JP2000199852A - Off-axis catoptric system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form an extremely excellent image though a system is compact. SOLUTION: This off-axis catoptric system for forming the image of a long- distance object at a position out of an optical axis is provided with a 1st concave mirror M1 having positive power, a convex mirror M2 having negative power and a 2nd concave mirror M3 having positive power in order from a light incident side. The mirrors M1 and M2 substantially constitute an afocal optical system, and a diaphragm S is arranged in an optical path between the mirrors M2 and M3, and the reflection surface shape of the mirrors M1, M2 and M3 is made aspherical.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reflection optical system for forming an image of a distant object.

[0002]

2. Description of the Related Art A three-reflection anastigmat telescope is known as an optical system having a large aperture, covering a wide field and a broad wavelength range. For example,
U.S. Pat. No. 4,240,707 discloses a reflecting optical system including three reflecting mirrors having concave and convex portions.

[0003]

However, the above-mentioned conventional off-axis reflecting optical system has a large aperture and good image forming performance over a wide angle of view, but generally the size of the optical system becomes large. There's a problem. For example, when observing the earth from a platform such as an artificial satellite, the weight and size of the observation equipment occupied by the satellite are limited, and it is not always preferable to use a conventional off-axis three-reflection optical system. In the above-mentioned prior art reflecting optical system, the overall length is as large as about 1.1 times the focal length, and furthermore, since it is a system without off-center shielding used off-axis, there is a problem that the size in the radial direction is large.

Accordingly, an object of the present invention is to provide an off-axis reflecting optical system capable of forming an extremely excellent image while being a compact optical system having a short overall length and a small size in a radial direction.

[0005]

In order to achieve the above object, a reflection optical system according to the present invention is an off-axis reflection optical system for forming an image of a distant object at a position off the optical axis. A first concave mirror having a positive power, a convex mirror having a negative power, and a second concave mirror having a positive power, wherein the first concave mirror and the convex mirror substantially form an afocal optical system. The diaphragm is arranged in an optical path between the convex mirror and the second concave mirror, and the first concave mirror, the convex mirror, and the second concave mirror have an aspherical reflecting surface.

In the off-axis reflecting optical system according to the second aspect of the present invention, the stop is disposed at a position decentered with respect to the optical axis, based on the configuration of the reflecting optical system of the first aspect. An off-axis reflecting optical system according to a third aspect of the present invention is based on the configuration of the reflecting optical system of the first aspect, wherein a plane reflecting mirror is disposed at the position of the stop. Also,
The off-axis reflecting optical system according to the fourth aspect of the present invention satisfies the following conditions based on the configuration of the reflecting optical system according to the first to third aspects.

Φ> φ1 / (1 + M) where φ: power of the entire system φ1: power of the first concave mirror M: angular magnification of the afocal optical system.

In order to achieve the above object, a reflecting optical system according to the present invention is an off-axis reflecting optical system for forming an image of a distant object at a position off the optical axis. In order from the incident side, it has a first concave mirror having a positive power, a convex mirror having a negative power, and a second concave mirror having a positive power. The first concave mirror and the convex mirror substantially constitute an afocal optical system. A stop is disposed on the convex mirror at a position eccentric with respect to the optical axis, and the first concave mirror, the convex mirror, and the second concave mirror have an aspherical reflecting surface shape.

[0009]

As described above, in the off-axis reflecting optical system according to the first aspect of the present invention, the first concave mirror having a positive power and the convex mirror having a negative power substantially constitute an afocal system, and the convex mirror is formed. Since the stop is arranged between the second concave mirror and the second concave mirror, it is possible to achieve a much smaller size than the conventional three-piece reflecting optical system. Further, by making each reflecting mirror aspherical, it is possible to satisfactorily correct aberrations other than the Petzval sum and achieve excellent imaging performance.

In this configuration, it is preferable that the stop be disposed at a position decentered with respect to the optical axis of the off-axis reflecting optical system. With this configuration, it is possible to further improve the imaging performance. Further, in the first aspect of the present invention, if a plane reflecting surface is disposed at the position of the stop, the optical path can be folded back by the plane reflecting surface, and the entire optical system can be made compact.

In any one of the above-mentioned structures, the off-axis reflecting optical system preferably satisfies φ> φ1 / (1 + M). Where φ: power of the entire system φ1: power of the first concave mirror M: angular magnification of the afocal optical system

In the present invention, the powers of the first concave mirror, the convex mirror, and the second concave mirror are φ1, φ2, and φ3, respectively, the power of the entire system is φ, and each magnification of the afocal system constituted by the first concave mirror and the convex mirror is When M is set, (a) M = −φ2 / φ1 (b) φ3 = Mφ is satisfied. Here, (c) When the relationship of φ3> (φ1φ2) / (φ1 + φ2) is satisfied, it is possible to achieve a small Petzval sum and to achieve a reduction in the size of the entire system.

When the above equations (a) to (c) are put together, φ> φ1 / (1 + M) is obtained. Therefore, when the value is out of the range of the conditional expression, it is disadvantageous not only in reducing the size of the entire system, but also in increasing the angle of view because it becomes difficult to reduce the Petzval sum. .

In the off-axis reflecting optical system according to the fifth aspect of the present invention, since a stop eccentric to the optical axis is arranged on the convex mirror, the distance between the convex mirror and the second concave mirror is reduced. Thus, downsizing can be achieved.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, numerical embodiments according to the present invention will be described with reference to the drawings. The off-axis reflecting optical system of the following numerical examples is used for forming a two-dimensional image by scanning by a push bloom method (scanning a one-dimensional sensor arranged in an annular field in a direction perpendicular to the one-dimensional sensor). The F-number is 4 to 6, and the angle of view is 5 ° or more.

FIG. 1 is an optical path diagram of an off-axis reflecting optical system according to a first embodiment. In FIG. 1, the off-axis reflecting optical system has a first concave mirror M1, a convex mirror M2, and a second concave mirror M3. Here, the centers of curvature of the respective reflecting mirrors M1 to M3 are arranged on the optical axis of the off-axis reflecting optical system, forming a coaxial system. Further, the first concave mirror M1 and the convex mirror M2
Are substantially coincident with each other, and the first concave mirror M1 and the convex mirror M2 substantially form an afocal system.

In FIG. 1, a light beam from a distant object travels at a predetermined angle with respect to the optical axis toward an entrance pupil located behind the first reflecting mirror M1. After being reflected by M1, the light is reflected by the convex mirror M2, and is reflected by the second reflecting mirror M2 via the stop S. The light beam from the second reflecting mirror is condensed at a predetermined position outside the optical axis to form an object image. Here, the good image range in the image plane (the range in which a good image is formed in the image plane) is an annular field located at a position deviated from the optical axis. If a film or the like is arranged, an object image can be obtained.

The size and shape of the reflecting surface of each of the reflecting mirrors M1 to M3 are determined so that the light flux reaching the good image area on the image plane is not blocked by each of the reflecting mirrors. The stop S is disposed substantially at the front focal plane of the second concave mirror, and the off-axis reflecting optical system of the first embodiment is an image-side telecentric optical system. Tables 1 to 3 below show optical data of the off-axis reflecting optical system of the first embodiment. In Table 1, F represents the F number, f represents the focal length of the entire system, and the unit of the radius of curvature and the surface spacing is mm as an example, and the sign of the surface spacing is reversed in sign every time the light passes through the reflecting surface. And Also,
The aspherical optical surfaces are marked with * at the surface number, and the aspherical data is shown in Table 2. In each table, "En" represents 10 to the nth power. In this embodiment,
Considering the tangent plane at the vertex of the aspheric surface, when the distance measured from the optical axis on the tangent plane is y, and the displacement (sag amount) from the tangent plane in the direction along the optical axis is Z, the aspherical shape The equation is given by the following equation (d).

[0019]

(Equation 1)

Where r: radius of curvature of the vertex, κ: cone coefficient, A: fourth-order aspherical coefficient, B: sixth-order aspherical coefficient, C: eighth-order aspherical coefficient, D: tenth-order aspherical coefficient ,.

Table 3 shows numerical values corresponding to the conditions of the first embodiment. In Table 3, φ (= 1 / f) is the power of the whole system, φ1 is the power of the first concave mirror M1, φ2 is the power of the convex mirror, φ3 is the power of the second concave mirror, and M is the first concave mirror M1.
And the angular magnification of the afocal system constituted by the convex mirror M2.

[0022]

[Table 1]

[0023]

[Table 2] [First surface (first concave mirror M1)] κ: 0.465694 A: 0.140575E-09 B: 0.663412E-16 C: 0.529192E-22 D: 0.727873E-28 [Second surface (convex mirror M2)] ] Κ: -0.284650 A: 0.812558E-09 B: 0.000000E + 00 C: 0.000000E + 00 D: 0.000000E + 00 [Fourth surface (second concave mirror M3)] κ: 0.040075 A: 0.000000E + 00 B : 0.255857E-15 C: 0.000000E + 00 D: 0.000000E + 00

[0024]

[Table 3] φ1 = 1.768E-3 φ2 = -3.258E-3 φ3 = 2.188E-3 M = 1.842 φ1 / (1 + M) = 6.22E-4 φ = 0.001 Off-axis reflecting optical system of the first embodiment. Fig. 2 shows the lateral aberration diagram of
FIG. 3 shows a spot diagram.

In the transverse aberration diagram of FIG. 2, the optical axis of the off-axis reflecting optical system is the Z axis, the axis perpendicular to the Z axis, the axis in the plane of FIG. 1 is the Y axis, and the axis perpendicular to the YZ plane is the axis. Assuming the Z axis, FIG. 2A shows that the incident angle on the YZ plane is −7.10.
When the incident angle in the XZ plane is 2.50 °, the lateral aberration of the light beam in the YZ section in the light flux on the entrance pupil plane is shown,
FIG. 2B shows the lateral aberration of the light beam in the YZ section in the light beam on the entrance pupil plane when the incident angle on the YZ plane is −7.10 ° and the incident angle on the XZ plane is 1.75 °.
(C) shows that the incident angle on the YZ plane is −7.10 ° and the XZ
The horizontal aberration of the light beam in the YZ section in the light beam on the entrance pupil plane when the incident angle on the plane is 0.00 ° is shown. FIG.
(D) shows the case where the incident angle on the YZ plane is -7.10 [deg.] And XZ
FIG. 2 shows the lateral aberration of the light beam in the XZ section in the light beam on the entrance pupil plane when the incident angle on the plane is 2.50 °.
(E) shows that the incident angle on the YZ plane is −7.10 ° and the XZ
FIG. 2 shows the lateral aberration of the light beam on the XZ section in the light beam on the entrance pupil plane when the incident angle on the plane is 1.75 °.
(F) shows that the incident angle on the YZ plane is -7.10 ° and the XZ
The horizontal aberration of the light beam in the XZ section in the light beam on the entrance pupil plane when the incident angle on the plane is 0.00 ° is shown. And
FIG. 3A is a spot diagram on the image plane when the incident angle on the YZ plane is −7.10 ° and the incident angle on the XZ plane is 2.50 °, and FIG. At -7.10 °, the incident angle on the XZ plane is 1.
Spot diagram at the image plane at 75 °, FIG.
(C) shows that the incident angle on the YZ plane is −7.10 ° and the XZ
4 is a spot diagram on an image plane when an incident angle on a plane is 0.00 °.

As described above, it can be seen that the off-axis reflecting optical system according to the first embodiment achieves excellent imaging performance despite its compactness. FIG.
9 is an optical path diagram showing a modification of the first embodiment, in which a plane reflecting surface M4 is provided at the position of the stop S in the off-axis reflecting optical system of the first embodiment. As is clear from FIG. 4, the optical system according to the first embodiment is more compact. In this modification, the optical data is substantially the same as the optical data in Tables 1 to 3 described above (only the signs of the surface intervals after the third surface in Table 1 are reversed). The description of the imaging performance is omitted.

[Second Embodiment] FIG. 5 is an optical path diagram of an off-axis reflecting optical system of a second embodiment. In FIG. 5, the off-axis reflecting optical system has a first concave mirror M1, a convex mirror M2, and a second concave mirror M3. Here, the centers of curvature of the respective reflecting mirrors M1 to M3 are arranged on the optical axis of the off-axis reflecting optical system, forming a coaxial system. Further, the first concave mirror M1 and the convex mirror M2
Are substantially coincident with each other, and the first concave mirror M1 and the convex mirror M2 substantially form an afocal system.

In FIG. 5, a light beam from a distant object travels at a predetermined angle with respect to the optical axis toward an entrance pupil located behind the first reflecting mirror M1, and this first reflecting mirror After being reflected by M1, the light is reflected by the convex mirror M2, and is reflected by the second reflecting mirror M2 via the stop S. The light beam from the second reflecting mirror is condensed at a predetermined position outside the optical axis to form an object image. Here, the good image range in the image plane is within an annular field located at a position separated from the optical axis. The size and shape of the reflecting surface of each of the reflecting mirrors M1 to M3 is determined so that the light flux reaching the good image area on the image plane is not blocked by each of the reflecting mirrors.

In the second embodiment, the stop S is substantially on the front focal plane of the second concave mirror, is eccentric from the optical axis, and is inclined with respect to the optical axis. The off-axis reflecting optical system is an image-side telecentric optical system.
Tables 4 to 6 below show optical data of the off-axis reflecting optical system of the second embodiment. In Table 4, F represents the F number, f represents the focal length of the entire system, the unit of the radius of curvature and the surface interval is mm as an example, and the sign of the surface interval reverses the sign every time the light passes through the reflecting surface. And In addition, an aspherical optical surface is marked with * at the surface number, and the aspherical data is shown in Table 5. The position of the stop S is also shown in Table 5. In Table 5, α is a rotation direction having a positive counterclockwise direction in the plane of FIG. 5, and Y is a Y-axis direction having a positive upper part in the plane of FIG. It is. In addition, the aspherical shape is the above (d)
Given by the formula. In each table, "En" represents 10 to the nth power.

Table 6 shows numerical values corresponding to the conditions in the second embodiment.
In Table 6, φ (= 1 / f) is the power of the entire system, φ1 is the power of the first concave mirror M1, φ2 is the power of the convex mirror, and φ3
Represents the power of the second concave mirror, and M represents the angular magnification of the afocal system constituted by the first concave mirror M1 and the convex mirror M2.

[0031]

[Table 4]

[0032]

[Table 5] [First surface (first concave mirror M1)] κ: 0.490509 A: 0.140320E-09 B: 0.734189E-16 C: 0.419062E-22 D: 0.104637E-27 [Second surface (convex mirror M2)] Κ: -0.529962 A: 0.534990E-09 B: 0.000000E + 00 C: 0.000000E + 00 D: 0.000000E + 00 [3rd surface (aperture S)] α: -16.000000 ° Y: -8.178857 [4th Surface (second concave mirror M2)] κ: 0.074597 A: 0.000000E + 00 B: 0.268568E-15 C: 0.000000E + 00 D: 0.000000E + 00

[0033]

[Table 6] φ1 = 1.770E-3 φ2 = -3.871E-3 φ3 = 2.188E-3 M = 2.188 φ1 / (1 + M) = 5.55E-4 φ = 0.001 Off-axis reflecting optical system of the second embodiment. Fig. 6 shows the lateral aberration of
FIG. 7 shows a spot diagram.

In the transverse aberration diagram of FIG. 6, the optical axis of the off-axis reflecting optical system is the Z axis, the axis perpendicular to the Z axis, the axis in the plane of FIG. 5 is the Y axis, and the axis perpendicular to the YZ plane is the axis. Assuming the Z axis, FIG. 6A shows that the incident angle on the YZ plane is −7.10.
When the incident angle in the XZ plane is 2.50 °, the lateral aberration of the light beam in the YZ section in the light flux on the entrance pupil plane is shown,
FIG. 6B shows the lateral aberration of the light beam of the YZ section in the light beam on the entrance pupil plane when the incident angle on the YZ plane is −7.10 ° and the incident angle on the XZ plane is 1.75 °.
(C) shows that the incident angle on the YZ plane is −7.10 ° and the XZ
The horizontal aberration of the light beam in the YZ section in the light beam on the entrance pupil plane when the incident angle on the plane is 0.00 ° is shown. FIG.
(D) shows the case where the incident angle on the YZ plane is -7.10 [deg.] And XZ
FIG. 6 shows the lateral aberration of the light beam in the XZ section in the light beam on the entrance pupil plane when the incident angle on the plane is 2.50 °.
(E) shows that the incident angle on the YZ plane is −7.10 ° and the XZ
FIG. 6 shows the lateral aberration of the light beam on the XZ section in the light beam on the entrance pupil plane when the incident angle on the plane is 1.75 °.
(F) shows that the incident angle on the YZ plane is -7.10 ° and the XZ
The horizontal aberration of the light beam in the XZ section in the light beam on the entrance pupil plane when the incident angle on the plane is 0.00 ° is shown. And
FIG. 7A is a spot diagram on the image plane when the incident angle on the YZ plane is −7.10 ° and the incident angle on the XZ plane is 2.50 °, and FIG. At -7.10 °, the incident angle on the XZ plane is 1.
Spot diagram at the image plane at 75 °, FIG.
(C) shows that the incident angle on the YZ plane is −7.10 ° and the XZ
4 is a spot diagram on an image plane when an incident angle on a plane is 0.00 °.

As described above, it can be seen that the off-axis reflecting optical system according to the second embodiment achieves excellent imaging performance despite its compactness. In the off-axis reflecting optical system of the second embodiment, it is also possible to dispose a flat reflecting surface at the position of the stop S as in the modification of the first embodiment. Third Embodiment FIG. 8 is an optical path diagram of an off-axis reflecting optical system according to a third embodiment. In FIG. 8, the off-axis reflecting optical system includes:
It has a first concave mirror M1, a convex mirror M2, and a second concave mirror M3. Here, the centers of curvature of the respective reflecting mirrors M1 to M3 are arranged on the optical axis of the off-axis reflecting optical system, forming a coaxial system. Further, the centers of curvature of the first concave mirror M1 and the convex mirror M2 substantially coincide with each other, and the first concave mirror M1 and the convex mirror M2 substantially form an afocal system.

In FIG. 8, a light beam from a distant object travels at a predetermined angle with respect to the optical axis toward an entrance pupil located behind the first reflecting mirror M1. After being reflected by M1, the light is reflected by the convex mirror M2 and then reflected by the second reflecting mirror M2. The light beam from the second reflecting mirror is condensed at a predetermined position outside the optical axis to form an object image.
Here, the good image range in the image plane is within an annular field located at a position separated from the optical axis. The size and shape of the reflecting surface of each of the reflecting mirrors M1 to M3 are determined so that the light flux reaching the good image area on the image plane is not blocked by each of the reflecting mirrors.

In the third embodiment, the stop S is substantially on the convex mirror M2 and is arranged eccentrically from the optical axis. With this configuration, the distance between the convex mirror M2 and the second concave mirror M3 can be reduced. The total length of the off-axis reflecting optical system of this embodiment is reduced to about the focal length of the second concave mirror M3. Tables 7 to 9 below show optical data of the off-axis reflecting optical system of the third example. In Table 7, F
Denotes the F number, f denotes the focal length of the entire system, and the unit of the radius of curvature and the surface spacing is mm as an example, and the sign of the surface spacing is reversed every time the light passes through the reflecting surface. In addition, an aspherical optical surface is marked with * at the surface number, and the aspherical data is shown in Table 8. Table 8 also shows the position of the stop S. In Table 8, Y is the Y-axis direction with the upper part in the plane of FIG. 8 being positive. The aspheric shape is given by the above equation (d). In each table, "En" represents 10 to the nth power.

Table 9 shows numerical values corresponding to the conditions of the third embodiment.
In Table 9, φ (= 1 / f) is the power of the entire system, φ1 is the power of the first concave mirror M1, φ2 is the power of the convex mirror, and φ3
Represents the power of the second concave mirror, and M represents the angular magnification of the afocal system constituted by the first concave mirror M1 and the convex mirror M2.

[0039]

[Table 7]

[0040]

[Table 8] [First surface (first concave mirror M1)] κ: 0.305421 A: 0.166975E-09 B: 0.409171E-16 C: 0.164100E-21 D: -0.315285E-27 [Second surface (Aperture S )] Y: 67.142857 [Third surface (convex mirror M2)] κ: 0.658059 A: 0.252641E-08 B: 0.000000E + 00 C: 0.000000E + 00 D: 0.000000E + 00 [Fourth surface (second concave mirror M3) )] Κ: -0.349477 A: 0.000000E + 00 B: 0.977684E-15 C: 0.000000E + 00 D: 0.000000E + 00

[0041]

[Table 9] φ1 = 1.848E-3 φ2 = -4.041E-3 φ3 = 2.188E-3 M = 2.188 φ1 / (1 + M) = 5.80E-4 φ = 0.001 Off-axis reflecting optical system of the third embodiment. Fig. 9 shows the lateral aberration diagram of
FIG. 10 shows a spot diagram.

In the transverse aberration diagram of FIG. 9, the optical axis of the off-axis reflecting optical system is the Z axis, the axis perpendicular to the Z axis, the axis in the plane of FIG. 8 is the Y axis, and the axis perpendicular to the YZ plane is the axis. Assuming the Z axis, FIG. 9A shows that the incident angle on the YZ plane is −7.10.
When the incident angle in the XZ plane is 2.50 °, the lateral aberration of the light beam in the YZ section in the light flux on the entrance pupil plane is shown,
FIG. 9B shows the lateral aberration of the light beam in the YZ section in the light beam on the entrance pupil plane when the incident angle on the YZ plane is −7.10 ° and the incident angle on the XZ plane is 1.75 °.
(C) shows that the incident angle on the YZ plane is −7.10 ° and the XZ
The horizontal aberration of the light beam in the YZ section in the light beam on the entrance pupil plane when the incident angle on the plane is 0.00 ° is shown. FIG.
(D) shows the case where the incident angle on the YZ plane is -7.10 [deg.] And XZ
FIG. 9 shows the lateral aberration of the light beam on the XZ section in the light beam on the entrance pupil plane when the incident angle on the plane is 2.50 °.
(E) shows that the incident angle on the YZ plane is −7.10 ° and the XZ
FIG. 9 shows the lateral aberration of the light beam on the XZ section in the light beam on the entrance pupil plane when the incident angle on the plane is 1.75 °.
(F) shows that the incident angle on the YZ plane is -7.10 ° and the XZ
The horizontal aberration of the light beam in the XZ section in the light beam on the entrance pupil plane when the incident angle on the plane is 0.00 ° is shown. And
FIG. 10A shows that the incident angle on the YZ plane is −7.10 °.
, The spot diagram on the image plane when the incident angle on the XZ plane is 2.50 °, and FIG. 10B shows the case where the incident angle on the YZ plane is −7.10 ° and the incident angle on the XZ plane is 1.75 °. FIG. 10C is a spot diagram on the image plane when the incident angle on the YZ plane is −7.10 ° and the incident angle on the XZ plane is 0.00 °.

As described above, it can be seen that the off-axis reflecting optical system according to the third embodiment achieves excellent imaging performance despite its compactness.

[0044]

According to the present invention, the first concave mirror / convex mirror
By making each reflecting mirror of the off-axis reflecting optical system of the second concave mirror having no center shielding and having an aspherical surface and the first concave mirror and the convex mirror being an afocal system, the size of the optical system can be reduced as compared with the conventional one. Further, it has become possible to provide an off-axis reflecting optical system having excellent optical performance.

[Brief description of the drawings]

FIG. 1 is an optical path diagram of a first embodiment according to the present invention.

FIG. 2 is a lateral aberration diagram of the first example.

FIG. 3 is a spot diagram of the first embodiment.

FIG. 4 is an optical path diagram showing a modification of the first embodiment.

FIG. 5 is an optical path diagram of a second embodiment according to the present invention.

FIG. 6 is a lateral aberration diagram of the second embodiment.

FIG. 7 is a spot diagram of a second embodiment.

FIG. 8 is an optical path diagram of a third embodiment according to the present invention.

FIG. 9 is a lateral aberration diagram of the third example.

FIG. 10 is a spot diagram of the third embodiment.

[Explanation of symbols]

 M1: first concave mirror M2: convex mirror M3: second concave mirror M4: plane reflecting mirror S: stop

Claims (5)

[Claims] 1. An off-axis reflecting optical system for forming an image of a distant object at a position off the optical axis, a first concave mirror having a positive power, a convex mirror having a negative power, and a A second concave mirror, wherein the first concave mirror and the convex mirror substantially constitute an afocal optical system, and an aperture is disposed in an optical path between the convex mirror and the second concave mirror; An off-axis reflecting optical system, wherein the reflecting surfaces of the first concave mirror, the convex mirror, and the second concave mirror are aspherical. 2. The off-axis reflecting optical system according to claim 1, wherein said stop is disposed at a position decentered with respect to said optical axis. 3. The off-axis reflecting optical system according to claim 1, wherein a plane reflecting mirror is disposed at a position of said stop. 4. The off-axis reflecting optical system according to claim 1, wherein the following condition is satisfied. φ> φ1 / (1 + M) where φ: power of the entire system φ1: power of the first concave mirror M: angular magnification of the afocal optical system 5. An off-axis reflecting optical system for forming an image of a distant object at a position off the optical axis, wherein a first concave mirror having a positive power, a convex mirror having a negative power, and a A first concave mirror and the convex mirror substantially constitute an afocal optical system, and an aperture is disposed on the convex mirror at a position eccentric to the optical axis. The off-axis reflecting optical system, wherein the first concave mirror, the convex mirror, and the second concave mirror have an aspherical reflecting surface shape.