Enhanced performance Schmidt telescope, associated detection devices and methods

Abstract

A compact Schmidt telescope (1), with an aperture number less than 2, a wide spectral range, and an extended field of view, capable of providing the optical image quality necessary for detecting the presence of small space objects. The telescope comprises, aligned and centered on the same optical axis (O), a concave mirror (10), a sphericity corrector (12), and a field corrector (14). The sphericity corrector (12) comprises an aspheric-achromatic lens doublet, i.e., two aspheric lenses (12A, 12B), each comprising two faces, at least one of which is curved and at least one of which has an aspheric profile. The two lenses are composed of different optical glass so as to form an achromatic doublet. The two lenses further have a central hole (120) centered on the optical axis (O) in which the field corrector (14) is placed. Figure for the abstract: Fig. 1

Description

Title of the invention: Improved performance Schmidt telescope, associated detection devices and method. Technical field

[0001] The invention relates to a Schmidt telescope.

[0002] It finds applications in the field of space observation or surveillance, in particular for detecting small space objects, such as nanosatellites or debris present in space, from a ground station (first application) or from a satellite in space (second application).

[0003] Thus, the invention also relates to a ground observation station and a satellite, each comprising said telescope for detecting such objects. The invention further relates to a detection method associated with the ground station or the satellite comprising the telescope.

[0004] In space, satellites operate in an environment with an ever-increasing density of space objects, such as satellites or space debris. Space debris originates from objects sent into space by humans. This debris consists primarily of fragments or pieces of satellites that have broken off, for example, following collisions between satellites, between a satellite and space debris, or even between pieces of space debris themselves. The size of this debris can be small, for example, on the order of 10 cm, or even less than 10 cm in diameter. Most space debris orbits the Earth, mainly at a distance of between 1,000 and 1,500 km, but also in geostationary orbits at approximately 36,000 km.

[0005] The presence of space debris and / or nanosatellites in the vicinity of one or more satellites must be detectable to prevent potential collisions that could damage or destroy the satellites. Depending on the intended application, such detection must be possible from a ground station or from a satellite in space.

[0006] To this end, there is a need for a telescope capable of imaging with sufficiently high optical quality to detect small space objects. Prior art

[0007] Of all known telescopes, the Schmidt telescope is the most promising for meeting this need. It is generally used to observe space from Earth.

[0008] According to one embodiment, this telescope comprises, aligned on the same optical axis, a concave (e.g., spherical) mirror, a sphericity corrector configured for correct the spherical aberration due to the mirror and a field corrector.

[0009] The field corrector is designed to flatten the field exiting the telescope, so as to allow the use of planar photodetectors. The field corrector is arranged between the concave mirror and the sphericity corrector.

[0010] The sphericity corrector is designed to correct spherical aberration, also known as "spherical aberrations," created by the spherical mirror, which has the detrimental effect of degrading the image quality provided by the telescope. In practice, spherical aberration occurs when the spherical mirror, more generally concave, does not focus all incident light rays onto a single focal point but onto a plurality of points along the optical axis. These aberrations manifest themselves for light rays far from the telescope's optical axis, i.e., where Gaussian conditions do not apply, and are therefore more pronounced the larger the telescope's entrance pupil (or aperture).

[0011] In a conventional Schmidt telescope, the sphericity corrector consists of an optical plate with parallel, flat faces, called a "Schmidt plate," which is appropriately aspherized. This plate is placed at the center of curvature of the mirror to correct spherical aberration over a wide field of view, which has the disadvantage of making the telescope bulky, particularly due to its considerable length. To correct spherical aberration, at least one face of the Schmidt plate has an aspheric profile obtained by an aspherization process, so that the Schmidt plate is aspheric.

[0012] By moving the sphericity corrector closer to the concave mirror to reduce the size of the telescope, aberrations appear, primarily coma and astigmatism, which degrade the resolution of the images provided. Consequently, the telescope is unable to provide sufficient optical image quality to detect small space objects, such as space debris or nanosatellites. Therefore, there is a need to correct such aberrations, at least to avoid degrading the optical quality of the image output by the telescope, and even to improve the optical quality to allow the detection of even smaller space objects and thus gain in precision.

[0013] Hereafter, "optical quality" will refer to the telescope's ability to focus light rays emanating from a point on the imaged object onto the smallest possible area of ​​an optoelectronic image sensor or photodetector, for example, a CCD (Charge-Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor) type. This optical quality reflects the purity with which the image of an object is obtained in the optical domain, i.e., through the optical components constituting the telescope. In practice, optical quality is limited by the phenomenon light diffraction. The optical quality will subsequently be described as "high" or "improved" when it approaches the limits due to diffraction, concentrating the incident light onto photosensitive pixels of the smallest possible size. In the context of the present invention, we are therefore interested in the quality of the optical image before photodetection. Thus, the invention does not relate to devices for improving image quality through digital image processing techniques after photodetection.

[0014] Furthermore, to observe space, the telescope's entrance pupil, through which light rays from the objects to be imaged enter, must have a sufficiently large diameter to capture a sufficient number of photons, so as to detect even the faintest objects as quickly as possible. Thus, the larger the entrance pupil, the better the telescope is able to detect small objects, such as satellite debris or nanosatellites.

[0015] To increase the resolution of the images provided, the telescope must have a resolving power (or resolving power) high enough to distinguish the two closest points. The higher the resolution, the smaller the detected space objects can be. As is known, the resolving power depends directly on the size of the entrance pupil and, more specifically, on the aperture diameter of the optics used. Thus, the resolution of a telescope is defined for a given optical aperture. The larger the aperture (e.g., 100, 200, 400 mm), the finer the resolution (e.g., 0.5, 0.25, 0.125 arcseconds).

[0016] In addition to optical constraints, the telescope must also meet mechanical constraints in terms of weight and compactness, these constraints being more or less strong depending on the intended application.

[0017] For example, stricter constraints apply in the case of the second application, where the telescope must be able to be integrated into a satellite as easily and economically as possible. In this case, the telescope must have a compromise between its weight, volume, and inertia that is sufficiently advantageous to allow it to be carried on a satellite.

[0018] The telescope's overall size is essentially determined by its aperture diameter (height) and its focal length (length). When the telescope is intended to be mounted on a satellite, its inertia must be limited. This inertia is higher the larger the telescope. More precisely, the inertia is a function of the cube of the telescope's length.

[0019] Thus, a conventional Schmidt telescope, whose length is typically equal to the radius of the mirror, and which was carried on a satellite, would exhibit an inertia that would severely penalize the satellite in terms of electrical and / or fuel consumption, insofar as the satellite is powered for to make it perform micro-propulsions, for example to allow in-orbit shots with long exposures requiring the satellite to be tilted to compensate for its rotation around the earth.

[0020] With regard to the first application, although the telescope's compactness is not a critical constraint, it is also desirable, particularly to improve the telescope's agility during movement, for example, to scan the sky more quickly. Generally, the telescope has an aperture diameter between 5 cm and 80 cm. Regarding the second application, which is more demanding in terms of compactness, it is desirable to reduce the aperture diameter, ideally to values ​​between 20 cm and 30 cm, typically corresponding to the payload of nanosatellites, and with a weight that should not exceed ten kilograms.

[0021] Numerous adaptations of the Schmidt telescope have been proposed to date in the scientific and technical literature. Among all these adaptations, the variants of the sphericity corrector most relevant to the present invention will now be reviewed.

[0022] According to a first variant, the Schmidt plate is replaced by two spherical lenses of opposite powers, thus forming a corrector known as a "Houghton corrector".

[0023] It is recalled that a spherical lens is an optical element made of a vitreous material characterized by a refractive index n and contained between two spherical surfaces, i.e. each having a profile corresponding to that of a sphere with a respective radius of curvature RI, R2 and having a non-zero optical power q> expressed as follows (Eq. 0): <p=l / f =(n-l)(l / Rl -1 / R2) où f’ désigne la longueur focale de la lentille.

[0024] In the Houghton corrector, the two spherical lenses are made of the same glass with refractive index n and have opposite optical powers, so that the cumulative paraxial optical power of the corrector is zero, as in the case of a Schmidt plate.

[0025] The Houghton corrector has the disadvantage that the high individual powers and curvatures of each of the spherical lenses only partially correct spherical aberration and coma within the telescope's reduced field of view and across a limited spectral range. Furthermore, these lenses are not suitable for correcting astigmatism specifically caused by the proximity of the spherical corrector to the mirror. Moreover, depending on the telescope's aperture diameter, these lenses can be bulky and heavy, thus hindering the optimization of both the telescope's size and weight.

[0026] According to a second variant, the Schmidt blade is replaced by a so-called corrector of the "Baker Nunn". This corrector consists of a triplet of three aspherical lenses placed at the center of curvature of the spherical mirror. However, due to the large number of lenses and the significant distance between these lenses and the mirror, this sphericity corrector is also not suitable for reducing the volume and size of the telescope.

[0027] According to a third embodiment, the Schmidt corrector plate is replaced by a doublet of Schmidt corrector plates made of two different types of glass, each plate ashered according to the properties of the glass constituting each plate, and still positioned at the center of curvature of the mirror. However, these are not suitable for reducing the size of the telescope, given the still considerable distance of the plates from the mirror. As described previously, bringing this doublet of achromatic plates closer to the mirror would have the disadvantage of causing optical aberrations.

[0028] The inventor has found that none of the variants of the sphericity corrector described above allows the telescope to provide sufficient optical quality over a wide field of view as well as through extended spectral range and for an axial footprint significantly smaller than the radius of curvature of the main mirror.

[0029] A parameter that can be used to estimate image quality is the average size of the image spot over the entire field of view, for example, approximately 6° for the telescope according to the invention. Hereafter, the field of view (FOV) will refer to the maximum viewing angle of the telescope on either side of its optical axis, expressed in degrees (°), minutes ('), and seconds (") of arc (1° = 60' = 3600").

[0030] Furthermore, having observed that the most efficient Schmidt telescopes have an aperture number N at best equal to 2.2 and that it is not possible to achieve aperture numbers lower than this limit value in particular because of the resulting optical aberrations, the inventor therefore became interested in reducing the aperture number, for a field of view of 6° or more, while ensuring that maximum compactness is optimized.

[0031] By definition, the aperture number N denotes the ratio of the focal length (or distance) F of the telescope to the aperture D of the telescope, such that N = F / D. In practice, the aperture D corresponds to the diameter of the telescope's entrance pupil. The entrance pupil is the limiting optical surface through which light enters the telescope. This aperture number N is often denoted "f / N" and commonly referred to as the "focal ratio."

[0032] Furthermore, the aperture number N determines the amount of light received by a sensor placed at the image plane of the telescope. Thus, this aperture number N also characterizes the speed of image acquisition. Indeed, the higher the number The lower the aperture number (N), the shorter the required exposure time and consequently the faster the acquisition. Conversely, the higher the aperture number (N), the longer the required exposure time and the slower the acquisition.

[0033] None of the prior art variants described above makes it possible to provide the telescope with an aperture number N less than 2, allowing in particular to achieve increased compactness and / or speed of image acquisition, while having sufficient optical quality to detect small space objects, such as space debris or nanosatellites moving at very high speeds, from a ground station or from a satellite in space.

[0034] In this regard, the inventor has observed that a Houghton type sphericity corrector is not suitable, not only because it does not allow sufficient correction of aberrations but also because it does not allow the telescope to achieve a sufficiently low aperture number N, in particular to reduce the size, weight and / or acquisition time of the telescope.

[0035] Indeed, the inventor has observed that by seeking to reduce the aperture number N to values ​​less than 2, the sphericity corrector of the telescope (Schmidt plate or its alternatives described above) is not optimal for the intended application, insofar as it does not allow to fully correct the chromatic aberrations, spherical aberration and its chromatic variations, residual and high-order coma and astigmatism due by the sphericity corrector itself, through the entire field of view of the telescope and over a wide spectral band (e.g. between 450 nm and 900 nm).

[0036] The appearance of these optical aberrations has the disadvantage of degrading the optical quality of the objects imaged by the telescope. Such a deterioration in optical quality is all the more critical when the size of the space objects to be imaged is small. The inventor therefore sought to develop a solution to correct all or part of the residual optical aberrations mentioned above, so as to be able to reduce the number of apertures without degrading image quality.

[0037] Ideally, such a correction aims to ensure that each point of the object to be imaged corresponds in the image provided by the telescope to a single image point identical to the point of the object, resulting in perfect image quality. In practice, the image of a point is not a point but a blotch, notably due to optical aberrations, such as chromatic aberration and higher-order spherical aberrations, coma, astigmatism, which persist in the sphericity correctors of prior art telescopes. More precisely, the optical quality is limited by diffraction, whereby a point is imaged by an Airy disk, but this disk is degraded by optical and field aberrations (e.g., chromaticity, coma, sphericity) introduced by the telescope.

[0038] In other words, one of the objectives of the invention is to effectively limit residual aberrations in the simplest way possible when the aperture number is reduced to values ​​less than 2, i.e., by reducing the size of the spots of each image point as much as possible, so that they can be considered points as closely as possible and thus contribute to increased optical quality of the images provided. The size of these spots must be as small as possible within the limits of diffraction in order to detect space debris of the smallest possible size.

[0039] Although Schmidt telescopes exist with an aperture number less than 3, this number cannot fall below 2.2 without encountering the aforementioned drawbacks. For example, for an aperture number equal to the known limit of 2.2, the Schmidt telescope, regardless of the variant used for the sphericity corrector, does not sufficiently correct residual aberrations to achieve adequate image optical quality, and moreover, the spectral range is too limited (e.g., not allowing the inclusion of, in particular, an IR and / or UV range close to the visible).

[0040] In an article entitled “On the capabilities of survey of telescopes of moderate size” published in The Astronomical Journal, 152:121, 2016 November, V. Yu Terebizh reviews various telescope designs used for ground-based space surveillance. However, none of these designs allows for a reduction in the telescope's aperture number without degrading the optical quality of the images provided, thus enabling the detection of space debris.

[0041] Moreover, these designs are not suitable for carrying a telescope according to the second application at least for the following reasons.

[0042] Some designs proposed by V. Yu Terebizh use Mangin mirrors and large spherical corrective lenses, which pose serious problems for installation on an orbiting satellite. Mangin mirrors are bulky, and the high-power spherical lenses are relatively heavy. The other optical elements described by Terebizh, as well as the structures designed to hold them, are also relatively heavy. Furthermore, due to temperature variations, the thermal sensitivity of such large, heavy corrective lenses combined with the mirror can induce defocusing of the image at the telescope's focal point and a loss of image quality.

[0043] Furthermore, the sunlight received by the telescope generally solarizes the materials, inducing a loss of optical transmission over time directly related to the thickness of these lenses. In this respect, spherical lenses, such as those used in a Houghton corrector, are not suitable.

[0044] Finally, the components are naturally deformed under the effect of their weight (gravity), which biases the measurements and alignments made on the ground before the satellite is launched into space. These deformations are all the more pronounced when the components of the telescope on board a satellite are heavy. In this respect, spherical lenses, such as those used in a Houghton corrector, are not suitable.

[0045] Technical problem

[0046] In view of the above, it is desirable to provide a telescope which makes it possible to overcome at least some of the problems and disadvantages of the aforementioned prior art Schmidt telescopes.

[0047] In particular, a technical problem that the present invention proposes to solve is to reduce the number of apertures of a Schmidt telescope, without deteriorating the optical image quality provided, at best improving it, so that the image quality is high enough to detect space debris and / or nanosatellites (i.e. optical quality close to the limitations due to diffraction) over a large field of view (e.g. greater than 2°, preferably greater than 5°, ideally on the order of 6°) and in a wide optical spectrum (e.g. including the visible range and the near-visible infrared (IR) range and / or the near-visible ultraviolet (UV) range).

[0048] Technical solution

[0049] In response to this technical problem, a compact Schmidt telescope with significantly improved optical performance is therefore proposed. This telescope, configured to form an image in a focal plane, comprises a concave mirror, a sphericity corrector adapted to correct the spherical aberration of the concave mirror, a field corrector comprising an optical entrance surface, the concave mirror, the sphericity corrector and the field corrector being centered on the same optical axis of the telescope.

[0050] The sphericity corrector comprises two aspheric lenses such that: - each aspheric lens comprises two faces, at least one of which is curved and at least one of which has an aspheric profile, - the two aspheric lenses are composed of different optical glass so as to form an achromatic doublet, preferably of the Flint-Crown type - the two aspherical lenses have a central hole centered on the optical axis and in which the field corrector is placed; - the two aspheric lenses are placed between: • a first plane located at a first distance from the focal plane of the telescope as one moves away from the concave mirror; and • a second plane located at a second distance from the optical entrance surface of the field corrector as one approaches the concave mirror, where the first and second distances are equal to 1 / 10 of a distance separating the concave mirror from the focal plane of the telescope;

[0051] In addition, the telescope according to the invention is configured so that it has an aperture number less than or equal to 2, preferably less than or equal to 1.5 or 1.3.

[0052] Optionally, for at least one of the two aspheric lenses, one of the two faces is curved, preferably spherical in shape, and the other of the two faces has an aspherical profile; or - at least one of the two faces is curved and also has an aspherical profile.

[0053] Thus, the two aspheric lenses have low paraxial optical power and limited volume and weight.

[0054] The aspherical nature of the two lenses of the sphericity corrector according to the invention advantageously allows joint correction of chromatic aberrations and aperture aberrations of the telescope, such as spherical aberration and axial chromatic aberration.

[0055] The two aspheric lenses have the effect of creating an aspheric and achromatic optical wavefront at the output sphericity corrector, so that the spherical aberration is corrected over a wider range of wavelengths compared to the case where the lenses were not aspheric.

[0056] Due to their curvature and aspheric nature, the volume and mass of each aspheric lens can be advantageously reduced compared to those of a spherical lens which has two spherical faces.

[0057] Consequently, the paraxial optical power of each of the two aspherical lenses is non-zero, unlike in a Schmidt corrector. Furthermore, this optical power is reduced compared to that of the typical spherical lenses of a Houghton doublet, particularly due to the aspherical profile. Thus, the assembly formed by the two aspherical lenses of the telescope according to the invention makes it possible to overcome much better the optical aberrations encountered with a conventional Schmidt corrector or a conventional Houghton corrector.

[0058] Surprisingly, the two aspheric lenses made of different types of glass correct not only chromatic aberrations, particularly along the optical axis, over the entire focal length of the telescope, but also the chromatic variation of spherical aberration, such that the correction due to the aspheric nature of the lenses is wavelength-invariant. Thus, the com Achromatic combinations of aspheric lenses made from two different types of glass, such as Crown and Flint, add a multiplying effect compared to aspheric lenses made from a single glass. This advantageously allows for a reduction in the number of apertures while broadening the spectral range across which the correction is performed.

[0059] Consequently, the use of two different glasses to form the two aspheric lenses makes the corrections of aberrations, particularly spherical ones, perfectly achromatic, achieved by the set of the two achromatic lenses, at a level of correction never before achieved by the various variants of the Schmidt telescope of the prior art, so that the optical quality of the image provided is improved.

[0060] Unlike an aspheric Schmidt plate, the aspheric wavefront generated by the two aspheric lenses composed of different glasses does not vary with wavelength, thus allowing correction of aberrations and consequently an improvement in optical quality over an extended spectral band compared to the prior art, which can cover the visible and near-visible infrared (IR) and / or near-visible ultraviolet (UV) ranges.

[0061] Such effects cannot be achieved by a prior art Houghton doublet since the lenses of this doublet are spherical and are made of the same optical glass.

[0062] Surprisingly, the inventor found that the selection of different glasses, for example of Crown and Flint type respectively, for the two lenses of the sphericity corrector taken in combination with the aspheric character of the lenses and their small variation in thickness makes it possible in combination to increase the optical image quality in a wide band of optical operating frequencies of the telescope, while correcting the various residual chromatic, spherical and / or coma aberrations, up to higher orders and over a wide field of view.

[0063] According to one principle of the invention, the two lenses thus form a so-called "aspheric-achromatic" doublet of low paraxial optical power, allowing the telescope to achieve a higher resolving power while reducing optical aberrations across a wide spectral range and in an extended field of view.

[0064] Consequently, the combination of these features makes it possible to configure the telescope to achieve such an aperture number N as less than 2, or even less than or equal to 1.3, which is significantly reduced compared to the limiting value of 2.2 obtained for the highest-performing Schmidt telescopes of the prior art. Thus, the telescope according to the invention can achieve a significantly reduced compactness and / or acquisition speed compared to the prior art.

[0065] Surprisingly, the aspheric nature of the lenses contributes advantageously to correcting coma at higher orders as well as astigmatism, in com combination with the field corrector. In other words, coma correction is carried out jointly by the field corrector and the sphericity corrector to achieve a level of correction never before achieved in earlier Schmidt telescopes, especially those using a Houghton corrector.

[0066] Co-locating the aspheric-achromatic doublet of the sphericity corrector and the field corrector near the focal plane P of the telescope allows for the correction of optical aberrations as described above, including field aberrations (e.g., coma, astigmatism, distortion) thanks in particular to the field corrector. This positioning of the sphericity corrector and the field corrector makes it advantageous to make the telescope even more compact and with low inertia, which is particularly beneficial when the telescope is placed in orbit according to the second application. Although the constraints of compactness and inertia are less critical in the case of the first application, such positioning would also be beneficial for integrating the telescope into a ground station, which would then be more agile in scanning the sky.

[0067] The combination of the distinctive features presented above advantageously allows the Schmidt telescope to achieve an aperture number less than 2, preferably less than 1.5, or even less than 1.3, while increasing its optical quality sufficiently to detect small space objects over a wide field of view, e.g., on the order of 6°, and over an extended spectral range that may include part of the UV and / or IR spectrum. Depending on the intended application, the aperture diameter D and the focal length F of the telescope can be freely fixed so that the aperture number N=F / D is less than 2, 1.5, or 1.3.

[0068] A major difficulty which has been successfully overcome by the inventor is having managed to find a solution that precisely meets all of these aforementioned optical and mechanical constraints.

[0069] Optionally, each aspheric lens has a maximum thickness variation between a center and an edge of said lens, which is non-zero and less than 5%, preferably non-zero and less than 3% of a diameter of said lens. More particularly, this thickness variation is between 1% and 5% of the diameter of the lens, preferably between 1% and 3% of said diameter.

[0070] Due to this constrained maximum thickness variation, the two aspherical lenses of the sphericity corrector have low paraxial optical power. This has the effect, in particular, of providing correction of chromatic aberration, spherical aberration, and coma near the optical axis at higher orders, thus improving the optical quality of the image provided.

[0071] This thickness constraint advantageously extends the field of view of the telescope through which the corrections are made, unlike the case of Schmidt correctors, which lack optical power, or two Houghton-type spherical lenses, are examples of this. Indeed, as described previously, in the case of a Houghton doublet, each lens has high individual optical power, resulting in residual aberrations.

[0072] Thus, the residual paraxial optical power of the two aspheric lenses introduced by limiting their variation in thickness makes it possible to increase the quality of the image formed by the telescope in its focal plane, through the entirety of its field of view, while avoiding the introduction of lenses that are too thick and therefore too heavy in order to optimize the compactness and weight of the telescope.

[0073] Surprisingly, the low paraxial optical power of the two lenses as described above, combined with their aspheric nature, allows for the joint correction of axial chromatic aberration, spherical aberration, and coma. Combined with the field corrector, these two aspheric lenses achieve remarkable total optical correction.

[0074] In particular, the inventor has found that this combination of characteristics (i.e. asphericity and low variation in lens thickness) has the effect of substantially improving mainly chromatic and aperture optical aberrations (i.e. spherical aberrations and axial chromaticism) to higher orders, in particular at aperture numbers below 1.5 and more particularly below 1.3.

[0075] Such effects cannot be obtained by known sphericity correctors of the prior art, in particular those using one or more Schmidt plates (aspherized or not) or Houghton lenses.

[0076] Optionally, the telescope according to the invention may further comprise all or part of the following features, taken alone or in combination: - the aperture number N=F / D is not equal to 1.31, more specifically the aperture diameter D is not equal to 190 mm and the focal length F is not equal to 250 mm; and - the field corrector includes a set of at least two lenses configured to convert a curved optical field at the output of the concave mirror into a planar optical field and to correct the coma aberration and astigmatism aberration of the telescope; - the field corrector further includes at least one additional lens configured to allow distortion correction; - at least one lens of the field corrector is aspheric; - the lenses of the field corrector are made of the same optical glass; - the concave mirror is spherical; - the concave mirror is aspherical; - the telescope is configured to operate in a visible spectral range and an infrared spectral range and / or an ultraviolet range, depending on the intended application; - the telescope also includes an optical sensor positioned in the focal plane of the telescope opposite the field corrector.

[0077] The invention also relates to a satellite comprising a telescope according to the invention as described above, in particular for the detection of space debris and / or nanosatellites from space according to the second application.

[0078] The invention also relates to a ground station comprising a telescope according to the invention as described above, in particular for the detection of space debris and / or nanosatellites from Earth according to the first application.

[0079] The invention also relates to a method for detecting at least one space object, such as space debris or a nanosatellite, comprising the following steps: - provision of a telescope according to the invention in a satellite or in a ground station; - acquisition of at least one image by the telescope; - detection of said at least one spatial object by processing said at least one image.

[0080] The invention will be better understood with the aid of the following description, given solely by way of example and made with reference to the accompanying drawings in which:

[0081] [Fig-1] [Fig.1] is a schematic view of a telescope according to a first mode of realization of the invention;

[0082] [Fig.2] [Fig.2] is a schematic view of the implemented sphericity corrector in the telescope of [Fig.1];

[0083] [Fig.3] [Fig.3] schematically illustrates an aspheric lens;

[0084] [Fig.4] [Fig.4] is a schematic view of a telescope according to a second mode of realization of the invention;

[0085] [Fig. 5] [Fig. 5] schematically illustrates a satellite in which is onboard the telescope according to [Fig.4];

[0086] [Fig.6] [Fig.6] illustrates in flowchart form a method for detecting space debris and / or nanosatellite debris; and

[0087] [Fig.7] [Fig.7] schematically illustrates a ground station comprising the telescope according to [Fig.4].

[0088] A telescope according to a first embodiment of the invention will now be described with reference to [Fig.1].

[0089] The telescope 1 comprises a concave mirror 10, a sphericity corrector 12 adapted to correct at least some spherical optical aberrations due to the concave mirror 10, and a field corrector 14 (here called a flattener) adapted to convert the reflected field by the concave mirror 10 in a planar field.

[0090] Each of the constituent elements of the telescope 1, including the mirror 10, the sphericity corrector 12 and the field corrector 14, is centered on the optical axis O of the telescope 1, so that all these elements are aligned along this same optical axis O.

[0091] Thus, the structure of telescope 1 is simple to implement and offers great ease of optical alignment. This configuration is particularly suitable for the second application since it has a good propensity to remain stable and robust in the face of mechanical disturbances that may occur during the launch of the satellite as well as in the face of thermal conditions when the satellite is in orbit.

[0092] Whatever the intended application, the telescope 1 is particularly well suited to form images of debris from satellites, nanosatellites or any other space objects projected or visualized in a focal plane P located upstream of the sphericity corrector 12 with respect to the incident rays.

[0093] In [Fig. 1], the direction of ray propagation is represented by dashed arrows. For each optical element, the terms "input" and "output" refer to the direction of ray propagation.

[0094] By definition, the focal plane P of the telescope 1 is the plane in which an image is formed to be captured at the telescope's output. In practice, the focal plane P is materialized by the presence of an optical sensor to capture such an image, such as a camera as illustrated in the embodiment of [Fig. 4].

[0095] By "concave mirror," it will be understood that the mirror has a curved and hollow reflecting surface as seen from the sphericity corrector 12, such that the incident rays from the sphericity corrector 12 are reflected towards the latter. In other words, the concave mirror 10 makes the incident rays converge towards the sphericity corrector 12, towards its optical focus.

[0096] In the present example, the concave mirror 10 is a spherical mirror, that is to say that its reflecting (or reflective) surface forms a spherical cap characterized by a radius of curvature (not shown).

[0097] However, in other embodiments or variants (not illustrated), the concave mirror may be an aspherical mirror, that is to say, its reflecting surface is curved but does not follow the shape of a sphere. For example, this is the case for mirrors whose reflecting surface shape is selected from among a hyperboloid, a paraboloid, an ellipsoid, or a vertex flattened ellipsoid.

[0098] In one embodiment (not shown), the surface of the concave mirror 10 is aspherized or deformed according to optical powers increasing with the distance of the point on the surface from the optical axis O. The term "aspherized" is used to To designate any optical surface exhibiting an aspheric profile, i.e., a curved surface deformed so that it does not conform to the contour of a sphere. A more precise definition of the aspheric nature of a surface will be given below with reference to [Fig. 3].

[0099] Such aspheric deformation or aspherization of an initially spherical concave mirror can thus be advantageously used to obtain higher-quality images by incrementally correcting optical aberrations to higher orders within the scope of the present invention. In particular, the inventor has demonstrated that the use of an aspheric (or aspherized) mirror improves the image quality of the telescope by a factor of at least 2 compared to the case where a spherical mirror is used.

[0100] The sphericity corrector 12 is designed to correct spherical aberrations due to the spherical mirror 10, taking into account, in particular, the size of the telescope's entrance pupil. For this purpose, it comprises two lenses 12A, 12B, namely a first lens 12A serving as the entrance pupil of the telescope 1, through which the incident light rays enter, followed by a second lens 12B.

[0101] As illustrated in [Fig. 1], the lenses 12A, 12B are cut (for example by drilling) in their center along the optical axis O, so as to form a central hole 120 centered on the optical axis O and adapted to receive the field corrector 14. The field corrector 14 can be fixed to the wall of the central hole 120 or to the external structure of the telescope 1 by means of a "spider" type attachment device or any other equivalent attachment device. These two lenses 12A, 12B will be described in more detail below with reference to [Fig. 2].

[0102] According to the example in [Fig.1], the field corrector 14 comprises a set of six lenses 14A, 14B, 14C, 14D, 14E, 14F configured to convert a curved optical field supplied at the output of the concave mirror 10 into a planar optical field and corrected for one or more optical field aberrations selected from among coma, astigmatism, distortion, optical field curvature.

[0103] In the present example, a first piano-spherical lens 14A, located at the input of the field corrector 14, is selected as diverging. It is followed by a second spherical lens 14B, which is converging. The assembly formed by the first 14A and second 14B spherical lenses corrects the coma, astigmatism, and curvature of the optical field remaining at the output of the concave mirror 10.

[0104] A third spherical 14C lens is provided to improve the correction of higher-order geometric aberrations as well as chromatic aberrations and also to allow the correction of distortion.

[0105] A fourth 14D spherical lens is provided to further improve the correction of chromatic aberrations and higher-order geometric aberrations and obtain image spots on the order of a few pm.

[0106] A fifth spherical 14E lens is provided to achieve very high quality and uniform aberration correction over a field of view up to 6° for a total optical aperture of 230 mm and a detector with pixels of only 4.5 pm.

[0107] A sixth 14F spherical convergence lens is provided to further increase image quality in the field, in the case of spherical lenses only.

[0108] In the present example, the set of six 14A-14F lenses significantly improves the image quality by correcting field aberrations (coma, achromatic aberration, distortion, curvature). These 14A-14F lenses are all spherical and made of the same optical glass to simplify optical manufacturing and standardize the chromatic variation of residual aberrations. Consequently, the telescope structure is simplified and its manufacturing costs are reduced.

[0109] For a given number of lenses, it will be possible to play on different parameters of the lenses, such as the spherical character (radius of curvature) or aspheric character (rate of asphericity), the choice of the glass constituting it and its thickness to further improve the corrections of optical aberrations.

[0110] According to other embodiments or variants (not illustrated), all or part of the lenses of the field corrector 14 may be aspheric and / or composed of different types of glass. Thus, the number and configuration of the lenses (i.e., convergent / divergent nature, spherical / aspheric shape, selection of glass types) of the field corrector 14 may be adjusted according to the aberrations and distortions to be corrected and the detection performance to be achieved, based on the general knowledge of a person skilled in the art. For example, the field corrector 14 may comprise only spherical lenses, or only aspheric lenses, or a mixture of spherical and aspheric lenses.

[0111] Once the configurations of the field corrector 14 and the sphericity corrector 12 are selected, the focal length F of the telescope 1 and the position of the focal plane P of the telescope 1 are fixed accordingly. The field corrector 14 is then arranged through the central hole 120 of the sphericity corrector 12, for example, arranged at least partially inside the lens doublet 12A, 12B, so as to limit the volume of the telescope to achieve greater compactness. The positioning of the sphericity corrector 12 can then be adjusted according to the focal plane P of the telescope and the field corrector 14 as described below.

[0112] The field corrector 14 has an optical entrance surface 14A.1 corresponding to the entrance surface of the first lens 14A, through which the light from the concave mirror 10 enters the field corrector 14 and presents an exit surface from which light exits in the direction of the focal plane P of the telescope 1.

[0113] Since at least one of the lens surfaces is curved (i.e. non-planar), a plane tangent to these surfaces and perpendicular to the optical axis O is defined to express distances with respect to these surfaces as illustrated in [Fig. 1], in order to define the positioning of the sphericity corrector 12.

[0114] Thus, we define a plane Tl tangent to the input surface 12A.1 of the first lens 12A, a plane T2 tangent to the output surface 12B.2 of the second lens 12B of the sphericity corrector 12, a plane T3 tangent to the input surface 14A.1 of the first lens 14A of the field corrector 14. Thus, the planes Tl, T2, T3 are all perpendicular to the optical axis O of the telescope 1 and consequently parallel to each other.

[0115] The sphericity corrector 12 is positioned so that it lies between a first extreme plane P' and a second extreme plane T4, these two planes also being perpendicular to the optical axis O. The first extreme plane P' is located at a first distance ei from the focal plane P when moving away from the concave mirror 10. The second extreme plane T4 is located at a second distance e2 from the optical entrance surface 14A.1 of the field corrector 14 when moving towards the concave mirror 10. Preferably, the first ei and second e2 distances are equal to 1 / 10 of the focal length F of the complete telescope separating the concave mirror 10 from the focal plane P of the telescope 1.

[0116] In the ideal case where the manufacture of the telescope and the positioning of its constituent elements are perfectly executed, the first ei and second e2 distances are zero, i.e. ei=e2=0. In this case, the sphericity corrector 12 is entirely contained between the focal plane P of the telescope 1 and the optical entrance surface 14A.1 of the field corrector 14. This condition ei=e2=0 makes it possible to have no optical blocking of rays and to benefit from an arrangement of an image sensor (or photodetector) at the focal plane which can extend further than the sensitive surface of the sensor.

[0117] However, in practice, it is necessary to provide margins of adjustment thanks to the first ei and second e2 distances, so that the two lenses 12A, 12B of the sphericity corrector 12 are fully contained in a space E contained between the two extreme planes P', T4 (when e^0 and e2^0) and more particularly between the planes P and T3 (when ei=e2=0).

[0118] In other words, to position the sphericity corrector 12, - the entrance surface 12A.1 of the first lens 12A can be separated from the concave mirror 10 by a distance xi without the plane Tl exceeding the plane P' and more particularly the focal plane P (i.e., separation by a distance maximum Xi+eO; and / or - the exit surface 12B.2 of the second lens 12B can be brought closer to the concave mirror 10, by a distance x2 without the plane T2 exceeding the plane T4 and more particularly the plane T3 (i.e. approaching a maximum distance x2+e2).

[0119] The sphericity corrector 12 of the telescope 1 will now be described in more detail with reference to [Fig.2].

[0120] The two lenses 12A, 12B of the sphericity corrector 12 are arranged opposite each other, perpendicular to the optical axis O of the telescope 1 and centered on this same optical axis O. For example, the two lenses are joined together as a single unit to ensure greater stability. Preferably, they are separated by a minimum distance of approximately 1 mm to minimize the risk of impact during assembly.

[0121] In the present example, the two lenses 12A, 12B have the same diameter D defining the size of the entrance pupil of the telescope 1. In the present example, this diameter D corresponds to the diameter of the aperture of the telescope.

[0122] In other embodiments (not illustrated), the lenses 12A, 12B may have distinct diameters, in which case the entrance pupil of the telescope will be defined by the lens with the smallest diameter.

[0123] According to a specific feature of the invention, the two lenses 12A, 12B are aspheric lenses of low optical power forming an achromatic doublet, called an "asphero-achromatic doublet" which will now be described.

[0124] By definition, a lens is an optical component exhibiting a non-zero variation in thickness between its center and its edges (or extremities). Therefore, plates with flat, parallel faces, such as Schmidt lenses, are excluded from this definition, as are menisci whose two faces are curved but whose thickness varies almost nothing between their center and extremities. The term "curved" or "bent" refers to a surface that is not planar (or flat).

[0125] A "spherical" surface has a contour corresponding to that of a sphere, a sector, or a portion of a sphere, whose curvature is defined by a constant radius of curvature. Thus, a spherical surface has a uniform curvature that can be convex or concave. A spherical surface is a particular example of a curved shape, in the case where the radius of curvature is constant. In general, an aspherical surface is curved but not spherical.

[0126] In this regard, it is necessary to clearly distinguish a lens from an optical plate (whose two faces are planar) and in particular an aspheric plate of which at least one of the faces is aspherized (i.e. deformed according to an aspheric profile).

[0127] In general, the aspheric profile of a surface of an optical component, such

[0128]

[0129]

[0130]

[0131]

[0132]

[0133]

[0134]

[0135]

[0136] A lens or a blade is obtained by applying an aspheric coating process to a surface of the component. This surface may initially be curved or planar. Thus, the aspheric profile resulting from the aspheric coating process is superimposed on a principal curved or planar profile of the surface being coated. For example, in the case of a lens, at least one face of which is coated, the aspheric profile should be understood by those skilled in the art as being superimposed on a principal curved or planar profile. Within the framework of the present invention, the two lenses of the sphericity corrector 12 may be selected, for example, from the following types: - a plano-convex lens, that is to say comprising a planar face and a curved face of convex shape, at least one of these faces having an aspheric profile; - a plano-concave lens, that is to say comprising a planar face and a curved face of concave shape, at least one of these faces having an aspherical profile; - a bi-curve lens, that is to say comprising two curved faces, at least one of which has an aspheric profile; for example one of the faces is spherical, concave or convex and the other face is aspheric. The two lenses selected are not necessarily of the same type. For example, a plano-convex lens and a bi-curve lens can be combined within the sphericity corrector; or a plano-concave lens and a bi-curve lens, or more generally any possible combination of the aforementioned lens types. On the other hand, we will exclude any optical component that does not fall within the definition of a lens as provided above, for example a meniscus-shaped lens or a parallel-sided optical plate (i.e. Schmidt plate). Hereafter, the term "aspheric lens" will refer to any lens (as defined above) comprising at least one aspheric (or aspherized) surface, that is, one exhibiting an aspheric profile. The lens surface can be aspherized using known aspherization processes (e.g., machining, polishing, etc.). The aspheric profile will now be described in more detail with reference to [Fig. 3]. Figure 3 schematically illustrates an aspheric lens in a two-dimensional orthonormal coordinate system (X,Y), where X corresponds to the optical axis O of the lens (or telescope). One of the lens faces (e.g., the entrance face S) is configured according to an aspheric profile generally defined by the following equation (Eq. 1): z(r) - ——7==7 + ILr 2 ) J - where z(r) defines, along the X-axis, the distance between the plane Tv tangent to the vertex V of the lens (perpendicular to the optical axis O) and the face S of the lens as a function of the radial distance r along the Y-axis with respect to the optical axis O, - R denotes the radius of curvature at the vertex V of the lens (corresponding to the radius of curvature of a perfect sphere coinciding with the surface of the sphere at its vertex V), - k denotes a conicity constant, such that k>0 for an oblate ellipse, -l <k<0 pour une ellipse prolate, k="-l" sphère, parabole, k<-l hyperbole, - {A„} denote the coefficients of a polynomial asphericity correction corresponding to higher order deformation terms n in even powers of r, where n is a natural number greater than 2 (i.e. asphericity terms An .(r2)n ; e.g. for n=2, A2.r4, for n=3, A2.r6, etc).

[0137] Within the framework of the present invention, it is assumed that the taper constant k is non-zero and / or that the asphericity coefficients {An] are non-null, so as to exclude any spherical profile from the definition of an aspherical profile.

[0138] Thus, any lens, of which at least one face (i.e. entrance face and / or exit face) is shaped according to a profile defined by equation 1 above, is an aspheric lens.

[0139] The differences in thickness between the center and the edges of the aspheric lens illustrated in [Fig. 3] have been exaggerated compared to reality, solely to clearly visualize the aspheric nature of the lens on one of its faces. In particular, it is clearly observed that, according to the aspheric profile, the surface S of the aspheric lens deviates from that of the profile of a spherical lens as indicated by the dashed lines.

[0140] By adjusting the parameters defined by equation 1 above, it is possible to reduce the amount of glass used to form the aspheric lens, since the radius of curvature is gradually reduced unlike the case of the spherical lens for which the radius of curvature remains constant over the entire surface for a given face.

[0141] An example of an aspheric lens implemented in the sphericity corrector 12 will now be described in more detail with reference to [Fig.2].

[0142] According to the example in [Fig.2], the surfaces 12A.1, 12A.2, 12B.1, 12B.2 of the aspheric lenses 12A, 12B are all curved in the direction of the focal plane P of the telescope 1. In other embodiments (not illustrated), other orientations of these curvatures may be envisaged, for example with different orientations for at least two surfaces.

[0143] In the present example, each of the two lenses 12A, 12B is asphericized only on one of its two faces, e.g., on the exit face 12A.2 of the first lens 12A and on the entrance face 12B.1 of the second lens 12B. In other words, only the surfaces 12A.2, 12B.1 have a profile (or contour) according to equation 1. For each lens 12A, 12B, the aspheric profile can be adjusted in such a way specific by modifying the parameters of equation 1, depending on the performance to be achieved based on the nature of the aberrations to be corrected.

[0144] In general, the two aspheric lenses 12A, 12B of the sphericity corrector 12 each have an aspheric profile on their input surface 12A.1, 12B.1 and / or on their output surface 12A.2, 12B.2, this profile being able to be differentiated for each of these surfaces (i.e., by using distinct parameters in equation 1 above). It suffices that each of the two lenses be aspherized on one of its two faces.

[0145] The deformations caused by the aspheric surfaces of the lenses 12A, 12B cause the transmitted light wave to compensate for the spherical aberration caused by the concave mirror 10.

[0146] As illustrated in [Fig.2], each aspheric lens 12A, 12B has a thickness e ib, e2b at its ends bh b2 respectively (i.e. at a maximum distance from the optical axis O) and a different thickness e'ic, e'2c in its center cb c2, i.e. proximal thickness along the optical axis O before the cutting of its central area to form the hole 120 intended to receive the field corrector described with reference to [Fig.1].

[0147] The portions of the aspheric lenses 12A, 12B cut to form the hole 120 appear as dashed lines in [Fig.2]. Thus, each lens 12A, 12B has a thickness eic, e2c near its center cb c2ji.e. along the optical axis O after cutting its central area to form the hole 120.

[0148] In the present example, one of the lenses is thicker at its center than at its ends so that it is converging, while conversely, the other lens is thicker at its ends than at its center so that it is diverging. The order of the two lenses is indifferent to the first order.

[0149] According to a particular feature of the invention, each of the two aspheric lenses 12A, 12B has a maximum thickness variation Aeb Ae2 between its edges bb b2 and its center cb c2 which is non-zero and less than 5% of the diameter D of the lens, more particularly less than 3.33% of D or preferably less than 3% of the diameter D, so that each aspheric lens 12A, 12B has a low paraxial optical power. For example, this thickness variation is between 1% and 5%, or between 1% and 3.33%, or between 1% and 3% of the diameter D.

[0150] For example, this thickness variation is determined, for each lens, without taking into account the aspherization of the lens, that is to say by abstracting from the aspheric profile that the face or faces of the lens may present, as described previously with reference to [Fig.3] according to equation 1. In other words, it is the thickness variation before aspherizing one or both faces of the lens to make it aspheric by application of any type of known aspherization process.

[0151] This maximum thickness variation, excluding aspherization, can be adjusted according to the aspherization profile of the lenses. Indeed, the lower the aperture number N of the telescope, particularly to values ​​close to 1.3 and / or below 1.3, the more significant the aspherization of the sphericity corrector lenses becomes and influences considerations regarding lens thickness variation.

[0152] In other words, according to an example of the invention, two support elements are used to form an aspheric lens (i.e., lenses) whose thickness variations between the center and the edge are less than 5% of the diameter of these elements. Thus, the invention differs from known prior art aspheric lens support elements, such as a Schmidt corrector plate or any other support whose entrance and exit faces are parallel to each other. This thickness limitation on lenses advantageously allows for a simultaneous reduction in the weight of the telescope and the residual aberrations that would be due to excessive optical power of the lenses.

[0153] In the present example, the two aspheric lenses 12A, 12B obey the same condition of maximum thickness variation expressed by the following equation: 0.01xD < Ae < 0.05xD (Eq. 2), where D denotes the diameter of the entrance pupil (corresponding to the diameter of each lens), Ae corresponds to Aei which denotes the modulus of the thickness difference between the center Ci and one of the edges bid of the first lens 12A such that Aei=leic- eibl or Ae corresponds to Ae2 which denotes the modulus of the thickness difference between the center c2 and one of the edges b2 of the second lens 12B such that Ae2=le2c- e2bl. In other words, the thickness of each lens 12A, 12B varies at most, between its center and one of its edges, by a value between 0.01xD and 0.05xD. If the aspheric lenses 12A, 12B have different diameters, designated Db and D2 respectively, the aspheric lenses 12A, 12B will obey the following relationships respectively: 0.01xDi <aei< 0,05xdi (eq.3) et 0,01xd2 <ae2< 0,05xd2 (eq. 4). en pratique, les deux lentilles ont des diamètres sensiblement identiques, en particulier, di="D2=D."

[0154] In certain embodiments (not illustrated), the two aspheric lenses 12A, 12B may obey different maximum thickness variation conditions, provided that these thickness variations remain less than or equal to 0.05xD (or 0.05xDi or 0.05xD² in the case where Di^D²), in particular between 0.01xD and 0.05xD. Thus, for example, the two lenses 12A, 12B may respectively satisfy the following equations: 0.01xD <aei< 0,05xd avec aei="leic-" eibl (eq. 5) et 0,01xd <ae2< 0,03xd ae2="le2c-" e2bl (eq.6). autrement dit, la variation d’épaisseur maximale de chaque lentille asphérique 12a, 12b pourra être limitée par un seuil supérieur différent pour (e.g. l’une des lentilles l’autre), dès lors que ce reste inférieur ou égal à 0,05xd. toutefois, lorsque les ont le même diamètre d, il n’est pas nécessaire différentier upper thresholds. In other implementation examples, the coefficient 0.01 of the lower bound 0.0 IxD can be reduced as long as it does not reach the value 0.

[0155] For example, at least one of the aspheric lenses is thickened in its center, so that it has a greater thickness in its center than at its ends, like a converging lens, as illustrated in [Fig.2], which has the effect of providing it with a paraxial light convergence power.

[0156] In other embodiments, at least one of the aspheric lenses is thickened at its ends, so that it has a greater thickness at its ends than at its center, like a diverging lens.

[0157] In all cases, the difference in thickness gives each aspheric lens 12A, 12B a low paraxial optical power, compared to a doublet of spherical lenses such as those used in the Houghton corrector or one or two Schmidt plates.

[0158] Thus, limiting the maximum thickness variation as described above makes it possible to limit the paraxial optical power of the aspheric lenses, which has the effect of improving quality across the entire field of view at the exit of the concave mirror 10. In particular, the thickness variation ranges described above make it possible to ensure that the corrections made by the doublet of aspheric lenses 12A, 12B are made over a large field of view, especially compared to the case where a Houghton type corrector would be used.

[0159] From a mechanical point of view, limiting the maximum thickness variation between the edges and the center of the aspheric lenses 12A, 12B to less than 5%, or preferably to 3.33% or 3% of their diameter D, has the effect of reducing the weight and inertia of the telescope, which is particularly advantageous for mounting it in an orbiting satellite according to the second application.

[0160] From an optical point of view, the doublet of aspheric lenses 12A, 12B with low paraxial optical power, as described above, makes it possible in particular to improve the correction of chromatic aberration near the optical axis, spherical aberration, and coma, especially compared to the case of a simple Schmidt corrector plate or a doublet of two Houghton-type spherical lenses. This improvement is mainly due to the aspherization of the surfaces and the low paraxial optical power introduced by the lenses 12A, 12B, the maximum thickness variation of which is limited to 5% of the diameter D, preferably to 3.33 or 3% of D as described above.

[0161] Thus, by combining the low paraxial optical power and the aspheric profile of the two lenses 12A, 12B, the inventor has demonstrated that excellent correction of optical aberrations, primarily chromatic and aperture aberrations, can be achieved, unsurpassed by prior art telescopes, particularly at higher optical orders, thereby increasing image quality through the entire field of view of the telescope while allowing a reduced aperture number N to reach 1.5 or even 1.3.

[0162] Such a reduction in the aperture number is significant compared to the prior art limit of 2.2. This reduction not only makes the telescope more compact but also significantly increases the image acquisition speed. In particular, an aperture number N reduced to 1.5 allows the incident light to be concentrated onto a significantly smaller image spot, thus improving resolution and increasing the telescope's detectivity, compared to the best prior art Schmidt telescopes for which the aperture number is limited to 2.2. This is particularly advantageous for detecting even smaller space objects without degrading the optical quality of the images produced.

[0163] In particular, the inventor has found that the asphericity of the lenses of the sphericity corrector taken in combination with the condition of variation of thickness of these lenses according to the invention as described above has the effect of correcting a chromatic variation of the spherical aberration as well as a component of coma and residual astigmatism, which combine with the corrections made by the field corrector.

[0164] When the sphericity corrector is placed between the P' and T4 planes as described previously with reference to [Fig.1], this combination of features has the effect of correcting the residual spherical, coma, and astigmatism aberrations of the telescope to a level never before achieved by the prior art.

[0165] The two aspheric lenses 12A, 12B are further made of two different glasses selected such that they exhibit complementary refractive index dispersions as a function of wavelength. Thus, the two lenses 12A, 12B form an "aspheric-achromatic" doublet which corrects the spherical aberration caused by the concave mirror 10, while reducing chromatic and coma aberrations, regardless of the wavelength of the incident light, or at least over a wide spectral range.

[0166] The inventor has observed that the aspheric character of lenses 12A, 12B taken in combination with the use of different glasses provides a correction of spherical aberrations which does not vary or varies negligibly as a function of the wavelength of the incident light over a relatively wide spectral range compared to Schmidt telescopes of the prior art.

[0167] This broad spectrum correction is all the more advantageous as the telescope is intended to collect and concentrate light over an extended spectral range to obtain the best possible detection efficiency.

[0168] In practice, to form this aspheric-achromatic doublet, the dispersion profile of The refractive index of the glass and the aspheric profile of the aspheric lenses 12A, 12B can be selected so that the doublet corrects the spherical aberration of the concave mirror 10 with the minimum of chromatic variation.

[0169] Preferably, one of the aspheric lenses is selected as a "Flint" type, i.e., exhibiting a high dispersion of refractive index as a function of wavelength, and the other aspheric lens is selected as a "Crown" type, i.e., exhibiting a low dispersion of refractive index. More specifically, a "Crown" type glass has an Abbe number greater than 55 and a low refractive index, indicating low chromatic dispersion, while a "Flint" type glass has an Abbe number less than 50 and a relatively high refractive index.

[0170] For example, the first aspheric lens 12A is made of the glass referenced BK7 having an Abbe number v1=64, while the second aspheric lens 12B is made of the glass referenced F2 having an Abbe number v2=32.

[0171] Thus, the selection of these two glasses makes it possible to significantly broaden the spectral range in which telescope 1 can operate, in particular in the visible optics range (e.g. between 450 nm and 750 nm) and in an infrared range close to the visible (e.g. between 475 nm and 900 nm) and / or in an ultraviolet (UV) range close to the visible (i.e. below 450 nm, e.g. between 200 nm and 370 nm).

[0172] The wider the spectral range of operation of the telescope, the more photons the telescope is able to capture, which also helps to increase the detectivity of the telescope, in particular to enable the detection of even smaller space objects.

[0173] The low paraxial optical power of the two aspherical lenses resulting from the thickness condition described above makes it possible to act on the other aberrations of axial (or lateral) chromatism, of coma, in combination with the lenses of the field corrector in order to improve the correction as already described above.

[0174] Returning to [Fig. 1], the telescope 1, according to a specific feature of the invention, has an aperture number N=F / D less than or equal to 2, preferably less than or equal to 1.30, where F denotes the focal length of the telescope and D denotes the diameter of the telescope's aperture. In this example, the diameter of the aperture D is the diameter of the first aspheric lens 12A serving as the entrance pupil of the telescope 1. For example, the first 12A and second 12B aspheric lenses have the same diameter D.

[0175] Thanks to its aspheric-achromatic doublet, the telescope according to the invention 1, having an aperture number less than 2, preferably less than 1.5, ideally less than 1.3, is capable of achieving an image quality far superior to that which state-of-the-art telescopes could achieve, if they were configured with such a number of openings.

[0176] For example, the telescope 1 according to the invention can be configured to achieve an aperture number less than 1.7, which advantageously allows light to converge into small pixels, provided that the image quality corresponds to the size of said pixels.

[0177] It is precisely the aspheric-achromatic doublet of the sphericity corrector 12 which makes it possible to achieve such a low aperture number, despite its colocalization with the field corrector 14 which, due to its position at the focal plane, does not allow the advantages of a pupil position at the center of curvature of the mirror as is the case for the conventional Schmidt telescope.

[0178] The telescope 1 according to the invention also makes it possible to contain an extended image field, ideally with an angle of about 6°, on a two-dimensional sensor placed in the focal plane P of the telescope, for example of the latest generation CCD or CMOS type available on the market.

[0179] In doing so, an aperture number less than 1.5, preferably less than 1.3, also reduces the overall volume and weight of the telescope and consequently its inertia. This is particularly advantageous, especially in the case of the second application, in order to optimize the energy consumption of the satellite carrying the telescope, as required for nanosatellites or "cubesats". Thus, the telescope according to the invention offers an excellent compromise between its size, weight, and optical power, through a reduced SWaP (Size, Weight, and Power) factor compared to prior art telescopes.

[0180] For a telescope configuration conforming to that of [Fig. 1], the inventor has demonstrated that the aperture diameter D (or entrance pupil of the telescope) can be advantageously enlarged to 400 mm, 600 mm, 800 mm, or even to values ​​greater than 800 mm, while preserving a field of view greater than 6° and an image quality compatible with opto-electronic sensors, for example of the large format CCD or CMOS type used in commercially available cameras.

[0181] According to the various designs set out in the aforementioned Terebizh article, going up to such telescope aperture diameter values ​​would lead to very high mass Mangin-type lenses or mirrors, making the concept virtually unfeasible in practice for integrating the telescope into a satellite.

[0182] The present invention retains, however, all its potential for image quality and feasibility in large apertures (e.g., aperture diameter of the order of 800 mm) due to the clever design of the sphericity corrector based on aspherical lenses having a small variation in thickness between the center and the edge and made of different glasses (cf. aspheric-achromatic doublet as described above).

[0183] Thus, the telescope according to the invention makes it possible to increase the detectability of objects of small size and / or low brightness, particularly due to their small size, such as space debris or nanosatellites.

[0184] For example, a telescope configured according to the example in [Fig. 1] is capable of capturing space images in a wide field of view with a field of view typically between 3° and 8°, in particular 6°. Such a field of view allows the telescope to explore the geostationary arc from a low Earth orbit (LEO) as quickly as possible. The image quality obtained in the focal plane P of the telescope is near-perfect, with an image spot diameter of less than 4 pm, in the wavelength range between 475 nm and 900 nm, with residual distortion of less than 0.005%, for an aperture diameter of 230 mm and a focal length of 311 mm, i.e., an aperture number N=F / D=1.35.

[0185] Advantageously, the telescope is configured to operate in the spectral range of visible optics, for example between 380 nm and 780 nm. This spectral range is the most effective for detecting light scattered by satellites and satellite debris illuminated by sunlight. Thus, the optical components of the telescope 1, in particular the asphero-chromatic lens doublet 12A-12B of the sphericity corrector 12 and the lenses 14A-14F of the field corrector 14, are made of a material suitable for the visible optics range (e.g., capable of transmitting light at wavelengths between 380 nm and 780 nm).

[0186] In other embodiments, the optical components of the telescope according to the invention may be selected to operate in the near-visible infrared spectral range (e.g., at wavelengths strictly less than 2.5 pm), the mid-infrared (i.e., at wavelengths between 3 pm and 5 pm), the thermal infrared (i.e., at wavelengths between 8 pm and 12 pm), or even longer wavelengths. In this case, refractive elements made of glass adapted to operate in the desired wavelength range will be used. The telescope according to the present invention is of particular interest for compact infrared imagers, insofar as it has a low aperture number N=F / D, typically less than 1.5, as is generally desired in the infrared range.

[0187] In other particular embodiments, the telescope according to the invention may be configured to operate in the spectral range of ultraviolet (UV) radiation close to visible light, for example at wavelengths between 200 nm and 370 nm.

[0188] Thus, depending on the intended application, the telescope according to the invention can be configured to operate in the visible, infrared and / or ultraviolet ranges.

[0189] In the present example, the field corrector 14 comprises six lenses. However, according to alternative embodiments not shown, the number of lenses may be adapted, in particular depending on the lens configuration (i.e. choice of glass, asphericity, etc.).

[0190] The inventor has demonstrated that at least two lenses are necessary to enable the field corrector 14 to jointly perform the field curvature correction enabling the conversion of the curved image field from the concave mirror into a planar field in the focal plane and the corrections of coma and astigmatism aberrations (i.e., aberration correction of order 3) resulting from the distance between the aspheric lenses 12A, 12B and the concave mirror 10 (i.e., a distance less than 1.2F, preferably less than F, where F denotes the wavelength of the telescope).

[0191] In general, the addition of corrective optical elements within the field corrector 14, such as the lenses described above, advantageously allows geometric and / or chromatic aberrations to be corrected to higher orders, provides additional distortion correction and consequently achieves a higher level of overall image quality over the largest possible field of view.

[0192] Indeed, the addition of such lenses creates additional degrees of freedom, making it possible to correct more aberrations and at higher orders. Thus, the overall correction provided by all these lenses is the result of an optimal combination of the contributions of the various optical surfaces or interfaces. The more surfaces or interfaces the field corrector has, the more correction possibilities there are, distributed across all these surfaces.

[0193] A second embodiment will now be described with reference to [Fig.4].

[0194] This embodiment corresponds to the first embodiment of [Fig. 1], in of which the telescope 4 further includes a camera 44 positioned upstream of the aspherical lenses 12A, 12B, at the level of the focal plane P of the telescope 4 and centered on the optical axis O.

[0195] As illustrated in [Fig.4], the camera 44 is positioned just at the output of the field corrector 14 with respect to the direction of propagation of the optical rays, i.e. in the focal plane P. For the sake of brevity, only the differences with respect to [Fig.1] will be described.

[0196] The camera 44 is placed at the output of the field corrector 14, opposite the latter, so that the light supplied at the output of the field corrector 14 is captured by the camera 44. Thus, the camera is configured to record images of the observed objects supplied at the output of the field corrector 14.

[0197] For example, the camera 44 includes a matrix optical sensor suitable for space applications. This sensor is selected to operate in the visible range, and more particularly in the wavelength range between 475 nm and 900 nm. This sensor is planar in shape and coincides with the focal plane P of telescope 4.

[0198] In alternative embodiments, this sensor can be adapted to operate in the infrared range, for example up to wavelengths of the order of 2.5 pm.

[0199] Advantageously, the camera 44 is located outside the assembly formed by the concave mirror 10 and the aspherical blades 12A, 12B, which is particularly advantageous for replacing the camera 44 or intervening on the camera 44 during maintenance operations, without having to handle the other components of the telescope 4 and consequently without risk of misaligning the other constituent elements of the telescope.

[0200] The telescope 4 further includes an entrance baffle 48 adapted to be attached to the sphericity corrector 12A, 12B and intended to reduce the level of stray light in order to ensure the best possible detectivity of the telescope 4. In addition, this baffle protects the camera 44 from excessive solar heating.

[0201] The telescope according to the invention has an aperture number less than 2, preferably less than or equal to 1.3, with an aperture diameter D ranging from 5 cm to 80 cm, depending on the availability of optical lenses in the required size. For example, for an aperture diameter D of approximately 20 cm, the telescope according to the invention weighs less than 10 kg, excluding the sensor at the focal plane P. Such a weight makes the telescope light enough to be carried on a satellite, while ensuring sufficient robustness for its launch into orbit. The telescope according to the invention can achieve a length of approximately 40-50 cm, which is about half the length of prior art Schmidt telescopes.

[0202] In other embodiments, the diameter D of the aperture may be increased to a value of 400 mm, 600 mm, or 800 mm in cases where the telescope is intended to be carried on larger satellites or where the telescope is intended to be ground-based, for example, in a ground observation station. A larger aperture diameter D may even be considered, for example, between 800 mm and 1000 mm.

[0203] The advantages of telescope 4 according to the invention are as follows.

[0204] First, the correction of optical aberrations of the telescope is facilitated, so that it is possible to achieve an aperture number of less than 2 while improving the optical quality of the images provided over a wide field of view and across an extended domain, thus making it possible to detect space debris of even smaller size than can be detected with prior art Schmidt telescopes.

[0205] Secondly, mass balancing is ensured for good mechanical stability of the telescope, particularly during its launch into orbit.

[0206] Thirdly, the focal plane P of the telescope is kept outside the assembly formed by the mirror 10, the asphericity corrector 12 and the field corrector 14, thus allowing an image sensor and its proximity electronics to be placed outside the assembly, which allows easy access to the sensor and its electronics without having to intervene on the assembly, as described with reference to [Fig.4].

[0207] A satellite 50 according to an embodiment of the invention will now be described with reference to [Fig.5].

[0208] Satellite 50 includes a telescope 4, as described previously with reference to [Fig.4] for detecting the presence of space objects, such as a nanosatellite 5a and a piece of space debris 5b, in the field of view a of telescope 4, with for example a=8°.

[0209] The telescope 4 according to the invention is sufficiently compact and lightweight that it can be easily mounted inside the satellite 50, regardless of the satellite's size. In practice, the telescope according to the invention can be configured to fit into a small space, typically between 300 mm and 400 mm.

[0210] Indeed, the aperture diameter D of the telescope can be freely fixed so that N=F / D<2, or N<1.5 or N<1.3, depending on the sizing constraints of the space available for the payload in the satellite in which the telescope is intended to be carried and depending on the minimum optical quality desired, for example taking into account the size of the space objects to be detected and / or the orbit on which the satellite is located.

[0211] When mounted on a nanosatellite, the aperture diameter D can be fixed between 100 mm and 200 mm. For example, an aperture diameter D=190 mm allows the telescope to be easily integrated into a Cubesat 12U type nanosatellite.

[0212] When mounted on a medium-sized satellite, the aperture diameter D can be set between 200 mm and 400 mm. For example, an aperture diameter D=230 mm allows the telescope to be easily integrated into a HEMERIA EOPD type platform.

[0213] In the case of onboard use in larger satellites, the aperture diameter D may be between 400 mm or 500 mm.

[0214] It is assumed that space objects, for example a nanosatellite 5a and a fragment of satellite 5b, are in the field of view of the telescope.

[0215] The satellite 50 includes computing means 52, such as a computer, for processing an image signal S supplied at the output of the telescope's image sensor. For this purpose, the computing means 52 of the satellite 50 are configured to process the image signal S, for example according to known image processing algorithms, so as to detect space objects 5a, 5b in the image.

[0216] The computing means 52 are further configured to identify among the detected space objects 5a, 5b, the one or those which could collide with the satellite 50.

[0217] Satellite 50 further includes warning means 54 configured to emit an alert signal A, in the event of positive identification of one or more space objects in the vicinity of satellite 50.

[0218] A method for detecting spatial objects according to an embodiment of the invention will now be described with reference to [Fig.6].

[0219] During a supply step E60, the telescope 4 according to the embodiment described above with reference to [Fig. 4] is mounted in the satellite 50 according to [Fig. 5]. During this step, the satellite 50 is also equipped with the computing means 52 and the warning means 54 described with reference to [Fig. 4].

[0220] During a sending step E62, satellite 50 equipped with telescope 4 is sent into space.

[0221] During an acquisition step E64, telescope 4 acquires one or more S images.

[0222] During a processing step E66, these images S are processed by the computing means 52 of the satellite 50, so as to detect space objects.

[0223] During an E68 alert step, an A alert signal is generated if at least one space object is detected that is likely to collide with satellite 50.

[0224] Advantageously, the optical quality of the image provided by the telescope 4 according to the invention is sufficiently high to allow the detection and identification of small space objects, such as fragments of satellites and / or nanosatellites.

[0225] A ground station 70 according to an embodiment of the invention will now be described with reference to [Fig.7].

[0226] The ground station 70 includes a telescope according to the invention, for example the telescope 4 as described above with reference to [Fig. 4]. This telescope is adapted to detect, in the vicinity of a monitored satellite 7a, space objects such as nanosatellites 7b, 7c, some of which 7b are located in the field of view a of the telescope.

[0227] The same process as described with reference to [Fig.6] applies in the case where telescope 4 is integrated during step E60 into ground station 70, with the obvious exception of step E62 sending into space.

[0228] For this purpose, the ground station 70 includes computing and processing means similar to those described with reference to satellite 50, as illustrated in [Fig.5].

[0229] In the detailed description of the invention given above, the terms used shall not be interpreted as limiting the invention to modes of implementation the interpretations set forth in this description, but must be interpreted to include all equivalents which can be predicted by a person skilled in the art by applying their general knowledge to the implementation of the teaching which has just been disclosed to them.

Claims

Demands

1. A Schmidt telescope (1; 4) configured to form an image in a focal plane (P), the telescope comprising a single mirror, the mirror being concave (10), a sphericity corrector (12) adapted to correct spherical aberrations of the concave mirror (10), a field corrector (14) comprising an optical entrance surface (14A.1) and an optical exit surface from which light is intended to exit in the direction of the focal plane (P), said optical exit surface being positioned at the focal plane (P), the concave mirror (10), the sphericity corrector (12) and the field corrector (14) being centered on the same optical axis (O) of the telescope, the telescope being characterized in that the sphericity corrector (12) comprises two aspherical lenses (12A, 12B) such that: - each aspheric lens (12A, 12B) comprises two faces (12A.1, 12A.2; 12B.1, 12B.2), at least one of which is curved, in particular spherical in shape, and at least one of which has an aspheric profile, - each aspheric lens (12A, 12B) has a maximum thickness variation (Ael; Ae2) between a center and an edge of said lens, which is between 1% and 5%, preferably between 1% and 3%, of a diameter (D; Dl, D2) of said lens (12A, 12B), - the two aspheric lenses (12A; 12B) are composed of different optical glass so as to form an achromatic doublet, preferably of the Flint-Crown type; - the two aspherical lenses (12A, 12B) have a central hole (120) centered on the optical axis (O) and through which the field corrector (14) is placed; - the two aspherical lenses (12A, 12B) are placed between: • a first plane (P') located at a first distance (ei) from the focal plane (P) of the telescope (1) moving away from the concave mirror (10); and • a second plane (T4) located at a second distance (e2) from the optical entrance surface (14A.1) of the field corrector (14) as one approaches the mirror concave (10), where the first (ei) and second (e2) distances are equal to 1 / 10 of a distance separating the concave mirror (10) from the focal plane (P) of the telescope (1); and in that the telescope is configured so that it has an aperture number (N) less than or equal to 2, preferably less than or equal to 1.5 or 1.

3.

2. Telescope (1; 4) according to claim 1, wherein the aperture number (N) is not equal to 1.

31.

3. Telescope (1; 4) according to any one of claims 1 or 2, wherein the field corrector (14) comprises a set of at least two lenses (14A, 14B, 14C, 14D, 14E, 14F) configured to convert a curved optical field at the output of the concave mirror (10) into a planar optical field and to correct coma aberration and astigmatism aberration of the telescope.

4. Telescope (1; 4) according to claim 3, wherein the field corrector (14) further comprises at least one additional lens configured to permit distortion correction.

5. Telescope (1; 4) according to claim 3 or 4, wherein at least one lens of the field corrector (14) is aspherical.

6. Telescope (1; 4) according to any one of claims 1 to 5, wherein the lenses of the field corrector (14) are composed of the same optical glass.

7. Telescope (1; 4) according to any one of claims 1 to 6, wherein the concave mirror is aspherical.

8. Telescope (1; 4) according to any one of claims 1 to 7, configured to operate in a visible spectral range and an infrared spectral range and / or an ultraviolet spectral range.

9. Telescope (4) according to any one of claims 1 to 8, further comprising an optical sensor (44) positioned in the focal plane (P) of the telescope (4) opposite the field corrector (14).

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