Korsch-type compact three-mirror anastigmat telescope

Abstract

A three-mirror anastigmat telescope comprises at least a concave first mirror, a convex second mirror and a concave third mirror, the three mirrors arranged so that the first mirror and the second mirror form, from an object at infinity, an intermediate image situated between the second mirror and the third mirror, the third mirror forming, from this intermediate image, a final image in the focal plane of the telescope. In the architecture of the telescope, at least the surface of the concave third mirror is a -polynomial surface.

Claims

1. A three-mirror anastigmat telescope comprising a concave first mirror, a convex second mirror and a concave third mirror, the three mirrors being arranged so that the first mirror and the second mirror form, from an object at infinity, an intermediate image situated between the second mirror and the third mirror, the third mirror forming, from this intermediate image, a final image in the focal plane of the telescope, wherein the surface of the concave third mirror is a -polynomial surface, a normal to a surface at a center of the concave first mirror being tilted on an optical axis of the telescope defined by the ray passing through a center of an input pupil and perpendicular to the input pupil, a normal to a surface at a center of the convex second mirror being tilted on the optical axis of the telescope and a normal to a surface at a center of the concave third mirror being inclined on the optical axis of the telescope, the pupil of the telescope being situated on the concave first mirror.

2. The anastigmat telescope according to claim 1, wherein the surface of the concave first mirror is a -polynomial surface.

3. The anastigmat telescope according to claim 1, wherein the surface of the convex second mirror is a -polynomial surface.

4. The anastigmat telescope according to claim 1, wherein the angular field of view is greater than 6 degrees in a direction of space.

5. The anastigmat telescope according to claim 1, wherein the angular field is greater than 2.5 degrees in two perpendicular directions of space.

6. A method for installing a three-mirror anastigmat telescope comprising a concave first mirror, a convex second mirror and a concave third mirror, the three mirrors being arranged so that the first mirror and the second mirror form, from an object at infinity, an intermediate image situated between the second mirror and the third mirror, the third mirror forming from this intermediate image a final image in the focal plane of the telescope, the pupil of the telescope being situated on the concave first mirror, the method being implemented by optical system computation software, wherein the method comprises at least the following steps: in a first step, determination of the paraxial parameters of the telescope; in a second step, installation of the optical system of the telescope in a Korsch-type configuration comprising the three aspherical mirrors, determination of the main field aberrations by the nodal aberration theory and determination of the corresponding Root Mean Square (RMS) Wave Front Error (WFE); in a third step, addition, to the definition of the aspherical surface of one of the mirrors of the optical system, of the Zernike polynomial coefficients corresponding to the computed aberrations, said surface thus being a -polynomial freeform surface; in a fourth step, elimination of the occluding of the primary mirror by a rotation of at least one of the mirrors and modification of the form of the mirror with freeform surface, so as to correct the aberrations created by the rotation of the mirror and modification of the Zernike polynomials so as to reduce the RMS WFE below a predetermined threshold.

7. The method for installing an anastigmat telescope according to claim 6, wherein the surface definition modifications made in the third step or in the fourth step also affect the surface of one of the other two mirrors of the telescope.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be better understood and other advantages will become apparent on reading the following description, given in a nonlimiting manner and by virtue of the attached figures in which:

(2) FIG. 1 represents a first optical architecture of three-mirror anastigmat telescopes called TMA telescopes according to the prior art;

(3) FIG. 2 represents a second optical architecture of three-mirror anastigmat telescopes called Korsch telescopes according to the prior art;

(4) FIG. 3 represents a front view of a primary Korsch telescope mirror with its central aperture in two different field configurations;

(5) FIG. 4 represents the modulation transfer function of the Korsch telescope with central aperture;

(6) FIG. 5 represents a third optical architecture of three-mirror anastigmat telescopes comprising a freeform mirror according to the prior art;

(7) FIG. 6 represents a fourth optical architecture of three-mirror anastigmat telescopes comprising a freeform mirror according to the prior art;

(8) FIG. 7 represents an optical architecture of three-mirror anastigmat telescopes according to the invention.

DETAILED DESCRIPTION

(9) By way of example, FIG. 7 represents an optical architecture of three-mirror anastigmat telescopes according to the invention. This architecture comprises a concave first mirror M1, a convex second mirror M2 and a concave third mirror M3. In this figure, the optical axis X passing through the centre of the pupil P is represented by dotted lines and the normals N.sub.M1, N.sub.M2 and N.sub.M3 to the surface of the mirrors M1, M2 and M3 are represented by arrows arranged at the centre of the mirrors.

(10) This architecture is derived from the Korsch-type architectures as described previously. However, it is demonstrated that the use of freeform surface mirrors makes it possible to notably increase the accessible anastigmat field. According to the architectures employed, the gain is substantially by a factor 2.

(11) The three mirrors are arranged so that the first mirror and the second mirror form, from an object at infinity, an intermediate image situated in a focussing plane P.sub.FI situated between the second mirror and the third mirror. The third mirror forms, from this intermediate image, a final image in the focal plane of the telescope where the detector D is situated.

(12) At least the surface of the concave third mirror is a -polynomial surface. The surfaces of the first and second mirrors can also be -polynomial.

(13) The pupil P of the telescope is situated at the level of the concave first mirror M1.

(14) As can be seen in FIG. 7, the normal N.sub.M1 to the centre of the surface of the concave first mirror M1 is tilted by a few degrees on the optical axis X of the telescope defined by the ray passing through the centre of the input pupil and perpendicular to this pupil, the normal N.sub.M2 to the centre of the surface of the convex second mirror M2 is tilted by a few degrees on the optical axis X of the telescope and the normal N.sub.M3 to the centre of the surface of the concave third mirror M3 is tilted by a few degrees on the optical axis X of telescope.

(15) This three-mirror configuration comprising an intermediate focal plane, a pupil situated at the level of the first mirror and mirrors with -polynomial surface weakly tilted on the axis makes it possible to obtain both a significant optical field, an open system and a bulk that is more reduced than the solutions of the prior art.

(16) The method for computing the optical system of the telescope relies on the analysis of the aberrations expressed in the form of Zernike polynomials in the field. This analysis makes it possible to determine the values of the Zernike coefficients to be applied to the different mirrors M1, M2 and M3.

(17) The method used rests on the nodal aberration theory, known as such and generalized to freeform surfaces. This method is described in Theory of aberration fields for general optical systems with freeform surfaces by K. Fuerschbach. It is installed by means of optical system computation software.

(18) In a first step, the paraxial parameters of the telescope, that is to say its focal length, its aperture and its field, are determined.

(19) In a second step, the optical system of the telescope is installed in a Korsch-type configuration with three simply aspherical mirrors. In this second step, no account is taken of any occultings due to the different mirrors. The main aberrations in the field are then determined by the nodal aberration theory, that is to say the astigmatism, coma and spherical aberrations, as well as the RMS WFE in the field of the Korsch with three aspherical mirrors.

(20) In a third step, Zernike coefficients corresponding to the computed aberrations are added to at least the surface of one of the mirrors of the optical combination so as to reduce them and/or eliminate them in all the field of the telescope. The optical solution found remains theoretical because the light is partly blocked by the mirrors.

(21) Finally, in a fourth and final step, this occulting is eliminated by a rotation of the mirrors. These rotations allow the optic to continue to work on the optical axis. However, this tilt adds astigmatism and coma. To correct the aberrations added, the form of the freeform mirror or mirrors is modified. By using the nodal aberration theory, it is possible to correct the aberrations created by the rotations of the mirrors, by directly modifying the Zernike polynomials applied to each of the three mirrors. The influence of the Zernike polynomials on the mirrors is different according to the position of the mirror relative to the pupil. Thus, the Zernike polynomials applied to a mirror in the vicinity of the pupil such that the mirror M1 of FIG. 5 have an influence on all the points of the field, which is not the case for the mirror M3 which is situated far from a pupil.

(22) Obviously, the third and the fourth steps can be performed simultaneously.

(23) In a telescope according to the invention, the angular linear field can be greater than 6 degrees in a direction of space or the angular field can be greater than 2.5 degrees in two perpendicular directions of space. The pupil aperture is between 7 and 25.

(24) Also, the absence of aperture in the primary mirror makes it possible to increase the useful surface area by 15 to 20%, to increase the modulation transfer function in the medium frequencies and to simplify the technical production.

(25) By way of example, a Korsch telescope of 10 metre focal length with f/4 aperture can have a linear field of 60.5. In this case, the root mean square error on the wavefront or RMS WFE, the acronym for Root Mean Square WaveFront Error does not exceed /24 in all the field of the telescope.