TMA-type telescope with two optical channels

Abstract

An anastigmatic three-mirror telescope comprising at least one primary mirror (1), a secondary mirror (2) positioned in front of the primary mirror (1) such that the primary and secondary mirrors have optical surfaces (1.1, 2.1) opposite each other, a tertiary mirror (3) having an optical surface (3.1) opposite the optical surface (2.1) of the secondary mirror (2), and a first optronic sensor (10) positioned opposite the tertiary mirror (3). The telescope includes a second optronic sensor (20) positioned behind the secondary mirror (2) and the optical surface of the secondary mirror (2.1) includes a dichroic treatment (2') arranged to separate an incident light flux reflected by the primary mirror (1) into a first light flux reflected towards the first optronic sensor (10) via the tertiary mirror (3) and a second light flux transmitted to the second optronic sensor (20), the first and second light fluxes belonging to different spectral bands and the secondary mirror (2) being transparent to the spectral band of the second light flux. FIGURE IN ABRIDGED ANALYSIS: Fig. 1.

Description

Title of the invention: TMA type telescope with two optical paths

[0001] The present invention relates to the field of optics and more particularly to TMA type telescopes, that is to say anastigmatic telescopes with three mirrors.

[0002] BACKGROUND OF THE INVENTION

[0003] It is known that TMA type telescopes, comprising at least one primary mirror, a secondary mirror placed in front of the primary mirror in such a way that the primary and secondary mirrors have optical surfaces opposite each other, a tertiary mirror having an optical surface opposite the optical surface of the secondary mirror, and an optronic sensor placed opposite the tertiary mirror to capture a luminous flux reflected successively by the primary mirror, the secondary mirror and the tertiary mirror.

[0004] Some telescopes of this type are adapted to have two separate optical paths leading to two sensors sensitive to distinct spectral bands, such as an infrared sensor and a visible sensor. To this end, dichroic plates are arranged between the mirrors to separate the incident light into an infrared light beam transmitted to the infrared sensor and a visible light beam reflected back to the visible sensor. This results in an increase in the telescope's overall size. Furthermore, the use of a dichroic plate in a converging beam (at the telescope's output) introduces aberrations in the wavefront of the light beam transmitted by the dichroic plate. These aberrations can be difficult to correct to obtain an image of the expected quality.

[0005] SUBJECT OF THE INVENTION

[0006] The invention aims in particular to provide a telescope that at least partially remedies the aforementioned drawbacks. Summary of the invention

[0007] To this end, the invention provides a three-mirror anastigmatic telescope comprising at least one primary mirror, a secondary mirror positioned in front of the primary mirror such that the primary and secondary mirrors have optical surfaces opposite each other, and a tertiary mirror having an optical surface opposite the optical surface of the secondary mirror. The telescope includes a first optronic sensor positioned opposite the tertiary mirror and a second optronic sensor positioned behind the secondary mirror. The optical surface of the secondary mirror includes a dichroic coating arranged to separate an incident light flux reflected by the primary mirror into a first light flux reflected towards the first optronic sensor via the third mirror and a second light flux transmitted to the second optronic sensor. luminous belonging to different spectral bands and the secondary mirror being transparent to the spectral band of the second luminous flux.

[0008] Thus, the secondary mirror will perform a splitting of the incident light beam reflected by the primary mirror into a first beam of light reflected towards the first sensor and a second beam of light transmitted towards the second sensor. Positioning the second sensor behind the secondary mirror helps to limit the transverse bulk. Furthermore, this arrangement simplifies the optical formula, avoids spectral separation downstream of the telescope, and provides a large aperture ratio for the second spectral band by placing it directly on the side of the primary mirror's focal point.

[0009] According to optional features, used individually or in whole or in combination: - the telescope includes at least one first field corrector positioned between the secondary mirror and the second sensor; - the first field corrector is a lens-based field corrector; - the secondary mirror has a rear surface shaped like a lens to shape the second beam of light; - the first luminous flux includes at least one wavelength from the visible range; - the second luminous flux includes at least one wavelength from the infrared range; - the primary mirror and the tertiary mirror are made of silicon carbide and the secondary mirror is made of silicon; - the telescope has an aperture diaphragm located at the secondary mirror.

[0010] Other features and advantages of the invention will become apparent from the following description of a particular, non-limiting embodiment of the invention. Brief description of the drawings

[0011] Reference will be made to the attached drawings, among which:

[0012] [Fig.1] is a schematic axial cross-sectional view of a telescope according to the invention;

[0013] [Fig.2] is an isolated view of the first optical path of the telescope according to the invention;

[0014] [Fig.3] is an isolated view of the second optical path of the telescope according to the invention. DETAILED DESCRIPTION OF THE INVENTION

[0015] With reference to the figures, the telescope according to the invention is of the TMA type and comprises at least one primary mirror 1, one secondary mirror 2, and one tertiary mirror 3. The primary mirror 1 and the secondary mirror 2 have optical surfaces 1.1 and 2.1 facing each other. The tertiary mirror 3 has an optical surface 3.1 facing the surface Optical surface 2.1 of secondary mirror 2. The primary mirror 1, secondary mirror 2, and tertiary mirror 3 have rear surfaces opposite optical surfaces 1.1, 2.1, and 3.1 (only the rear surface 2.2 of secondary mirror 2 is referenced in the figures). The mirrors, and all the components of the telescope of the invention, are mounted, in a manner known per se, in a frame not shown in the figures.

[0016] The primary mirror 1 is here made of silicon carbide. The optical surface 1.1 of the primary mirror 1 is arranged to reflect light radiation in the visible and infrared spectral bands towards the secondary mirror 2. A reflective coating in the visible and infrared is applied to the optical surface 1.1 of the primary mirror 1, here a metallization consisting, for example, of silver or aluminum.

[0017] The secondary mirror 2 is here made of silicon. The optical surface 2.1 of the secondary mirror 2 has a dichroic coating 2' comprising successive layers of selected materials and of known thicknesses, designed to reflect visible radiation towards the tertiary mirror 3 and transmit infrared radiation backwards. It is therefore necessary that the material constituting the secondary mirror 2 be transparent to infrared radiation, that is to say, that it has a sufficient infrared radiation transmission rate for the intended application. Hence the use of silicon, but other materials are possible, such as germanium, zinc sulfide, or zinc selenide. As a specific example, materials that can be used for 2' dichroic treatment include: ZnS, ZnSe, Ge, Si, SiO2, MgF2, YF3, YbF3, DiF3... The thicknesses are determined according to the wavelengths that we want to separate.The telescope is arranged here to have an aperture diaphragm positioned at the level of the secondary mirror 2.

[0018] The tertiary mirror 3 is here made of silicon carbide, like the primary mirror 1. The optical surface 3.1 of the tertiary mirror 3 is arranged to reflect light radiation in the visible spectral band towards a first optronic sensor 10. A visible-reflective coating is applied to the optical surface 3.1 of the tertiary mirror 3; in this case, a metallization made, for example, of silver or aluminum.

[0019] It is worth noting the advantage of using silicon for the secondary mirror 2 in combination with primary mirrors 1 and tertiary mirrors 3 made of silicon carbide. Indeed, the thermal expansion of silicon is close to that of silicon carbide. The mirror combination remains stable regardless of temperature variations.

[0020] The first optronic sensor 10 is sensitive to visible radiation and is placed opposite the optical surface 3.1 of the tertiary mirror 3 to capture a first luminous flux corresponding to the visible radiation reflected by the secondary mirror 2 towards the tertiary mirror 3.

[0021] A second optronic sensor 20, sensitive to infrared radiation, is placed behind the secondary mirror 2 to capture a second luminous flux corresponding to the infrared radiation transmitted by the secondary mirror 2.

[0022] In the present embodiment, the optronic sensor 20 extends respectively along an image focal plane perpendicular to the optical axis of the secondary mirror 2 and the optronic sensor 10 extends along an image focal plane parallel to the image focal plane of the optronic sensor 20.

[0023] The three-mirror telescope makes it possible to reproduce good image quality over a relatively large field of view, i.e., over a large viewing angle. Therefore, it is not necessary to add a field corrector for the visible spectrum when using the three mirrors.

[0024] On the other hand, in order to obtain good quality images over a large field in the infrared domain, the telescope includes a field corrector 30 on the infrared channel.

[0025] The field corrector 30 is positioned between the secondary mirror 2 and the second sensor 20. The field corrector 30 is composed of lenses. These lenses are made of materials specific to the wavelengths in the infrared range, such as semiconducting glasses or crystals like silicon, germanium, zinc sulfide, or zinc selenide. Depending on the desired image quality and field width, a greater or lesser number of lenses will be required, typically between two and five, but a larger number of corrective lenses can be considered. Preferably, the lenses are treated with anti-reflective coatings specific to both the spectral band of the corresponding channel and the lens materials, as the coating is calculated based on the refractive index of the material to which it is applied.

[0026] Furthermore, the rear surface 2.2 of the secondary mirror 2 is here shaped into a lens to shape the second light beam. The rear surface 2.2 thus forms the entrance lens of the field corrector 30.

[0027] This results in a bispectral telescope comprising an optical surface 2.1 acting as both a mirror and a transmission diopter by means of a dichroic coating 2' creating two optical paths in different spectral bands, namely, a visible path by reflection on the diopter and an infrared path by transmission from the diopter (the substrate supporting the dichroic coating being transparent to infrared radiation). Preferably, the optical elements of the visible path are arranged so that the visible path has an aperture of f / 5.4, and the optical elements of the infrared path are arranged so that the infrared path has an aperture of f / 2.7, which is a relatively large aperture. Of course, other apertures are possible.

[0028] We have seen that the telescope is preferably arranged so that the aperture diaphragm is located at the secondary mirror. Other TMA-type architectures exist with an aperture diaphragm located elsewhere, for example at the primary mirror, or downstream of the three mirrors, that is, between the tertiary mirror and the image focal plane. The location at the secondary mirror is the most advantageous within the framework of this invention because it allows for a more compact infrared path, with smaller lenses.

[0029] Of course, the invention is not limited to the embodiment described but encompasses any variant falling within the scope of the invention as defined by the claims.

[0030] In particular, the telescope may have a different structure from that described.

[0031] The materials used may differ from those mentioned. Mirrors primary and tertiary can, for example, be made of metal, glass-ceramics or glass, and do not need to be transparent, either in the visible or infrared range.

[0032] The field corrector may be a mirror field corrector, although a lens solution is easier to implement than a mirror solution. Alternatively, the field corrector may be omitted.

[0033] The dichroic treatment of the secondary mirror can also be arranged to separate other spectral bands than those mentioned, for example: near-infrared and mid-infrared; near-infrared and far-infrared; or others.

[0034] The optronic sensor 20 may not extend along an image focal plane perpendicular to the optical axis of the secondary mirror 2 and / or may not extend along an image focal plane parallel to the image focal plane of the optronic sensor 20.

Claims

Demands

1. Anastigmatic three-mirror telescope, comprising at least one primary mirror (1), a secondary mirror (2) positioned in front of the primary mirror (1) such that the primary and secondary mirrors have optical surfaces (1.1, 2.1) opposite each other, a tertiary mirror (3) having an optical surface (3.1) opposite the optical surface (2.1) of the secondary mirror (2), and a first optronic sensor (10) positioned opposite the tertiary mirror (3), characterized in that the telescope comprises a second optronic sensor (20) positioned behind the secondary mirror (2) and the optical surface of the secondary mirror (2.1) includes a dichroic treatment (2') arranged to separate an incident light flux reflected by the primary mirror (1) into a first light flux reflected towards the first optronic sensor (10) via the tertiary mirror (3) and a second light flux transmitted to the second optronic sensor (20), the first light flux and the second light flux belonging to different spectral bands and the secondary mirror (2) being transparent to the spectral band of the second light flux.

2. Telescope according to claim 1, having an aperture diaphragm located at the level of the secondary mirror (2).

3. Telescope according to claim 1 or 2, comprising at least one field corrector (30) disposed between the secondary mirror (2) and the second sensor (20).

4. Telescope according to claim 3, wherein the field corrector (30) is a lens field corrector.

5. Telescope according to any one of the preceding claims, wherein the secondary mirror (2) has a rear surface (2.2) shaped as a lens to shape the second light beam.

6. Telescope according to any one of the preceding claims, wherein the first luminous flux comprises at least one wavelength in the visible range.

7. Telescope according to any one of the preceding claims, wherein the second luminous flux comprises at least one wavelength in the infrared range.

8. Telescope according to claims 6 and 7, wherein the primary mirror (1) and the tertiary mirror (3) are made of silicon carbide and the secondary mirror (2) is made of silicon.

Images