JPS6233570B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Technical Field> The present invention relates to an improvement of a reflecting objective optical system of a Cassegrain type reflecting telescope mainly used for astronomical observation.

<Prior art> Cassegrain type and its variants are known as reflective objective optical systems whose mechanical barrel length is significantly shorter than the effective focal length, and examples of catadioptric systems include Schmidt-Cassegrain type, Maksutov-Cassegrain type,
Variants of these are known.

Theoretically, the Cassegrain system has no aberration on the axis, but the off-axis aberrations of coma, astigmatism, and curvature of field are extremely large, making it difficult to use for visual and photographic observation in a wide field of view. There is. Therefore, it is limited to applications with a narrow field of view, and it is customary in practice to set the F value to 15 or more. Furthermore, the secondary mirror is a convex hyperboloid surface mirror, which is very difficult to manufacture. A variant of the Cassegrain design in which the primary mirror has a concave elliptical surface and the secondary mirror has a convex spherical surface is usually called the Doll-Kirkam design. Although this secondary mirror is easy to manufacture, it has extremely large off-axis aberrations, so it does not contribute to improvement in optical performance.

The Schmidt-Cassegrain and Maksutov-Cassegrain types have incomplete correction of axial and off-axis aberrations and chromatic aberrations, resulting in somewhat poor image sharpness, but overall a relatively good aberration balance can be obtained.
However, in the Schmidt-Cassegrain system, a correction plate consisting of a quartic aspherical surface and a flat surface and a convex hyperboloid secondary mirror play an important role, and in the Maksutov-Cassegrain system, a thick meniscus lens called a correction lens plays an important role. , their production is technically difficult and costs considerably high.

<Object of the Invention> The object of the present invention is to provide a catadioptric objective optical system that has the characteristics of a short tube and has excellent optical performance, which can be manufactured without significant technical difficulty. There is a particular thing.

<Technical Means for Achieving the Object> In order to achieve the above object, the optical system of the present invention includes a surface reflecting primary mirror arranged with a concave surface facing the object world, and a surface reflecting primary mirror arranged with a convex surface facing the primary mirror. It consists of a secondary mirror and a concave and convex meniscus lens arranged near the primary mirror with the concave surface facing the secondary mirror,
The following optical conditions are satisfied.

0.44f≦｜R1｜≦0.80f −0.85≦k1≦−0.65 Here, R1 is the radius of curvature of the primary mirror, and k1 is the conical quadratic curve coefficient of the primary mirror.

0.16f≦｜R2｜≦0.25f 0.16f≦｜d1｜≦0.25f 0.10f≦｜R3｜≦0.20f 0.03f≦｜R4｜,｜R5｜≦0.06f 0.05f≦｜R6 ｜≦0.10f 42 ≦ν3 ν5≦38 Here, f is the effective focal length of the entire system, R2, R3,
R4, R5, and R6 are the radius of curvature of each surface of the secondary mirror, convex lens, and concave lens in the order of ray incidence, d1 is the distance between the primary mirror and secondary mirror, and ν3 and ν5 are the Atsube numbers of the convex meniscus lens and concave meniscus lens, respectively. be.

To explain the optical system of the present invention in the order in which light rays advance based on FIG. converge towards. A secondary mirror L2 with its convex surface facing the primary mirror L1 is arranged in front of the primary mirror focal point, and the light beam incident on it is reflected on the surface of the secondary mirror L2, and is gently converged due to the diverging effect of the secondary mirror until it reaches the vicinity of the primary mirror. The light enters the correction lens system located at . This correction lens system has meniscus lenses L3 and L4 close to each other, each having a concave and a concave surface, and is arranged with each concave surface facing the secondary mirror L2, and has a weak convergence effect. A hole is made in the center of the primary mirror for emitting the light beam, and the light beam that passes through the correction lens system forms an image behind the primary mirror and the correction lens system with a practically appropriate rear focal length.

The conditions of the optical system of the present invention will be explained.

The conditions for the primary mirror L1 are as follows.

(a) 0.44f≦｜R1｜≦0.80f (b) −0.85≦k1≦−0.65 (a) is the necessary condition to keep the mechanical cylinder length of the entire system sufficiently short, and (b) is the main condition. This is a necessary condition to obtain the best aberration correction when all reflective and refractive surfaces other than mirrors are spherical.

The conditions for the secondary mirror L2 are as follows.

(c) 0.16f≦｜R2｜≦0.25f (d) 0.16f≦｜d1｜≦0.25f Both conditions correct the spherical aberration, coma aberration, and astigmatism of the primary mirror while extending the focal length of the system. Also,
This is necessary to make the diameter of the secondary mirror an appropriate size.

The conditions for the correction lens systems L3 and L4 are as follows.

(e) 0.10f≦｜R3｜≦0.20f (f) 0.30f≦｜R4｜,｜R5｜≦0.06f (g) 0.05f≦｜R6｜≦0.10f (h) 42≦ν3 (i) ν5 ≦38 These are necessary conditions for correcting the remaining aberrations of the primary and secondary mirrors, correcting the chromatic aberration of the correction lens system itself, and providing the desired effective focal length f and rear focal length.

The secondary mirror L2 blocks the light beam incident on the primary mirror L1, but in order to minimize the loss of light quantity and the reduction in resolution due to diffraction, the effective diameter of L2 is approximately one-third or less of that of L1. It is appropriate to keep it. Therefore, since the distance d1 between the primary mirror and the secondary mirror should be set to approximately two-thirds of the focal length (R1/2) of the primary mirror, condition (d) is given. The focal ratio of the entire system, that is, the ratio of the effective focal length to the primary mirror effective diameter, is assumed in advance to be a value of 8 to 12, which is the same as or brighter than the conventional types described above. Here, regarding the primary mirror L1, condition (a) is given as a condition for R1 so that good aberration correction can be realized and the mechanical cylinder length of the entire system can be kept sufficiently short. Condition (c) together with (a) and (b) means that the effective focal length f
and necessary to give a suitable buggy focal length.
This shows the range of R2. The spherical aberration of the primary mirror L1 substantially cancels out the aberration of the secondary mirror L2 in the opposite direction by setting the conic coefficient k1 in the range (b).
The spherical aberration of L1 and L2 remains slightly, and the spherical aberration of L1 and L2 remains slightly.
Comatic aberration and astigmatism that are not corrected in 2 are corrected by the correction lens systems L3 and L4, but among these, it is both R3 and R6 that mainly contribute to the correction, (e) (g) Strong correction effect of R6 due to the conditions of R3
control and maintain a proper balance. The chromatic aberrations of L3 and L4 themselves are sufficiently corrected by the conditions (h) and (i) for the Abbe number and (f) for R4 and R5.

Next, in the optical system shown in Fig. 1, f=100, F
An example will be shown where =10.

Example (k1=-0.7861) R1=-60.00 R2=-19.72 R3=-15.24 R4=-3.99 R5=-3.48 R6=-7.54 d1=-22.00 d2=21.60: Distance between L2 and L3 d3=0.40 : Thickness of L3 d4 = 0.51: Distance between surfaces of L3 and L4 d5 = 0.27: Thickness of L4 N3 = 1.64328: Refractive index of L3 (d line) N5 = 1.63980: Refractive index of L4 (d line) ν3= 47.94: Atsube number of L3 (d line) ν5 = 34.57: Atsube number of L4 (d line) Effective diameter (radius) L1 = 5.00 Effective diameter (radius) L2 = 1.67 Effective diameter (radius) L3 = 1.00 Effective diameter (radius) ) L4=1.00 The aberration correction state of this example is excellent as shown in FIG. In the figure, d, C, F,
All g symbols represent spectral lines. or,
This example has excellent coma aberration as shown in FIG. 3, astigmatism as shown in FIG. 4, and distortion as shown in FIG. 5. In the figure, W represents the angle of view of the incident light beam.

<Effects of the Invention> As described above, the first effect of the optical system of the present invention relates to optical performance, and good correction can be obtained for all axial and off-axis aberrations and chromatic aberrations. The second effect is related to manufacturing, and the reflective and refractive surfaces of the secondary mirror and each unevenness correction lens are all spherical, and in addition, the dimensions are small and common materials are sufficient, so there are difficulties in manufacturing technology. There is no problem with this method, and it is also advantageous in terms of cost. Therefore, it can be said that the optical system of the present invention is significantly superior to conventional optical systems in terms of performance, manufacturing technology, and cost.

[Brief explanation of the drawing]

FIG. 1 is a side view showing the arrangement of an optical system according to an embodiment of the present invention, FIGS. 2 to 5 are diagrams showing aberration states of the optical system in FIG. 1, and FIG. 3 shows comatic aberration, FIG. 4 shows astigmatism, and FIG. 5 shows distortion. In the figure, L1 is a primary mirror, L2 is a secondary mirror, L3 is a convex meniscus lens, and L4 is a concave meniscus lens.

Claims (1)

[Scope of Claims] 1. A surface-reflecting primary mirror disposed with its concave surface facing the physical world, a surface-reflecting secondary mirror disposed with its convex surface facing the primary mirror, and a surface-reflecting secondary mirror disposed with its convex surface facing the primary mirror, each of which is located close to the primary mirror. and a concave and convex meniscus lens arranged with each concave surface facing the secondary mirror, and a light beam exits from a hole in the center of the primary mirror to the outside and forms an image behind the primary mirror. Then, the radius of curvature and conic coefficient of the primary mirror are R1 and k1, the effective focal length of the optical system is f, the radius of curvature of the secondary mirror is R2, the distance between the surfaces of the primary mirror and the secondary mirror is d1, and the The radius of curvature of the convex meniscus lens on the object side and anti-object side is R3 and
R4, the radius of curvature on the object side and anti-object side of the concave meniscus lens are R5 and R6, and the Atsube numbers of the concave and convex meniscus lens are ν3 and ν5, then the following relational expression, 0.44f≦｜R1｜≦0.80f −0.85≦k1≦−0.65 0.16f≦｜R2｜≦0.25f 0.16f≦｜d1｜≦0.25f 0.10f≦｜R3｜≦0.20f 0.03f≦｜R4｜≦0.06f 0.03f ≦｜R5｜≦ 0.06f 0.05f≦｜R6｜≦0.10f 42≦ν3 ν5≦38 A catadioptric objective optical system.