Telescope with optical alignment system

Abstract

A telescope having an optical alignment system, the telescope having a segmented primary mirror, the multiple mirror segments having multiple facets that are used to direct metrology light for aligning the telescope.

Description

[Technical field]

[0001] The present invention relates generally to optical alignment techniques for telescopes, such as space telescopes. [Background technology]

[0002] For a telescope to work properly, its optical systems must be precisely aligned with respect to one another. Alignment can involve positioning individual optical systems, positioning segments of an optical system (large telescopes often have segmented primary mirrors), controlling the shape of deformable mirrors, etc. In the case of diffraction-limited telescopes, some of the optical elements must be positioned with respect to one another to within a fraction of a wavelength of light. In this specification, "light" also refers to non-visible light, such as infrared (IR) and ultraviolet (UV).

[0003] This specification details alignment techniques for telescopes, which are useful for all telescopes, but are particularly useful for space telescopes.

[0004] Telescopes may require initial alignment, such as when deployed and deployed in space. After the telescope is precisely aligned, the alignment must be maintained. Alignment drift is caused by a variety of factors, such as temperature changes, changes in the gravity vector, wind buffeting, and acoustic vibrations. Precise alignment is especially important for imaging applications that require large area flat fields.

[0005] Most telescope designs employ passive techniques to counter these changes. Some telescopes use distant external sources (e.g., stars) to measure the wavefront error, i.e. the alignment error. However, this technique is not suitable for space telescopes imaging, for example, the Earth.

[0006] A telescope can be aligned by placing a plane mirror in front of it and passing light from an internal light source back through it. If the plane mirror is perfect, the only wavefront error measured will be that due to the telescope. However, this is inconvenient, and the telescope cannot be used to view distant objects with the test plane mirror in place.

[0007] McCann et al. ("A new mirror alignment system for the VERITAS telescopes", Astroparticle Physics32(2010),325-329) detail a system for aligning telescopes using stars. The telescope has a segmented mirror, where the mirror segments are called "facets". Vazquez-Montiel ("Method For Facets' Alignment For The High-Flux Solar Furnace At Cie-Unam In Temixco, Mexico. First Stage" (http: / / www.concentracionsolar.org.mx / images / pdf / memorias / 05_sp12_sv.pdf)) also detail a solar concentrator with a segmented mirror, where the mirror segments are called facets. None of these systems have mirror facets as defined herein. Kishner ("High-bandwidth alignment sensing in active optical systems", Proc. SPIE 1532, Analysis of Optical Structures, December 1, 1991) details an optical alignment system that attaches a retroreflector to the primary mirror. These reflections return along the incident optical path and do not require angular alignment. Zhao et al. ("Simultaneous multi-piston measurement method in segmented telescopes", Optics Express 24540, Vol. 25(20), October 2, 2017) details the use of modulation transfer function side lobes to measure piston errors of mirror segments. WO2021 / 018634 details a grating-based alignment system for telescopes with segmented mirrors. Further background prior art is described in US 5,274,479 and US 2018 / 0074236. Summary of the Invention

[0008] Thus, in one aspect, a telescope having an optical alignment system is detailed. The telescope includes a segmented primary mirror having a plurality of mirror segments. Each mirror segment has a segment surface that defines a portion of an image surface of the primary mirror. The image surface of the primary mirror may be, but is not necessarily, a first surface of the telescope.

[0009] Each mirror segment has a mirror facet. The direction of light reflected from the mirror facet defines the orientation of the segment surface of the mirror segment. That is, the facet has a fixed orientation with respect to the segment surface. Therefore, the orientation of the segment surface of the mirror segment can be determined from the direction of light reflected from the mirror facet.

[0010] In an embodiment, each mirror facet is configured to reflect light in a direction different from the light reflected from a segment surface of the mirror segment. Typically, the light reflected from the mirror facets converges towards the metrology system.

[0011] A light source (in an embodiment an internal light source) is configured to illuminate the mirror facets of each mirror segment, and in this regard, the internal light source may be a light source positioned not beyond the front aperture of the telescope.

[0012] As described above, a metrology system is positioned to receive light from the light source after reflection by the mirror facets of each mirror segment. The received light characterizes the optical alignment of the multiple mirror segments. The metrology system may include an imaging system, e.g., a camera, to capture the positions of multiple reflections from the multiple mirror facets and / or an image of the received light, such as an interference pattern produced by superimposing the multiple reflections.

[0013] Some embodiments of the telescope provide excellent sensitivity to various types of mirror segment misalignment and may be configured to automate the alignment process.

[0014] In another aspect, there is provided a method of aligning a telescope, particularly as described above, comprising the steps of illuminating a mirror facet of each mirror segment using a light source, and moving one or more of the plurality of mirror segments to align reflections from the mirror facets of each mirror segment.

[0015] The aligning step may include correcting tilt misalignments of the plurality of mirror segments by tilting one or more of the plurality of mirror segments such that reflections from a mirror facet of each mirror segment coincide with one another, or alternatively, the aligning step may include correcting translational misalignments of the plurality of mirror segments along an optical axis of the telescope by pistonically moving one or more of the plurality of mirror segments such that reflections from a mirror facet of each mirror segment produce interference fringes.

[0016] The mirror segment surfaces may also be provided with a diffraction grating that is illuminated by a monochromatic light source to generate a diffraction pattern. The method may then include determining one or more characteristics of the diffraction pattern and correcting misalignment of the mirror segments by moving one or more of the mirror segments in response to the one or more characteristics. Depending on the correction performed, different characteristics of the diffraction pattern may be measured. [Brief description of the drawings]

[0017] These and other aspects of the invention will now be described in detail, by way of example only, with reference to the accompanying drawings.

[0018] [Figure 1] FIG. 1 shows a prior art metrology system with an internal light source. [Diagram 2] FIG. 2 shows a schematic diagram of a telescope with an optical alignment system. [Diagram 3] FIG. 3 shows an embodiment of a mirror segment having facets at the ends of the segment. [Figure 4] FIG. 4 shows the image acquired by the measurement system. [Figure 5a] FIG. 5a shows an optical alignment system with a defocused image plane. [Figure 5b] FIG. 5b shows an embodiment of the optical alignment system. [Figure 6] Figures 6a-6d show the stages of progressive alignment of the telescope. [Figure 7] 7a-7d respectively show a simulation of a pupil of a pair of circular mirror segments, the point spread function (PSF) of the pair of mirror segments, the modulation transfer function (MTF) of the pair of mirror segments, and a cross section through the MTF showing the side lobes of the MTF. [Figure 8] FIG. 8 shows that the MTF side lobes for monochromatic and broadband light increase as the optical path difference (OPD) between the pair of mirror segments in FIG. 7 increases. [Figure 9a] Figure 9a shows the MTF sidelobe height as a function of OPD for a simulated dual-wavelength source. [Figure 9b] FIG. 9b shows the MTF sidelobe height as a function of the OPD of the broadband source. [Figure 9c] FIG. 9c shows the MTF sidelobe height as a function of OPD for the example experiment. [Figure 10] 10a and 10b show the interference fringes and corresponding MTF between each segment pair of a three mirror segment telescope, and the composite interference fringes and corresponding MTF. [Figure 11] Figure 11 shows an example of a prototype telescope.

[0019] In the various figures, like elements are designated with like reference numerals. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] 1 shows a schematic diagram of a telescope 50 with an alignment system. A metrology system 60 is located at the center of curvature 54 of a segmented primary mirror 52. The metrology system 60 is twice as far from the mirror as an imaging system 58, which is located at the focal point 58 of the primary mirror.

[0021] FIG. 2 shows a schematic diagram of a telescope 100 having an optical alignment system using the techniques described herein. In FIG. 2, a segmented primary mirror 102 is formed from a set of mirror segments 102a-c, each having a segment surface that defines a portion of the image surface of the primary mirror 102. Each of the multiple mirror segments, e.g., mirror segments 102a,b in FIG. 2, has at least one mirror facet 104. The facets have a fixed orientation with respect to the segment surface, so that the direction of light reflected from the mirror facet defines the orientation of the segment surface of the mirror segment. The facets can be thought of as defining a reference surface for the corresponding segment surface. This reference surface can be used for alignment of the multiple mirror segments, as will be described below.

[0022] In Figure 2, facet 104 is in the center of the mirror segment, but multiple facets may be located anywhere on the surface. Figure 3 shows an example of a mirror segment where facet 104 is at the end of the segment, which may be easier to manufacture.

[0023] The facets may have flat or curved surfaces. For example, the surface of the facets may be curved, e.g., spherical, to focus light from the light source to the measurement system. Typically, the area of ​​the facets 104 is small compared to the total reflective surface area of ​​the mirror segment, e.g., less than 10% of the total surface area. Thus, a facet, as used generally in the present invention, is a part of a segment surface that directs light in a direction different from the segment surface. For example, the facet may have a focal point located away from the focal plane of the telescope. In this specification, the segment surface refers to a part of the segment surface that defines a part of the image plane of the primary mirror, i.e., a part other than the facet. This may also be called the main part of the primary mirror segment surface.

[0024] A variety of techniques can be used to manufacture the mirror segments with their mirror facets, such as diamond turning or other precision freeform CNC (Computer Numerical Control) manufacturing methods.

[0025] Light rays 114 represent light from distant objects observed by the telescope, which are focused by the segmented primary mirror 102 towards the telescope focal point 106 where, for example, an imaging system 108 and / or other optical systems are located.

[0026] The telescope includes a light source 109 (in an embodiment, an internal light source) and a metrology system 110, where the internal light source is a light source that is positioned so as not to extend beyond the front aperture of the telescope. The light from the light source 109 is referred to as metrology light. In an embodiment, the light source is a point source of metrology light.

[0027] 2, the light source is co-located with the metrology system, and multiple light beams 112 show a measurement light path from light source 109 to primary mirror 102 and back to metrology system 110. The light source is positioned to illuminate the mirror facets of each mirror segment, and the light reflected from the facets characterizes the optical alignment of the mirror segments. In an embodiment, metrology system 110 includes a detector, e.g., an image sensor, for acquiring an image of the measurement light reflected from the facets.

[0028] In FIG. 2, both the imaging system 108 and the metrology system 110 are at the focal point 106, but in other arrangements the metrology system may be located away from the focal plane of the telescope. The techniques described herein can be used with many different telescope designs, including catadioptric designs. For example, a Cassegrain design can facilitate placing the source 109, metrology system 110, and focal point 106 at different locations along the optical axis of the telescope. In general, however, as can be seen by comparing FIG. 1 with FIG. 2, the techniques described herein can reduce the length of the telescope by approximately half. This is particularly important in space applications.

[0029] The optical path defined by reflection of the light rays 114 off of the primary mirror 102 is different from the optical path defined by reflection of the light rays 112 off of the primary mirror 102 because the measurement light is reflected by the facets, whereas the light rays 114 from the distant object are reflected by the major portion of each mirror segment. In general, the mirror facets 104 are configured to reflect light in a different direction than the light reflected from the major portion of the mirror segment surface.

[0030] Typically, the image surface of the primary mirror 102 is configured to focus light onto a collection surface or focal plane. Typically, but not by way of limitation, the image surface of the primary mirror 102 is the first surface of the telescope and defines the collection aperture of the telescope. Many optical elements may be between the collection aperture of the telescope and the imaging region, but these optical elements have been omitted from the drawings herein for simplicity.

[0031] For example, one advantage of using reflective facets to direct the measurement light rather than a diffractive approach is that reflection is compatible with the use of broadband light sources, which, as will be described below, are useful for aligning multiple mirror segments in the piston direction.

[0032] The metrology light can be used to determine the alignment of the mirror segments 102a-c. In an embodiment, the metrology light can also be used to automatically align the mirror segments with respect to one another. Thus, in an embodiment, the telescope includes a set of actuators 120a-c, where each of the mirror segments (or at least all but one of the mirror segments) has a respective actuator for moving the mirror segment. For example, the mirror segments may be tip / tiltable and / or pistonable by the actuator.

[0033] In an embodiment, the actuator of the mirror segment comprises a three-point ("Maxwell-type") mount with three linear actuator devices spaced around the end of the mirror segment, such that movement of any one linear actuator device rotates the mirror segment in the tip and / or tilt directions about an axis passing through the contact points of the other two linear actuator devices, and movement of all three linear actuator devices together can provide adjustment of the piston direction.

[0034] In an embodiment, the actuators 120a-c are controlled by control signals from an actuator control system 130 connected to the metrology system 110 to align the multiple mirror segments. More specifically, the metrology system is configured to acquire images of light from a light source after reflection by each of the multiple mirror facets. Further, the actuator control system is configured to control the set of actuators in response to the acquired images to move the multiple mirror segments to align multiple reflections from the multiple mirror facets of the multiple mirror segments.

[0035] The mirror segments may be aligned to stack the sub-images from the mirror segments and / or to in-phase the mirror segments, i.e., to define an optical path difference (OPD) between the segment faces of the mirror segments and the focal point of the telescope as λ. op To make it smaller, multiple mirror segments are aligned, where λ op is the operating wavelength of the telescope, which will be discussed later.

[0036] As a preliminary step to the telescope alignment procedure, depending on the initial degree of alignment, each of the mirror segments may be scanned in the tip / tilt direction so that the image sensor of the metrology system sees a reflection from each facet. The mirror segments may also be focused, i.e., pistoned, together to align the mirror segments to within the focal depth of the mirror segments (e.g., ∼0.1 mm). In this case, the reflected light from the mirror facets may be used, but need not be. For example, if the facets have curved surfaces to focus the measurement light onto the metrology system, the mirror segments may be pistoned to maximize the contrast of the image of the measurement light source from each facet, e.g., by minimizing the slope of the modulation transfer function (MTF) of the image of the measurement light source from the facet.

[0037] The automatic alignment procedure begins by controlling, using the actuator control system, a set of actuators to tilt one or more mirror segments in response to an image of the measurement light acquired by the metrology system to align reflections from the mirror facets of the mirror segments such that the reflections from the mirror facets of the mirror segments are laterally coincident with one another in the acquired image, i.e., sub-images from the mirror segments overlap. (Note that, although the terms "tip" and "tilt" are typically used herein to refer to rotation about two orthogonal axes, for simplicity of explanation, "tilt" may be used to refer to rotation about either axis.)

[0038] FIG. 4 shows an image acquired by the metrology system 110, including sub-images 402a-c from respective mirror facets of the mirror segments 102a-c. The actuator control system controls the actuators to tilt the corresponding mirror segments to overlap the sub-images, i.e. to laterally align the sub-images so that they coincide with each other. The tip / tilt angular movement α of the mirror segments results in a displacement αν of the sub-images in the image plane at a distance ν, or a displacement Mαν if the metrology system has a magnification M. This displacement can be corrected, for example, by α=δ / s from a linear displacement δ of the linear actuator device about a notional pivot at a distance s from the linear actuator device. In this way, the tilt of each of the mirror segments can be adjusted to move the sub-images until they coincide with each other.

[0039] In some embodiments, the position error may be calculated and the corresponding mirror adjusted to correct it. In some embodiments, a closed loop control system may be used to control multiple actuators to align the multiple sub-images.

[0040] Ideally, the tip / tilt misalignment of multiple mirror segments is λ op / D, where D is the (maximum) lateral dimension of the primary mirror 102 to obtain diffraction-limited angular resolution. To improve the correction of the misalignment, a fine-alignment process can be used. This involves separating each of the multiple sub-images so that their lateral positions can be measured more precisely. Precise measurement of the positions of the multiple sub-images becomes difficult when the multiple sub-images start to overlap.

[0041] One way to do this is to use a shutter to select individual mirror segments and determine the location of the sub-images. Another way is to use an additional detector (e.g., a second image sensor) to spatially separate the multiple sub-images in the defocus plane. This latter method has the advantage that all mirror segments can be measured and corrected simultaneously, and also facilitates fine alignment processes using a closed-loop control system. In such an arrangement, the metrology system can include a first image sensor that captures an image of the light received from the light source after reflection by each of the multiple mirror facets, and a second image sensor configured to capture a defocused image of the received light. When the multiple reflections are coincident with each other, the multiple reflections captured by the defocused image are spatially separated from each other.

[0042] Fig. 5a shows a system in which sub-images 402a,b correspond to defocused sub-images 502a,b in a plane called defocused image plane 520. The sub-images 402a,b are moved to coincide at a position 500 in the image plane, which corresponds to a target position 504a,b in the defocused image plane 520. Fig. 5b shows an embodiment in which a pair of beam splitters 522 is used to obtain the focused image plane 510 and the defocused image plane 520 (for clarity, the image sensor is not shown). In the example of Fig. 5b, the (point) light source 109 is co-located with the measurement system, although other arrangements are possible.

[0043] When the sub-images are superimposed, the mirror segments may be phased to reduce OPD (optical path difference) between the mirror segments due to pistonic misalignment, i.e., misalignment along the optical axis, e.g., the z-axis, of the telescope. More specifically, the actuator control system controls a set of actuators in response to an acquired (focused) image of the light from the light source to pistonically move the mirror segments to obtain interference fringes in the acquired image.

[0044] The interference fringes are generated by interference between overlapping sub-images of the measurement light from each of the mirror segments. In an embodiment, to use the interference fringes and reduce the OPD between the mirror segments, a broadband light source, e.g., a white light source such as a white LED, is employed as the light source, so that the fringes appear only when the OPDs coincide.

[0045] More specifically, in embodiments, the light source has a coherence length of less than 1000 μm, 100 μm, or 10 μm, for example, on the order of 1 μm. Fringes will only occur if the OPDs are matched within the coherence length of the light source. In embodiments where the mirror segments are already aligned within the focal depth of the telescope, the light source may have a coherence length that is less than the focal depth of the telescope. As one example, if the facets define a spherically curved surface, the fringes will occur at the center of curvature of the facets.

[0046] In an embodiment, the actuator control system is configured to control the set of actuators to maximize a measure of visibility of the interference fringes. The visibility of the interference fringes can be defined in standard ways, for example, as follows: max -I min ) / (I max +I min ), where I max and I min and are the maximum and minimum fringe light intensities, respectively. As described below, in some embodiments, the measure of visibility of the multiple interference fringes includes a measure of the height of one or more side lobes of the modulation transfer function of the multiple interference fringes.

[0047] In principle, monochromatic measurement light of only two different wavelengths would generate multiple interference fringes with different visibility, but the occurrence of multiple fringes would not define the unique position of each mirror segment. Using multiple monochromatic wavelengths reduces the uncertainty range. Using a broadband light source eliminates the degeneracy, since multiple fringes will only occur if the OPDs coincide within a very small optical coherence length. Thus, multiple OPDs can be measured at λ opThe system can be positioned so that multiple fringes do not occur unless the following are matched: The broadband light source may have a FWHM (full width at half maximum) bandwidth of, for example, greater than 10 nm, 50 nm or 100 nm. A light source with a shorter coherence length and a wider bandwidth allows for more accurate piston error measurements.

[0048] To determine the position when the OPDs match, all mirror segments except one may be fixed and the position of the one of the mirror segments may be scanned in the piston direction until fringes are generated. The fringes are formed when the OPD between the scanned mirror segment and one of the other mirror segments is small enough. The fringes include parallel fringes oriented perpendicular to the line between the mirror segments. As the mirror segment is scanned, another set of parallel fringes is obtained, which is rotated at a predetermined angle corresponding to the rotation angle between the pairs of mirror segments that generate the interference fringes, and crosses the first set of parallel fringes to generate a cross pattern. When all the OPDs match, the next mirror segment is scanned in the same way, generating a pattern of dots.

[0049] Thus, the actuator control system may control the set of actuators to obtain overlapping sets of stripes, each set of stripes having an orientation direction defined by the orientation of a corresponding pair of mirror segments. The actuator control system may control the set of actuators to obtain multiple overlapping stripes of multiple different orientations.

[0050] Figure 6 shows multiple images acquired by the metrology system of an exemplary telescope during alignment, with each image showing a stage of the alignment process: Thus, Figure 6a shows three sub-images 402a-c before superimposition, Figure 6b shows the sub-images superimposed at position 500, Figure 6c shows multiple interference fringes from multiple facets of a pair of mirror segments with matched multiple OPDs, and Figure 6d shows multiple interference fringes from multiple facets of a set of three mirror segments with matched multiple OPDs.

[0051] As described above, when the mirror segments are scanned, an interference pattern appears: if the light source includes a broadband light source, the zero order fringe has a maximum visibility at a unique location, making the zero order fringe uniquely identified.

[0052] However, in one variation, rather than using broadband light, the light source may include three or more different monochromatic wavelength sources, or at least two separate wavelength sources, one of which is tunable. Two of the wavelengths may then be used to search for interference fringes, for example by the scanning process described above, and when interference fringes are found, one of the wavelengths may be changed by substituting another of the monochromatic wavelengths or by changing the tuning of the wavelength. If the zero order position is found, the visibility of the fringes should not change substantially, but if fringes of different orders are found, the visibility of the fringes will decrease or the fringes will disappear.

[0053] Thus, if the light source includes two different monochromatic wavelengths of light, the degeneracy can also be removed by changing one of these wavelengths. For example, the actuator control system may be configured to control the set of actuators in response to the acquired image to piston the mirror segments such that the interference fringes remain substantially stationary when one of the different monochromatic wavelengths, e.g., the wavelength of the tunable light source, is changed.

[0054] As mentioned above, one way to measure the visibility of the multiple fringes is to use the modulation transfer function (MTF) of the measurement system. The MTF is defined by the Fourier transform (magnitude) of the PSF (point spread function), so if the light source is a point source, the MTF can be determined from the Fourier transform of the light acquired by the image sensor (in the focus image plane) of the measurement system. More specifically, the height of the MTF side lobe is a measure of the visibility of the multiple fringes, and maximizing the side lobe height maximizes the visibility.

[0055] Figure 7 shows the results of a simulation of monochromatic light of wavelength λ. Figure 7 shows the pupil of a pair of circular mirror segments (Figure 7a), the PSF of the segment pair (Figure 7b), the MTF of the segment pair (Figure 7c), and a cross section along a dashed line across the MTF showing the MTF side lobes (Figure 7d). In the example of Figure 7a, the pair of mirror segments has a diameter d and a separation distance D, the measurement system has a magnification M, the measurement image plane is located a distance v away from the pair of mirror segments, and p is the pixel size of the image sensor.

[0056] For a phase difference β between a pair of mirror segments, the OPD between the mirror segments due to piston misalignment (due to reflection) is 2β. As β increases, the phase (position) of the fringes changes cyclically with a period of λ / 4, and the fringes become increasingly smeared when broadband light is used. Figure 8 shows the side lobes for both monochromatic and broadband light. From left to right, β=0, λ / 8, λ / 4, 3λ / 8, λ / 2. From Figure 8, it can be seen that with broadband light, the side lobe height decreases along with the visibility of the fringes.

[0057] The side lobe height H can be calculated as follows:

number

[0058] Here, A(λ) is the spectrum of the measurement light intensity. The integral is calculated over the wavelength range of the measurement light, e.g., the minimum wavelength λ min from maximum wavelength λ max , which at the same time defines the wavelength limit of the measurement source. Figure 9a shows the side lobe height H as a function of the piston error β in the dual wavelength spectrum (λ1 = 633 nm, λ2 = 635 nm). The dual wavelength produces many visibility maxima at intervals of λ / 2. Figure 9b shows the calculated side lobe height H as a function of the piston error β for the spectra of real white, yellow, green and lime LEDs. The FWHM of the curves are 0.73 μm, 1.9 μm, 2.9 μm and 1.3 μm, respectively. This corresponds to the coherence length of each LED. The broadband source produces a single global maximum of H when the OPD is zero. Figure 9c shows an experimental example, showing how multiple fringes arise at β = 0.4 μm.

[0059] FIG. 9c shows the fringes of a pair of mirror segments. The fringe visibility can be measured for multiple mirror segments, for example, if the fringes have non-overlapping MTF side lobes when imaged by a metrology system. This can be conveniently achieved by positioning the mirror segments appropriately and / or by using masks and shutters. As an example, FIG. 10a shows the PSFs and corresponding MTFs between each pair of three mirror segments arranged at 120° to each other, and FIG. 10b shows the combined PSF and non-overlapping MTFs.

[0060] In one example of the alignment process, multiple mirror segment pairs are systematically scanned in the piston direction to find multiple interference fringes. For example, if N mirror segments are used, the reference mirror segment is kept stationary and the remaining N-1 mirror segments are scanned. In this way, a set of actuators is controlled to piston each of the N or N-1 mirror segments in sequence to obtain multiple interference fringes from each pair of mirror segments in the set of N.

[0061] After multiple fringes are found, the OPD of the mirror segment pair is minimized by maximizing the MTF side lobe height, e.g., by finely scanning the mirror segment positions. This is done by finding the maximum side lobe height H max This is done by scanning only one, i.e., the reference mirror segment, in the piston direction to identify where H is the piston error. For each of the other N-1 (fixed) mirror segments, the relative piston error between the scanned mirror segment and each of the other mirror segments is measured. For each fixed mirror segment, H max The piston position of λ (MTF side lobes corresponding to the interference between that mirror segment and the reference mirror segment) measures the piston error relative to the reference mirror segment (β=0). Thus, in an embodiment, the actuator control system moves one mirror segment while holding the other mirror segments fixed, and obtains position correction information for each of the other mirror segments. The actuator control system can use the position correction information to control the set of actuators to correct the relative positions of the mirror segments in the piston direction, i.e., to correct the measured error. This can reduce the OPD to β≪λ.

[0062] In some embodiments, the light source includes a fine alignment light source providing fine alignment light having a coherence length shorter than a coherence length of the measurement light initially used to find the fringes, and the actuator control system may then be configured to use the interference fringes from the fine alignment light to control the set of actuators to adjust the alignment of the mirror segments in the piston direction after an initial coarse alignment, i.e., after obtaining the interference fringes from the measurement light.

[0063] Optionally, during the phase alignment process described above, the tip / tilt alignment of the multiple mirror segments may be rechecked at intervals and corrected if necessary.

[0064] The advantage of multiple reflective facets is that they are compatible with the use of broadband measurement light. However, the diffractive approach can monitor the main image portion of each mirror segment surface, and in embodiments can monitor substantially the entire image surface of the primary mirror. In some embodiments, both reflective and diffractive metrology can be used to obtain complementary information. In this case, the multiple mirror facets and the diffraction grating are arranged such that the sub-images / spots generated by each approach are spatially separated from each other at the image sensor of the metrology system to avoid mixing of multiple signals from the reflective and diffractive approaches. Separation can also or instead be performed spectrally (using different wavelengths) or temporally (e.g. by time multiplexing).

[0065] Thus, one or more of the mirror segments in such a telescope may have a diffraction grating on the segment surface, e.g., part of a concentric circle pattern extending across the set of mirror segments as a whole. The diffraction grating may be weak, diffracting only a small percentage of the measurement light, e.g., less than 10%. Such a diffraction grating may also be formed by diamond turning, lithography (e.g., electron beam or UV / optical lithography), or by using holography to fabricate a coating. The light source may include a monochromatic light source (e.g., a point source, a laser source) for diffraction characterization of the primary mirror and a diffracted light metrology system, which may be positioned at conjugate optical positions defined by the diffraction grating. The line width and pitch of the diffraction grating may be selected to direct light from the light source to the diffracted light metrology system.

[0066] The metrology system may include a diffracted light metrology system that characterizes diffracted light from a monochromatic light source to determine a degree of optical alignment of the telescope and, optionally, to align the telescope.

[0067] For example, in an embodiment, the diffracted light from each mirror segment produces a diffraction pattern that includes spots in the measurement image plane, and these spots can be overlapped by moving the mirror segments on tip / tilt. This is similar to the previous approach using multiple reflective facets, but the diffraction grating can cover a larger area than the facets, producing smaller spots in the acquired image and facilitating more precise alignment.

[0068] As another example, the shape of the diffraction pattern, i.e. the shape of the image of the diffracted light captured by the image sensor of the metrology system, can be used to determine a measure of decentering of one or more of the mirror segments from the optical axis of the telescope. "Decentration" in this context refers to the translational misalignment of the mirror segments orthogonal to the optical axis of the telescope. Such decentering elongates the diffraction pattern (diffraction image / spot).

[0069] In another example, the diffracted light metrology system may include a wavefront metrology system, such as a Shack-Hartmann wavefront sensor, to characterize the wavefront of the diffracted light. The Shack-Hartmann wavefront sensor can measure the local tip-tilt of the wavefront from the displacement of the focused spot of the measurement light, and therefore can also be used to correct the tip / tilt of the multiple mirror segments.

[0070] FIG. 11 shows an example of a prototype telescope constructed as described above. The techniques detailed herein can be used with any telescope with a segmented primary mirror, and are useful for small and large telescopes. The techniques are particularly useful for telescopes that form an image of the object at which they are pointed, as they can be used to obtain resolution close to the diffraction limit. The techniques are not limited to visible light telescopes, but can also be used, for example, for infrared and ultraviolet telescopes of any wavelength range. The techniques can be used simultaneously when the telescope is imaging the target scene, and in some embodiments, the metrology and alignment system can run continuously. The techniques are particularly advantageous for space-based telescopes, such as, for example, Earth imaging telescopes.

[0071] Many other effective alternatives will occur to the skilled artisan. It will be understood that the invention is not limited to the described embodiments, but also encompasses modifications to the invention as defined in the appended claims that are obvious to a person skilled in the art.

Claims

1. 1. A telescope having an optical alignment system, comprising: A segmented primary mirror having a plurality of mirror segments, each of the plurality of mirror segments has a segment surface that defines a portion of an image plane of the primary mirror; each of the plurality of mirror segments has a mirror facet; an orientation of the light reflected from the mirror facet defines an orientation of the segment surface of the mirror segment; the segmented primary mirror, wherein the mirror facets are configured to reflect light in a direction different from light reflected from the segment surfaces of the mirror segments; a light source configured to illuminate a mirror facet of each of the plurality of mirror segments; a metrology system positioned to receive light from the light source after reflection by the mirror facets of each of the plurality of mirror segments; The telescope, wherein the received light characterizes the optical alignment of the plurality of mirror segments.

2. 10. The telescope of claim 1, wherein the mirror facets have curved surfaces for focusing light from the light source onto the metrology system.

3. 10. The telescope of claim 1, wherein the metrology system is located away from the focal plane of the telescope.

4. 10. The telescope of claim 1, wherein the light source comprises a source of measurement light having a coherence length less than a depth of focus of the telescope.

5. The telescope of claim 1 , wherein the light source comprises a source of metrology light having a coherence length of less than 100 μm or less than 10 μm.

6. a set of one or more actuators; an actuator control system connected to the set of one or more actuators and the measurement system; each of one or more of the plurality of mirror segments having a respective actuator for moving the mirror segment; the metrology system is configured to acquire an image of the received light from the light source after reflection by the mirror facet; 2. The telescope of claim 1, wherein the actuator control system is configured to control the set of one or more actuators in response to the acquired image to move one or more of the plurality of mirror segments to align a plurality of reflections from the mirror facets of the plurality of mirror segments.

7. each of the one or more actuators configured to tilt a corresponding one of the mirror segments; 7. The telescope of claim 6, wherein the actuator control system is configured to control the set of one or more actuators in response to the acquired image to tilt one or more of the plurality of mirror segments to align the plurality of reflections from the mirror facets of the plurality of mirror segments so that the reflections coincide with one another in the acquired image.

8. The measurement system includes: a first image sensor that captures the image of the received light from the light source after reflection by each of a plurality of the mirror facets; a second image sensor configured to capture a defocused image of the received light; 8. The telescope of claim 7, wherein the reflections captured in the defocused image are spatially separated from one another when the reflections coincide with one another.

9. each of the one or more actuators configured to pistonically move a corresponding one of the mirror segments; 7. The telescope of claim 6, wherein the actuator control system is configured to control the set of one or more actuators in response to the acquired image to pistonically move one or more of the plurality of mirror segments to acquire one or more interference fringes within the acquired image.

10. 10. The telescope of claim 9, wherein the actuator control system is configured to control the set of one or more actuators to maximize a measure of visibility of the one or more interference fringes.

11. The telescope of claim 10 , wherein the indication of the visibility of the one or more interference fringes comprises a measurement of the height of one or more side lobes of a modulation transfer function of the one or more interference fringes.

12. the actuator control system is configured to control the set of one or more actuators to obtain multiple overlapping sets of multiple stripes; each of the overlapping sets of the stripes has an orientation direction defined by an orientation of a corresponding pair of the mirror segments; 10. The telescope of claim 9, wherein the actuator control system is configured to control the set of one or more actuators to obtain a plurality of fringes having a plurality of different orientations.

13. The actuator control system includes: controlling the set of one or more actuators to initially move the plurality of mirror segments in a piston-like manner to acquire the plurality of interference fringes; Next, one of the plurality of mirror segments is moved while the other of the plurality of mirror segments is kept fixed, and position correction information for each of the other of the plurality of mirror segments is obtained; 10. The telescope of claim 9, further configured to use the position correction information to control the set of one or more actuators to correct the position of each of the plurality of mirror segments in a piston direction.

14. the light source further includes a fine alignment light source that provides fine alignment light having a coherence length shorter than a coherence length of the measurement light; 10. The telescope of claim 9, wherein the actuator control system is configured to, after acquiring a plurality of interference fringes of the measurement light, use a plurality of interference fringes of the fine alignment light to control the set of one or more actuators to perform piston-oriented adjustment of the plurality of mirror segments.

15. At least one of the plurality of mirror segments has a diffraction grating on a surface of the segment; the light source includes a monochromatic light source; the telescope further includes a diffractive optical metrology system; the light source and the diffracted light measurement system are positioned at conjugate optical positions defined by the diffraction grating; 10. The telescope of claim 1, wherein the diffracted light metrology system is configured to characterize the diffracted light from the monochromatic light source to determine a degree of optical alignment of the telescope.

16. 16. The telescope of claim 15, wherein the diffracted light metrology system is configured to characterize the diffracted light to determine an indication of decentering of one or more of the plurality of mirror segments from an optical axis of the telescope.

17. 1. A method of aligning a telescope, comprising: the telescope has a segmented primary mirror having a plurality of mirror segments; each of the plurality of mirror segments has a segment surface that defines a portion of an image plane of the primary mirror; each of the plurality of mirror segments has a mirror facet; an orientation of the light reflected from the mirror facet defines an orientation of the segment surface of the mirror segment; the mirror facet is configured to reflect light in a direction different from that of light reflected from the segment surface of the mirror segment; The method comprises: illuminating the mirror facets of each of the plurality of mirror segments with light using a light source; and moving one or more of the plurality of mirror segments to align reflections from the mirror facets of each of the plurality of mirror segments.

18. the aligning step includes correcting tilt misalignment of the plurality of mirror segments; 20. The method of claim 17, wherein moving one or more of the plurality of mirror segments comprises tilting one or more of the plurality of mirror segments such that multiple reflections from the mirror facets of each of the plurality of mirror segments coincide with one another.

19. the aligning step includes correcting translational misalignment of the plurality of mirror segments along an optical axis of the telescope; 20. The method of claim 17, wherein moving one or more of the plurality of mirror segments comprises pistoning one or more of the plurality of mirror segments such that multiple reflections from the mirror facets of each of the plurality of mirror segments produce multiple interference fringes.

20. the light source includes a monochromatic light source; the plurality of mirror segments have diffraction gratings on their surface to diffract light from the monochromatic light source to generate a diffraction pattern; The method comprises: determining one or more features of the diffraction pattern; 18. The method of claim 17, further comprising: correcting mirror segment misalignment by moving one or more of the plurality of mirror segments in response to the one or more characteristics.

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