Catadioptric bispectral telescope

Abstract

The invention provides a telescope 2 comprising a convex mirror 7, a concave mirror 6 perforated at its centre by an aperture, a separator lens 9, and two electromagnetic (EM) radiation sensors 12,13.

Description

DESCRIPTION TITLE: Catadioptric bispectral telescope Technical field The invention generally relates to multispectral imaging systems. A particularly interesting application of the invention covers a bispectral imaging system with a small bulk and using mirrors. Prior art Some imaging applications require multispectral imaging systems in order to be able to profit from different versions of the same scenery, with different objects or atmospheres highlighted according to these versions. In particular, a multispectral imaging system allows observing scenery with several wavelengths or different wavelength bands, each wavelength band highlighting some objects more than others. For example, multispectral imaging allows acquiring a wavelength band corresponding to the visible as well as a wavelength band corresponding to far-infrared. However, the existing bispectral systems do not allow observing scenery with the acquisition of the bispectral image performed simultaneously for both wavelength bands. In general, two images should be acquired one after another and there is no possibility to record the two at the same time or to be able to select at all times the wavelength band to operate with. In addition, the existing systems are often made based on fragile dioptres and the lenses placed at the end of these systems could be broken by impacts or projectiles, making the imaging system completely blind. Finally, the existing systems are not compact enough to be able to be embedded or encapsulated in some casings. Disclosure of the invention Hence, the present invention aims to overcome the drawbacks of the aforementioned systems and to provide a telescope acquiring two different wavelength bands simultaneously. Hence, an object of the invention is a telescope comprising, aligned on the same optical axis, a convex mirror, a concave mirror perforated at its centre, a separator lens, and two electromagnetic radiation sensors, the concave mirror and the convex mirror being configured to reflect first and second electromagnetic radiation bands, the electromagnetic radiation penetrating into the telescope through a hole formed at the centre of the concave mirror and hitting the convex mirror, the electromagnetic radiation hitting the concave mirror after being reflected on the convex mirror, the separator lens being configured to separate said two electromagnetic radiation bands towards two optical routes by reflecting the first electromagnetic radiation band and by transmitting the second electromagnetic radiation band, the two electromagnetic radiation sensors being placed on either side of the separator lens, each electromagnetic radiation sensor being configured to detect the first or the second electromagnetic radiation band corresponding to the side of the separator lens where it is placed. Thus, for the same scenery acquisition, the telescope enables light to follow two optical routes towards two different sensors in order to simultaneously acquire two different wavelength bands and to make a bispectral telescope out of this telescope. In addition, the mirrors are positioned at the end of the telescope, which guarantees robustness of the telescope. Even in the event where the latter is hit by a projectile on one of the mirrors, the telescope is still capable of acquiring an image. Finally, the telescope could be designed so as to be embedded in small-sized casings, the space of the two optical routes being reduced. Advantageously, the telescope comprises two sets of one or several lens(es) respectively placed upstream of each electromagnetic radiation sensor and configured to focus the electromagnetic radiations on each sensor. Advantageously, the telescope comprises a main lens common to the first and second electromagnetic radiation bands and placed in front of the separator lens. In one embodiment, the main lens comprises a central portion which belongs to the set of one or several lens(es) placed on the path of the first electromagnetic radiation band intended to be reflected by the separator lens or whose central portion leaves enough empty space to introduce therein the set of one or several lens(es) placed on the path of the first electromagnetic radiation band intended to be reflected by the separator lens. In one embodiment, the separator lens is manufactured in a material, or is covered with a multilayer dielectric treatment, configured to reflect the first electromagnetic radiation band and to transmit the second electromagnetic radiation band. Advantageously, the concave mirror is circularly perforated at its centre with a hole with a diameter substantially equal to the diameter of the convex mirror. Advantageously, the convex mirror has a diameter larger than the size of the electromagnetic radiation sensor placed on the path of the first electromagnetic radiation band intended to be reflected by the separator lens. Advantageously, the convex mirror and the concave mirror are spherical or conical or aspherical mirrors. In one embodiment, the convex mirror and the concave mirror are mirrors made of metal In one embodiment, the first and second electromagnetic radiation bands are bands corresponding to the visible or far-infrared or mid-infrared or near-infrared wavelengths. Brief description of the drawings Other aims, features and advantage of the invention will appear upon reading the following description, given only for non-limiting purposes, and made with reference to the appended drawings wherein: [Fig 1] illustrates a schematic section of a telescope according to the invention. Detailed disclosure of at least one embodiment In Figure 1, a section passing through the optical axis 1 of a telescope 2 according to the invention is represented. Electromagnetic rays 3 are also schematically represented, entering into the telescope 2 and covering the different available optical routes 4 and 5. The telescope 2 comprises on the same optical axis 1 a concave mirror 6, a convex mirror 7 with smaller dimensions than the concave mirror 6 in order to obtain an inverted Cassegrain configuration, a main lens 8, a separator lens 9 allowing separating different wavelengths or wavelength bands of the electromagnetic radiation 3, two sets 10 and 11 of two lenses placed on each side of the separator lens and two sensors 12 and 13 also placed on each side of the separator lens 9. The electromagnetic rays 3 penetrate into the telescope 2 through a hole 14 formed at the centre of the concave mirror 6 and hit the convex mirror 7. The convex mirror 7 has a diameter substantially equal to the diameter of the hole 14 formed in the concave mirror 6 in order to capture as much electromagnetic rays 3 coming from the hole 14 of the concave mirror 6 as possible. After being reflected on the convex mirror 7, the electromagnetic rays 3 will hit the concave mirror 6 on its reflective surface 6a directed towards the convex mirror 7. According to the configuration and the embodiment, the convex 7 and concave 6 mirrors are spherical or conical or aspherical in order to reflect the electromagnetic rays 3 in the desired direction. In one embodiment, the convex 7 and concave 6 mirrors are made of metal. In the event where a projectile hits the concave mirror 6 by its external surface 6b, only one portion of the concave mirror 6 is unusable but the electromagnetic rays 3 could be reflected on the rest of the concave mirror 6 and the telescope is still usable. In one embodiment, the external surface 6b of the concave mirror 6 comprises a coating or an additional protective device. After reflection on the concave mirror 6, the electromagnetic rays 3 are directed towards the separator lens 9. In one embodiment, a main lens 8 is inserted before the separator lens 9 in order to refract the electromagnetic rays 3 in the desired direction. The convex 7 and concave 6 mirrors as well as the main lens 8 are built so that each wavelength of the electromagnetic rays 3 whose trace should be obtained on one of the sensors 12 and 13 is reflected by the convex 7 and concave 6 mirrors and transmitted by the main lens 8. Conversely, the separator lens 9 is configured to reflect some wavelengths and to transmit others. Thus, two optical routes 4 and 5 are available at the exit of the separator blade 9. For example, the separator blade 9 separates pairs of wavelength bands such as, without limitation: the visible light band (from 400 nanometres to about 700 nanometres) and the near-infrared (from 700 nanometres to about 2.5 micrometres); the near-infrared and the mid-infrared (from 3 micrometres to about 5 micrometres); the visible and the mid-infrared; the near-infrared and the far-infrared (from 8 micrometres to about 14 micrometres); the visible and the far-infrared: the mid-infrared and the far-infrared. For example, the separator lens 9 is a lens manufactured in a specific material having transparency properties for some wavelengths, such as germanium or zinc sulphide. According to one embodiment, the separator lens 9 has undergone a dielectric treatment to be able to specifically separate wavelength bands of electromagnetic radiations, such as a multilayer treatment. Thus, the electromagnetic rays 15 of a first wavelength band are reflected according to a first optical route 4 towards a first sensor 12 placed behind the convex mirror 7. The first sensor 12 is a sensor adapted to detect the electromagnetic radiations 15 intended to be reflected by the separator lens 9 and corresponding to the first wavelength band. The first sensor 12 is placed behind the convex mirror 7, the diameter of the convex mirror 7 is larger than the size of the first sensor 12. In general, the convex mirror 7 is larger than the focal planes located downstream of the optical pathway of the electromagnetic radiation 3. Thanks to the first optical route 4, the space between the convex mirror 7 and the separator lens 9 within which the first sensor 12 is placed is efficiently used. Nevertheless, the space around the first sensor 12 is limited, the first sensor 12 does not therefore include any cooling system. In one embodiment, a first set of several lenses 10 is placed between the separator lens 9 and the first sensor 12 in order to focus as much rays as possible of the electromagnetic radiation 15 on the first sensor 12. For example, the first set of several lenses 10 includes two lenses. In one embodiment, the main lens 8 comprises a central portion belonging to the first set of several lenses 10. The central portion is a portion of the main lens 8 through which only the electromagnetic radiation 15 passes. In another embodiment illustrated in Figure 1, the main lens 8 is perforated at its centre so that its empty central portion 16 leaves space to the first set of several lenses 10 and not to refract the electromagnetic radiation 15 directed towards the first sensor 12. Other electromagnetic radiations 17 of a second wavelength band are transmitted by the separator lens 9 towards a second optical route 5. The second optical route 5 includes a second sensor 13 adapted to detect the electromagnetic radiations 17 of the second wavelength band and intended to be transmitted by the separator lens 9. The second sensor 13 having less size constraints, it consists for example of a cooled sensor, for example to detect mid- or far- infrared in a cooled manner and therefore more efficiently. In one embodiment, a second set of several lenses 11 is placed between the separator lens 9 and the second sensor 13 in order to focus as much rays as possible of the electromagnetic radiation 17 on the second sensor 13. For example, the second set of several lenses 11 includes two lenses. Each lens, amongst the first set of several lenses 10 or the second set of several lenses 11 or the main lens 8 or the separator lens 9, is freeform but is configured so that as much electromagnetic radiation 3 entering into the telescope 2 as possible is focused on one of the two sensors 12 and 13.

Claims

CLAIMS 1. A telescope (2) comprising, aligned on the same optical axis (1), a convex mirror (7), a concave mirror (6) perforated at its centre, a separator lens (9), and two electromagnetic radiation sensors (12; 13), characterised in that the concave mirror (6) and the convex mirror (7) are configured to reflect first and second electromagnetic radiation bands, the electromagnetic radiation penetrating into the telescope (2) through a hole (14) formed at the centre of the concave mirror (6) and hitting the convex mirror (7), the electromagnetic radiation hitting the concave mirror (6) after being reflected on the convex mirror (7), in that the separator lens (9) is configured to separate said two electromagnetic radiation bands (15; 17) towards two optical routes (4; 5) by reflecting the first electromagnetic radiation band (15) and by transmitting the second electromagnetic radiation band (17), and in that the two electromagnetic radiation sensors (12; 13) are placed on either side of the separator lens (9), each electromagnetic radiation sensor (12; 13) being configured to detect the first or the second electromagnetic radiation band (15; 17) corresponding to the side of the separator lens (9) where it is placed.

2. The telescope according to claim 1, comprising two sets of one or several lens(es) (10; 11) respectively placed upstream of each electromagnetic radiation sensor (12; 13) and configured to focus the electromagnetic radiations (15; 17) on each sensor (12; 13).

3. The telescope according to one of claims and 2, comprising a main lens (8) common to the first and second electromagnetic radiation bands and placed in front of the separator lens (9).

4. The telescope according to claims 2 and 3, wherein the main lens (8) comprises a central portion (16) which belongs to the set of one or several lens(es) (10) placed on the path of the first electromagnetic radiation band (15) intended to be reflected by the separator lens (9) or whose central portion (16) leaves enough empty space to introduce therein the set of one or several lens(es) (10) placed on the path of the first electromagnetic radiation band (15) intended to be reflected by the separator lens (9).

5. The telescope according to any one of claims _ to 4, wherein the separator lens (9) is manufactured in a material, or is covered with a multilayer dielectric treatment, configured to reflect the first electromagnetic radiation band (15) and to transmit the second electromagnetic radiation band (17).

6. The telescope according to any one of claims @ to 5, wherein the concave mirror (6) is circularly perforated at its centre with a hole (14) with a diameter substantially equal to the diameter of the convex mirror (7).

7. The telescope according to any one of claims . to 6, wherein the convex mirror (7) has a diameter larger than the size of the electromagnetic radiation sensor (12) placed on the path of the first electromagnetic radiation band (15) intended to be reflected by the separator lens (9).

8. The telescope according to any one of claims @ to 7, wherein the convex mirror (7) and the concave mirror (6) are spherical or conical or aspherical mirrors.

9. The telescope according to any one of claims _ to 8, wherein the convex mirror (7) and the concave mirror (6) are mirrors made of metal.

10. The telescope according to any one of claims @ to 9, wherein the first and second electromagnetic radiation bands (15; 17) are bands corresponding to the visible or far-infrared or mid-infrared or near-infrared wavelengths.

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