable, and it was found in prior work that it is important to balance the Petzval curvature using negative and positive mirrors to enable achieving a diffraction-limited design [4, 18], Thus, the location of the exit pupil is unconstrained, meaning for a three-mirror beam path system using the described method for switching between configurations, only uncompensated multiconfiguration systems are feasible to design with high performance.

[0034] In prior work, numerous fixed magnification freeform afocal systems were designed that meet the specifications for the high-magnification configuration of this system [4], For the present system design, we started with one of the designs, shown in Fig. 4, that could be corrected to well below the diffraction limit using only low-order Zernike terms (up to Z 10 in Fringe ordering), indicating that the design form was well- behaved and could be easily altered without sacrificing performance. We also chose such system as its volume (70 L) was large enough to accommodate a second configuration without causing obscuration challenges.

[0035] The first step to increasing the zoom ratio is to give the fixed magnification starting design a second but identical configuration. As the optimization progresses, the afocal magnification of the second configuration is slowly decreased from lOx via optimization constraints. As the afocal magnification decreases, the entrance pupil diameter must also decrease to maintain a constant exit pupil diameter. Similarly, the FOV must be increased as the afocal magnification is reduced to maintain a constant angle of light at the exit pupil. The zoomed parameters are the field points, the decenter and diameter of the aperture stop, the tilt of Ml , the 1 -M2 airspace, the surface shape and tilt of M2, the M2-M3 airspace, and M3’s Y-decenter and a-tilt (rotation about the local X-axis).

[0036] A preferred method of design optimization for freeform systems, both afocal and focal, has been well-established [18, 19]. However, it is instructive to list the constraints used during the optimization of the present systems due to their importance in determining the results. An afocal magnification constraint was implemented as described in [4], When a surface is shared between the two configurations, that surface is allowed to decenter to allow for the used area to shift, providing more independent aberration correction. However, the amount of decenter must be limited to ensure that the shared surface does not grow too large, fn the present systems, the decenter was limited to 45 mm. As is typical for off-axis geometries, clearance constraints were implemented for each configuration and between configurations, which is essential when using separate M2s or different sections of shared mirrors. Finally, it was found that the volume of the system preferred to increase indefinitely, so a volume constraint was applied to keep the system size reasonable. The final design specifications are shown in Table 1. While following the freeform aberration Correction principles of

[0018] , the afocal magnification of the second configuration was continually decreased from lOx until no further zoom ratio increase was possible while meeting the performance requirements. During the design phase, multiple methods were tried, including starting with identical configurations at a midpoint afocal magnification and increasing the afocal magnification of one configuration while decreasing the afocal magnification of the other configuration.Starting with two configurations that already gave a desired zoom ratio was also explored. Neither of these latter two methods produced results better than the chosen method.

[0037] Table 1. Design specifications for the three-mirror beam path systems.

[0038] After implementing the design methods as described above, we successfully were able to arrive at two-position zoom systems at 2x and 3x zoom ratios, as shown in Figs. 5A-5C, where the volumes of the respective designs arc 60 L (Fig. 5A), 75 L (Fig. 5B), and 100 L (Fig. 5C). Each design is diffraction-limited at X = 587 nm and, as described above, are limited to uncompensated exit pupils. A 3x zoom ratio is the maximum diffraction-limited zoom ratio that could be achieved without encountering major pupil quality issues. These systems were first optimized without constraints on pupil quality while minimizing the package size. The Fig. 5A 2x zoom ratio system met the pupil quality requirements without implementing constraints, but the pupil quality of the Fig. 5B 3x system quickly degraded. To remedy this shortcoming, exit pupil quality constraints were implemented to improve the exit pupil [4]. However, instead of trading off wavefront performance for a better pupil, we opted to illustrate a different type of trade-off. We increased the allowed decenter on the shared mirror (M3) from 45 mm to 60 mm, which subsequently required a larger volume system (75 L increased to 100 L). We also added more freeform terms (up to 14th order in FRINGE Zernike) to the shared mirror (M3), where the off-axis usage makes higher-order terms that typically provide negligible value useful. By allowing these design changes, we could control the exit pupil quality in the Fig. 5C 3x system to nearly within specification (6% pupil offset in the high-magnification configuration).

[0039] Comparing a freeform design in accordance with the present disclosure to an off-axis conic (OAC) design highlights the advantages of freeform optics. Off-axis conic optics have the potential to be easier to manufacture and measure (using nulling interferometry [20, 21]), so there must be a convincing reason to transition to freeformsurfaces. To highlight the benefits of freeform surfaces for this class of two-position multiconfiguration afocal telescopes, we completed an off-axis conic design with the exact specifications of the 2x zoom ratio system from the previous section (Fig. 5 A). The resulting off-axis conic design is shown in Fig. 6B, together with the corresponding freeform design in Fig. 6A. The maximum RMS WFE over the full FOV for the freeform design is 0.068X (high mag) and 0.065X (low mag). The maximum RMS WFE of the off- axis conic design over the full FOV is 0.15 (high mag) and 0.2 I (low mag) for X=587 nm. The freeform design achieved an RMS WFE approximately 2.7x better than the off- axis conic design, averaged over the two configurations. That degree of improvement is significantly greater than is typically seen for fixed focus or fixed magnification designs, often in the 1.5x improvement range. To understand the results, we fit the surface shapes in the freeform design with off-axis conics and looked at the magnitude of the residual departure, listed in Table 2. For Ml and M2 (both), the departure from an off-axis conic is not substantial, but the shared M3 substantially departs from an off-axis conic. This data highlights the importance of using freeform surfaces to describe the shared surface because they allow greater control across the full aperture of the surface where each configuration uses overlapping sub-apertures.

[0040] Table 2. Residual departures of the 2x zoom ratio freeform mirrors after fitting with an off-axis conic surface.

[0041] As described above, with a three-mirror beam path system, three first-order constraints must be held to get a high-performing and compensated design, yet there are only two free first-order variables. The lack of variables limits a 3-mirror beam path system using this multiconfiguration method to an uncompensated system, which is undesired for most applications. Another first-order variable that can change between configurations must be added to achieve a compensated design. A four-mirror beam path accomplishes that task and will be explored in the next section, but we can still achieve a compensated system with a three-mirror beam path by altering the multiconfiguration method. Instead of a shared M3, it is split into two mirrors (M3a and M3b), similar to M2.The separate M3 mirrors would have independent shapes and locations and would be stationary through zoom, meaning the only optic motion required to switch the configuration is the Ml tilt. One caveat is that the separate M3 mirrors cannot both lie on the same centerline with the exit pupil, so only the location of the exit pupil is the same between the configurations. The exit pupil tilt varies slightly between the configurations. However, by putting a fold mirror (not shown) at the location of the exit pupil whose tilt can be coordinated with the tilt of Ml, any imaging subsystem used with this afocal system can remain stationary when switching between the configurations as the fold mirror would properly direct the light. 2x, 3x, and 4x zoom ratio designs for this method are shown in Figs. 7A, 7B and 7C. The volumes of the systems are 55 L, 75 L, and 115 L for the 2x, 3x, and 4x zoom ratios, respectively. Each design is diffraction-limited and meets the exit pupil quality requirements. In this study, using high-order freeform terms did not appreciably help with performance or pupil quality, which can be attributed to the lack of off-axis shared mirrors. Thus, the only additional parameter to leverage for improved performance is the system volume, which is evident moving from the 3x to the 4x design (75 L to 115 L). Zoom ratios greater than 4x are not included as they require impractical large volumes to meet the design goals.

[0042] It was explained above that the original three-mirror beam path designs did not have enough degrees of freedom that affected the first-order properties between the configurations to allow for the exit pupils to be constrained to be coincident while achieving good wavefront performance. By additionally employing separate tertiary mirrors, another degree of freedom was added that allowed good wavefront performance and coincident exit pupils, but the tilt of the exit pupil could not be the same between the configurations, and the size of the systems is relatively large. A method to address both shortcomings of the three-mirror beam path variation is to introduce a fourth freeform mirror into the beam path. Specifically, the designs further described in this section share a single large primary mirror (Ml), use a separated secondary mirror (M2a, M2b), then share a single tertiary (M3) and a single quaternary (M4) mirror. The motion of the primary mirror is limited to tilts about its vertex, but the tertiary and quaternary mirrors can move in all dimensions when switching the configurations.

[0043] Multiple methods were investigated to find the optimal starting point. Our initial two paths tried to leverage the designs we had already completed. We first tried simply adding a fourth mirror to the already optimized three-mirror beam path designs and slowly moving the exit pupils closer together using design constraints. That methodwas unsuccessful as the inertia was too high to push the design out of the three-mirror beam path solution space. We next tried returning to the three-mirror fixed magnification starting point and adding a fourth mirror, then constraining the exit pupils to remain coincident as the zoom ratio increased. This method was mildly successful but still resulted in relatively large-volume systems. The method that produced the best designs was to create a new four-mirror beam path starting design from scratch. Each mirror could be fully utilized, and the design would not be stuck in a three-mirror beam path solution space. A selected four-mirror beam path starting design is shown in Fig. 8.

[0044] From this starting point, we used the same optimization strategy described above. We started with identical configurations with lOx afocal magnification. We then methodically worked the afocal magnification of the second configuration down as far as it would go while meeting the design specifications. When a certain zoom ratio was reached, we would then focus on reducing the volume of the system at that zoom ratio. The resulting designs arc shown in Figs. 9A-9D, and the design specifications arc shown in Table 3. We achieved compact volumes compared to any of the designs in prior Sections. For the 2x (Fig. 9A) and 3x (Fig. 9B) zoom ratios, diffraction-limited performance was achieved, and the pupil quality metrics were satisfied at a volume of 35 L. At a 4x (Fig. 9C) zoom ratio, the pupil quality metrics were becoming more challenging to maintain, so we allowed this design to have a larger offset of 75 mm between the center of the rays for each configuration on M3 and M4. This change resulted in a system with a slightly larger volume. Even with this change, however, the pupil offset emor was limiting, so we allowed it to be a somewhat larger 7%. Wavefront performance was not a limiting factor as we pushed the design to a 5x (Fig. 9D) zoom ratio, which was the project's ultimate goal. However, as with the 4x zoom ratio, pupil quality becomes the limiting factor, coming in at 10% for both offset and size error. These designs leverage the benefits of using higher-order terms (up to 14th-order Zernikes) on the shared mirrors to maximize performance while minimizing volume and exit pupil errors.

[0045] Table 3. Design parameters for the four-mirror beam path systems.i

[0046] If an application requires a 4x or 5x zoom ratio with better exit pupil quality than the designs in Figs. 9A-9D, the arrangement can be modified similarly as for thethree-mirror beam path designs. By changing the tertiary mirror M3 from a shared mirror to two separate mirrors (M3a, M3b), e.g., we can improve the quality of the exit pupil for the 4x and 5x zoom ratios to < 5% RMS while maintaining compact volume (50 L) and diffraction-limited performance. The four-mirror beam path with variation designs are shown in Figs. 10A and 10B, with the volume of each system being 50 L.

[0047] For the four-mirror beam path designs, we expected that corresponding off-axis conic (OAC) designs would not be able to achieve the same levels of performance as the freeform designs, especially for small volumes. Thus, we chose to compare the freeform design to OAC designs at two different volumes (50 and 100 L). Furthermore, OAC designs struggle to get beyond a 2x zoom ratio, so the zoom ratio was fixed at 2x to maintain a fair comparison. The results are shown in Figs. 11A-1 ID, with Figs. 11A (50L) and 11C (100 L) illustrating OAC designs, and Figs 1 IB (50L) and 1 ID (100L) illustrating corresponding freeform designs. The design specifications are shown in Table 4. The freeform surfaces shine at the 50 L volume, providing 12x better RMS wavefront error and roughly 2x better exit pupil quality, while at the 100 L volume, the OAC designs put up a better relative result, though they were still outmatched by 4.5x in wavefront and roughly 2x in exit pupil quality. Like in the earlier OAC comparison, we attribute the magnitude of improvement going from OACs to freeform to the ability to better describe the ideal surface shapes for the shared mirrors that have separated used apertures between the two configurations.

[0048] Table 4. Design parameters for Figs 11A-11D four-mirror beam path systems.

[0049] Bauer et al., Optics Express, Vol. 31, No. 15 / 17, pp. 24691-24701, July 2023, the disclosure of which is incorporated by reference herein, provides further details of design and exit pupil quality analysis and optimization techniques in the design of freeform afocal reflective telescopes as may be employed for the first and second configurations in various multiconfiguration embodiments of the present disclosure.

[0050] As described above, representative geometric tilts of freeform mirror surfaces for various three- and four-mirror beam path multiconfiguration systems are shown inFigs. 5A, 5B, 5C, 6A, 7A, 7B, 7C, 9, A, 9B, 9C, 9D, 10A, 1OB, 1 IB and 1 ID. Design parameters for the optical surfaces of the components of Fig. 7C for the illustrated 4x zoom ratio embodiment employing a common primary mirror Ml, separate secondary mirrors M2a and M2b, and separate tertiary mirrors M3a and M3b are further set forth in Tables 7.1-7.2 below. Design parameters for the optical surfaces of the components of Fig. 9D for the illustrated 5x zoom ratio embodiment employing a common primary mirror Ml, separate secondary mirrors M2a and M2b, a common tertiary mirror M3 and a common quaternary mirror M4 are further set forth in Tables 9. 1-9.2 below. Tn such tables, the optical surfaces for each embodiment are numbered increasingly from the entrance pupil to the exit pupil. For the 4x zoom ratio embodiment of Fig. 7C, mirrors Ml, M2a / M2b, and M3a / M3b correspond to Surface Nos. 2, 4, and 5, respectively, in Tables 7. 1 and 7.2. For the 5x zoom ratio embodiment of Fig. 9D, mirrors Ml , M2a / M2b, M3 and M4 correspond to Surface Nos. 2, 3, 4 and 5, respectively, in Tables 9.1 and 9.2.

[0051] Table 7.1 : Fig. 7C Design Parameters

[0052] Table 7.2: Fig. 7C Design Parameters

[0053] Table 9.1: Fig.9D Design Parameters

[0054] Table 9.2: Fig. 9D Design ParametersTable 9.2 (continued)

[0055] In this work, many variations of a multiconfiguration afocal telescope were conceived and designed. We first discussed and showed the ability to achieve a two- position zoom using a three-mirror beam path, with an uncompensated exit pupil. Then, by allowing the tertiary mirror to be split into two separate mirrors, we showed diffraction-limited designs up to a 4x zoom ratio with a compensated exit pupil. Adding a fourth mirror to the beam path allowed for a fully compensated system to be significantly minimized in size. Finally, splitting the tertiary mirror into separate mirrors in the four-mirror beam path design allowed further improvement of the exit pupil quality while maintaining a compact volume and diffraction-limited performance.

[0056] While this work related to demonstrating the possibilities for the optical design, any system to be built must be mechanically feasible. To that end, having a multiconfiguration system with a small volume, like the 4-mirror beam path systems, serves a dual purpose. The first and most apparent is that the system is smaller, weighs less, and requires smaller optical surfaces. The less obvious benefit is that the moving optics have less distance to travel when switching between configurations. Care was taken in these designs to ensure no collision points between the optics when they move. By utilizing these methods described herein, optical systems for remote sensing applications can be designed with the ultimate flexibility in their use.

[0057] All publications, patents, and patent applications cited herein are hereby incorporated by reference in their entirety for all puiposes.

[0058] Cited references:1. J. P. Rolland, M. A. Davies, T. J. Suleski, C. Evans, A. Bauer, J. C. Lambropoulos, and K. Falaggis, "Freeform optics for imaging," Optica 8(2), 161-176 (2021).2. J. Reimers, A. Bauer, K. P. Thompson, and J. P. Rolland, "Freeform spectrometer enabling increased compactness," Light Sci Appl. 6(1), el7026 (2017).3. E. M. Schiesser, A. Bauer, and J. P. Rolland, "Effect of freeform surfaces on the volume and performance of unobscured three mirror imagers in comparison with off-axis rotationally symmetric polynomials," Optics Express 27(15), 21750-21765 (2019).4. A. Bauer, C. Zhang, and J. P. Rolland, "Exit pupil quality analysis and optimization in freeform afocal telescope systems," Optics Express 31(15), 24691-24701 (2023).5. T. Kampe, I. Franka, P. Spuhler, N. Fan, Z. Ferkel, J. Soto, J. Fleming, J. Cole, and T. Finch, Development and test of 3-mirror freeform telescope for Earth remote sensing applications, SPIE Optical Engineering + Applications (SPIE, 2022), Vol. 12232.6. Z. Tang and H. Gross, "Improved correction by freeform surfaces in prism spectrometer concepts," Applied Optics 60(2), 333-341 (2021).7. C. Yoon, A. Bauer, D. Xu, C. Domer, and J. P. Rolland, "Absolute linear-in-k spectrometer designs enabled by freeform optics," Optics Express 27(24), 34593-34602 (2019).8. C. Liu, C. Straif, T. FlugeLPaul, U. D. Zeitner, and H. Gross, "Comparison of hyperspectral imaging spectrometer designs and the improvement of system performance with freeform surfaces," Applied Optics 56(24), 6894-6901 (2017).. Yang, J. Zhu, and G. Jin, "Starting configuration design method of freeform imaging and afocal systems with a real exit pupil," Applied Optics 55(2), 345-353 (2016). W. B. Wetherell, "AFOCAL SYSTEMS," in Handbook of Optics: Volume I - Geometrical and Physical Optics, Polarized Light, Components and Instruments, M. Bass, ed. (McGraw-Hill Education, 2010). L. G. Cook, "Five-mirror afocal wide field of view optical system," US 10,054,774 (2018). R. S. Kebo, "All-reflective zoom optical system," US5, 144,476 (1992). J. Johnson, C. Roll, A. Fontaine, T. Ulmer, and J. Roth, Wide field-of-view NURBS- based freeform afocal telescope, SPIE Defense + Commercial Sensing (SPIE, 2023), Vol. 12530. R. B. Johnson, Unobscured reflective zoom systems, SPIE's 1995 International Symposium on Optical Science, Engineering, and Instrumentation (SPIE, 1995), Vol. 2539. G. Xie, J. Chang, K. Zhang, J. Zhou, and Y. Niu, "Off-axis three-mirror reflective zoom system based on freeform surface," Frontiers of Optoelectronics 9(4), 609-615 (2016). C. Xu, D. Cheng, J. Chen, and Y. Wang, "Design of all-reflective dual-channel foveated imaging systems based on freeform optics," Applied Optics 55(9), 2353- 2362 (2016). J. Sasian, "Interpretation of pupil aberrations in imaging systems," Proc. SPIE 6342 (2006). A. Bauer, E. M. Schiesser, and J. P. Rolland, "Starting geometry creation and design method for freeform optics," Nature Communications 9(1), 1756 (2018). K. Fuerschbach, J. P. Rolland, and K. P. Thompson, "Theory of aberration fields for general optical systems with freeform surfaces," Optics Express 22(22), 26585-26606 (2014). T. J. Magner, J. Zaniewski, S. Rice, and C. Fleetwood, "Fabrication and testing of off-axis parabolic mirrors," Optics & Laser Technology 19(2), 91-96 (1987). D. Malacara, Optical Shop Testing (John Wiley & Sons, 2007).

Claims

CLAIMS1. A multiconfiguration afocal reflective telescope system switchable between (i) a first configuration providing a first relatively high afocal magnification for incoming light along a first beam path between a first configuration system entrance pupil and a first configuration system exit pupil and (ii) a second configuration providing a second relatively low afocal magnification for incoming light along a second beam path between a second configuration system entrance pupil and a second configuration system exit pupil; wherein each of the first and the second configurations employs a common primary mirror (Ml), a different one of two separate secondary mirrors (M2a, M2b), and either a common tertiary mirror (M3) or a different one of two separate tertiary mirrors (M3 a, M3b) arranged in asymmetrical optical geometries providing non-obscured optical paths, for directing and magnifying incoming light along each of the first and second beam paths in the first and second configurations; wherein the primary mirror (Ml) is tiltable when switching between the first and second configurations to direct incoming light to be incident on either of the two separate secondary mirrors (M2a, M2b), and if employed the common tertiary mirror (M3) is translatable and rotatable in space when switching between the first and second configurations; and wherein each of the primary mirror (Ml), the two separate secondary mirrors (M2a, M2b), and either the common tertiary mirror (M3) or each of the two separate tertiary mirrors (M3 a, M3b) comprise freeform reflective surfaces that correct optical aberrations induced by the asymmetrical optical geometries of the mirrors in each of the first and second beam paths.

2. A multiconfiguration afocal reflective telescope system of claim 1, wherein each of the first and the second configurations further employs a quaternary mirror (M4) arranged in the asymmetrical optical geometries providing nonobscured optical paths for directing and magnifying incoming light along each of the first and second beam paths in the first and second configurations, wherein the quaternary mirror (M4) is translatable and rotatable in space when switching between the first and second configurations; and wherein the quaternary mirror (M4) comprises a freeform reflective surface that further corrects optical aberrations induced by the asymmetrical optical geometries of the mirrors in each of the first and second beam paths.

3. A multiconfiguration afocal reflective telescope system of either claim 1 or 2, wherein the first configuration provides a first relatively high afocal magnification of at least 1 Ox, and the first and second configurations provide a magnification zoom ratio between the first relatively high afocal magnification and second relatively low afocal magnification of at least two, of at least 3, of at least 4, or of at least 5.

4. A multiconfiguration afocal reflective telescope system of any one of claims 1-3, wherein the first configuration employs a maximum entrance pupil diameter of 250 mm, the common primary mirror (Ml) has a maximum diameter of 250 mm, and the second configuration uses a portion of the common primary mirror (Ml) within the maximum diameter.

5. A multiconfiguration afocal reflective telescope system of any one of claims 1-4, wherein the first configuration system exit pupil and the second configuration system exit pupil are coincident.

6. A multiconfiguration afocal reflective telescope system of any one of claims 1-5, wherein the system has a volume of less than or equal to 100L, less than or equal to80L, less than or equal to 75L, less than or equal to 70L, less than or equal to 60L, less than or equal to 50L, less than or equal to 40L, or less than or equal to 35L, where the volume is calculated as the smallest box to encompass the system with one face normal to the incoming on-axis light beam.

7. A multiconfiguration afocal reflective telescope system of any one of claims 1-6, wherein both the first and the second configurations provide diffraction-limited performance for light wavelength of 587 nm, with RMS WFE of less than 0.07 waves.

8. A multiconfiguration afocal reflective telescope system of any one of claims 1-7, wherein the first and the second configuration exit pupils have RMS pupil size and RMS pupil offset errors of less than 10%.

9. A multiconfiguration afocal reflective telescope system of any one of claims 1-8, wherein the first and the second configuration exit pupils have RMS pupil size and RMS pupil offset errors of less than 5%.

10. A multiconfiguration afocal reflective telescope system of claim 2, wherein each of the first and the second configurations employs one of two separate tertiary mirrors (M3 a, M3b) arranged in the asymmetrical optical geometries providing non-obscured optical paths for directing and magnifying incoming light along each of the first and second beam paths in the first and second configurations,wherein the first configuration provides a first relatively high afocal magnification of at least 1 Ox, and the first and second configurations provide a magnification zoom ratio between the first relatively high afocal magnification and second relatively low afocal magnification of at least 5, wherein the first configuration employs a maximum entrance pupil diameter of 250 mm, the common primary mirror (Ml) has a maximum diameter of 250 mm, and the second configuration uses a portion of the common primary mirror (Ml) within the maximum diameter, wherein the first configuration system exit pupil and the second configuration system exit pupil are coincident, wherein the system has a volume of less than or equal to 100L, where the volume is calculated as the smallest box to encompass the system with one face normal to the incoming on-axis light beam, wherein both the first and the second configurations provide diffraction-limited performance for light wavelength of 587 nm, with RMS WFE of less than 0.07 waves, and wherein the first and the second configuration exit pupils have RMS pupil size and RMS pupil offset errors of less than 5%.

11. A multiconfiguration afocal reflective telescope system of claim 1 , wherein each of the first and the second configurations employs a common primary mirror (Ml), a different one of two separate secondary mirrors (M2a, M2b), and a different one of two separate tertiary mirrors (M3a, M3b) arranged in asymmetrical optical geometries providing non-obscured optical paths, for directing and magnifying incoming light along each of the first and second beam paths in the first and second configurations to first and second configuration system exit pupils, wherein the first configuration system exit pupil and the second configuration system exit pupil are coincident; and further comprising a fourth mirror located at the coincident exit pupils that serves as a fold mirror whose tilt is coordinated with the common primary mirror (Ml) providing a fully compensated system with only mirror tilts of the primary mirror (Ml) and the fold mirror and not requiring any mirror translations.

12. A multiconfiguration afocal reflective telescope system of any one of claims 1-11, wherein the field of view in the first relatively high afocal magnification configuration is 1x1 degrees.

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