JPH10111217A - Optical axis adjusting equipment for aspherical mirror - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical axis adjusting equipment for an aspherical mirror wherein optical axis adjustment of high precision is enabled to various kinds of aspherical mirrors by using a simple equipment. SOLUTION: This light axis adjusting equipment consists of the following; a light source part 50, an objective 60 which once converges the casted light from the light source part 50, disperses it and makes it enter an off-axis parabolic mirror 10 as a mirror to be adjusted, a plane mirror 70 which reverses a luminous flux which is dispersed via the objective 60 and reflected by the off-axis parabolic mirror 10 in a backward luminous flux traveling in the reverse direction and introduces it to the off-axis parabolic mirror 10 again, and an observation optical system 80 which can observe, via the objective 60, the condition of a converged spot CS wherein the backward luminous flux which is reflected by the plane mirror 70, and further reflected by the off-axis parabolic mirror 10 is converged again by the off-axis parabolic mirror 10. When the arrangement of the off-axis parabolic mirror 10 is so slightly adjusted that the converged spot CS is minimized, the optical axis OA of an inspection equipment 100 side is made to coincide with the optical axis of the off-axis parabolic mirror 10, and arrangement adjustment of the off-axis parabolic mirror 10 is completed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical axis adjusting device for an aspherical mirror (parabolic mirror, elliptical mirror, hyperboloid mirror, etc.)
The present invention relates to an optical axis adjusting device applied to both an axis center mirror and an off-axis mirror.

[0002]

2. Description of the Related Art A spectrophotometer for performing spectroscopic analysis, a mirror projection type semiconductor exposure apparatus, and
In a high-precision imagesetter, a high-precision aspheric mirror is used. In order to make full use of the performance of these highly accurate aspheric mirrors, highly accurate optical axis adjustment is indispensable. Further, the aspherical mirror used in these devices is often an off-axis aspherical mirror having no axis center, which makes it difficult to adjust the optical axis. In such a case, for example, as shown in FIG. Such an adjustment method is used.

As shown in the figure, when the mirror to be adjusted is an off-axis parabolic mirror 10, an off-axis paraboloid is provided by a collimator device 20 having an optical system 21 using a lens and a light source device 22 such as a laser. A parallel light having a size substantially equal to the aperture of the surface mirror 10 is generated, uniformly incident on the off-axis parabolic mirror 10, and the state of the condensed spot CS collected by the off-axis paraboloid 10 (frame) The state of occurrence of aberration) is magnified by the magnifying lens 30 and projected onto the projection plate 40 or the like. Then, the projection plate 40
Is adjusted by trial and error so that the converging spot CS is minimized while observing.

[0004]

However, in the above-described method, the larger the opening of the off-axis parabolic mirror 10 is, the larger the opening of the collimator device 20 must be. Becomes large, and the performance of the collimator device 20 affects the accuracy of optical axis adjustment, which may cause insufficient optical axis adjustment.

In addition to the method shown in FIG. 11, there are various methods for adjusting the optical axis using interference (for example, Japanese Patent Publication No. 8-27444). 10 is about the measurement wavelength (for example, 0.633 μm).
m) There is a situation that it can be applied only when the product has the following limited specifications.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical axis adjusting device for an aspherical mirror which can adjust the optical axis of various aspherical mirrors with high accuracy by a simple device.

[0007]

According to a first aspect of the present invention, there is provided an optical axis adjusting device for an aspherical mirror, wherein a light source and light emitted from the light source side are once focused and then diverged.
An objective lens to be made incident on an aspherical adjustable mirror arranged in a three-dimensionally adjustable manner, and the light flux reflected by the adjusted mirror via the objective lens is inverted into a backward traveling light flux that travels in the opposite direction, and is adjusted. A reflecting means for re-directing the light to the mirror, and an observation optical system for observing, via an objective lens, a state in which the backward traveling light beam reflected by the reflecting means and further reflected by the adjusted mirror is collected again by the adjusted mirror. And characterized in that:

According to a second aspect of the present invention, there is provided an optical axis adjusting device for an aspherical mirror, wherein the observation optical system causes the imaging device and the light emitted from the light source to be incident on the objective lens, and captures the backward traveling light beam passing through the objective lens. And a branching means for making the light incident on the device.

According to a third aspect of the present invention, there is provided an optical axis adjusting device for an aspherical mirror, wherein the observation optical system includes a magnification adjusting means for adjusting an imaging magnification of a backward traveling light beam incident on the imaging device.

According to a fourth aspect of the present invention, there is provided an optical axis adjusting device for an aspherical mirror, wherein the imaging device includes a solid-state imaging device and a display device for displaying an image signal from the solid-state imaging device.

According to a fifth aspect of the present invention, in the optical axis adjusting apparatus for an aspherical mirror, the mirror to be adjusted is an off-axis type aspherical mirror, and the observation optical system transmits the light emitted from the light source to the incident light of the objective lens. The pupil is incident on a part of the pupil deviated from the optical axis.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION

[First Embodiment] FIG. 1 is a perspective view illustrating the structure of an optical axis adjusting device for an aspherical mirror according to a first embodiment.

This optical axis adjusting apparatus is used for adjusting the optical axis of an off-axis parabolic mirror 10 incorporated in an optical recording head RH incorporated in a multi-beam recording apparatus. An objective lens 60 that once converges the emitted light from the side and then diverges the light and then enters the off-axis parabolic mirror 10 that is the adjusted mirror; and the off-axis parabolic mirror 10 that is diverged through this objective lens 60 The flat mirror 70 inverts the luminous flux reflected by the mirror to a reversing luminous flux traveling in the opposite direction and guides the reflected light back to the off-axis parabolic mirror 10, and further reflected by the off-axis parabolic mirror 10 after being reflected by the plane mirror 70. An observing optical system 80 is provided for observing, via the objective lens 60, the state of the converging spot CS where the reversely traveling light beam is condensed again by the off-axis parabolic mirror 10. Note that the observation optical system 80 is provided with the imaging device unit 8 that converts the image of the converging spot CS into an electric signal.
1 and an optical isolator which is a branching unit that causes light emitted from the light source unit 50 to be incident on the objective lens 60, and also allows the backward traveling light beam from the off-axis parabolic mirror 10 passing through the objective lens 60 to be incident on the imaging device unit 81. 82.

FIG. 2 simply explains the role and structure of the optical recording head RH incorporated in the multi-beam recording apparatus. A multi-beam light source 91 having two-dimensionally arranged laser diodes 91a, and an off-axis parabolic surface for reflecting and temporarily converging a recording light beam from the multi-beam light source 91 on a base 90 serving as an adjustment table. Mirror 10;
A stereoscopic projection lens 93 for forming a reduced projection image of the multi-beam light source 91 from the light beam reflected by the off-axis parabolic mirror 10
The zoom lens 94 for adjusting the magnification and the afocal reduction lens 96 for projecting the reduced projection image onto the photosensitive material FM attached around the rotary drum RD are fixed with their arrangement precisely adjusted. You. The rotating drum R
D rotates around the central axis RA for main scanning, and the base 90 moves in synchronization with the direction of the central axis RA for sub-scanning.

FIG. 3 is a view for explaining the arrangement of the optical system in the optical axis adjusting device of FIG. The light source unit 50 includes a He-Ne laser 51 that emits linearly polarized light, and a beam expander 52 that expands the beam diameter of the light emitted from the He-Ne laser 51 while maintaining the parallel light.

The light emitted from the light source unit 50 is guided to the objective lens 60 via the optical isolator 82, and the optical isolator 82 deflects the emitted light of a specific polarization component (S-polarized light) from the light source unit 50. It is composed of a polarizing beam splitter PBS and a λ / 4 plate 85 for changing the polarization state of the outgoing light reflected by the polarizing beam splitter PBS from linearly polarized light to circularly polarized light. The He-Ne laser 51 is set so that the polarization direction of the emitted light is parallel to the reflection surface of the polarization beam splitter PBS.

The outgoing light that has passed through the optical isolator 82 is converged through the objective lens 60, once forms a converging point at the front focal point of the objective lens 60, diverges, and is uniformly distributed on the off-axis parabolic mirror 10. Incident. The light that has entered the off-axis parabolic mirror 10 is reflected as a substantially parallel light beam and directly enters the plane mirror 70. Here, the objective lens 60, the off-axis parabolic mirror 10, and the plane mirror 70 function as a so-called collimator. The light that has entered the plane mirror 70 is reflected as a backward traveling light beam while maintaining a substantially parallel light beam, and again enters the parabolic mirror 10 off-axis from the opposite direction. Off-axis parabolic mirror 10
The reversely traveling light flux reflected again at the point is once focused near the front focal point of the objective lens 60 to form a condensed spot CS, and then enters the objective lens 60 from the opposite direction. The backward traveling light beam incident on the objective lens 60 is converted into substantially parallel light and
The light travels straight through the 板 plate 85 and the polarizing beam splitter PBS, and is incident on the imaging device section 81.

The imaging device section 81 forms an infinite conjugate ratio type image forming lens 87 for converging the backward traveling light beam having passed through the polarizing beam splitter PBS, and an enlarged image of the condensed spot CS formed by the image forming lens 87. And a CCD camera 88 having a solid-state imaging device CCD for photoelectric conversion. The image signal from the CCD camera 88 is displayed on the display device 89 so as to be easily observed when the optical axis of the off-axis parabolic mirror 10 is adjusted.

Returning to FIG. 1, a jig for adjusting the position and fixing the off-axis parabolic mirror 10 and the plane mirror 70 will be described. The former off-axis parabolic mirror 10 is attached to an arrangement adjusting jig 98 provided on a base 90, and allows fine adjustment of a three-dimensional position. The support plate 98a supporting the off-axis parabolic mirror 10 is movable in parallel with respect to the y-axis, and is rotatably supported on a pin 98b about an axis parallel to the x-axis. Also, the arrangement adjusting jig 98
The whole is movable in parallel with respect to each of the x and z axes, and is rotatable about an axis parallel to the y axis. On the other hand, the latter plane mirror 70 is fixedly attached to the fixed plate 70 for attaching the multi-beam light source 91 (see FIG. 2) in a later assembling process.
In a state where the reflecting surface 70a coincides with the reference surface of the multi-beam light source 91, that is, in a state where the reflecting surface 70a is perpendicular to the optical axis OA (coincides with the optical axis of the off-axis parabolic mirror 10 after the adjustment is completed), It will be fixed on the base 90.

The base 90 on which the off-axis parabolic mirror 10 and the plane mirror 70 are mounted is suitable for the inspection apparatus 100 including the light source unit 50, the objective lens 60, and the observation optical system 80. It is aligned using the reference plane,
After this alignment, the optical axis of the off-axis parabolic mirror 10 is adjusted. Further, before the base 90 is aligned with the inspection apparatus 100, the inspection apparatus 100 itself is also preliminarily adjusted with respect to the He-Ne laser 51, the beam expander 52, and the polarization beam splitter PB.
Precise positioning is performed for each element such as S.

FIG. 4 is a view for explaining the state of incident light IB which is incident on the objective lens 60 from the light source section 50 side. The observation optical system 80 causes the incident light IB to enter a part of the entrance pupil of the objective lens 60 that is shifted from the optical axis OA. As a result, the incident light IB focused by the objective lens 60 is incident on the focal point CP, which is the front focal point of the objective lens 60, from a direction shifted from the optical axis OA.
Only the off-axis parabolic mirror 10 can be uniformly illuminated without waste. Here, the position where the incident light IB is incident on the objective lens 60 is adjusted by finely moving the polarizing beam splitter PBS along the optical axis OA. Specifically, assuming that the optical path of the incident light IB reflected by the polarizing beam splitter PBS is initially on the optical axis OA and its vicinity, the polarizing beam splitter PBS is moved in a direction away from the objective lens 60. The position at which the incident light IB enters the objective lens 60 can be shifted from the optical axis OA by an amount d corresponding to this movement amount, and accordingly, the optical axis OA of the entrance pupil of the objective lens 60 is correspondingly shifted. The incident light IB can be made incident only on a part of the area deviated from the distance.

The optical axis adjustment using the optical axis adjustment apparatus shown in FIGS. 1 and 3 is basically performed by using a condensing spot CS formed in front of the objective lens 60 by the off-axis parabolic mirror 10.
The position of the off-axis parabolic mirror 10 is finely adjusted by using the arrangement adjusting jig 98 while observing the state by the observation optical system 80. That is, regarding the condensed spot CS formed by reflecting the condensed point CP initially formed by the objective lens 60 twice on the off-axis parabolic mirror 10 via the plane mirror 70, the state of occurrence of coma aberration Are observed using the display device 89. Then, the arrangement of the off-axis parabolic mirror 10 is finely adjusted so that the converging spot CS is minimized. When the focal spot CS becomes the smallest, the focal spot CS coincides with the focal point CP which is the front focal point of the objective lens 60, and the inspection apparatus 1
The optical axis OA on the 00 side coincides with the optical axis of the off-axis parabolic mirror 10, and the adjustment of the arrangement of the off-axis parabolic mirror 10 is completed.

FIGS. 5 to 7 are diagrams for explaining an outline of a concrete optical axis adjustment using the optical axis adjusting device of FIG. 1. FIG. This is a so-called spot diagram that simulates the relationship with the state of CS. FIG. 5 shows the state of the converging spot CS when the off-axis parabolic mirror 10 is displaced only in the direction of the optical axis OA with respect to the objective lens 60. FIG. X perpendicular to the optical axis OA direction with respect to the lens 60,
FIG. 7 illustrates a case where the off-axis parabolic mirror 10 is displaced in only one of the y direction and the off-axis parabolic mirror 10 in both the x and y directions perpendicular to the optical axis OA direction with respect to the objective lens 60. Is shown. In the drawing, the off-axis parabolic mirror 10 is assumed to have no aberration, and the inspection apparatus 100 is assumed to have no aberration. Further, in the case where the optical axis OA of the off-axis parabolic mirror 10 is displaced in only one of the x and y directions (FIG. 6), and in the case where it is displaced in both the x and y directions (FIG. 7). The optical axis OA of the off-axis parabolic mirror 10 is slightly inclined to make the light incident on the observation optical system 80.

As shown in FIG. 5A, when the off-axis parabolic mirror 10 is slightly closer to the objective lens 60 in the direction of the optical axis OA, the condensed spot CS spreads almost uniformly in a circular shape. On the other hand, as shown in FIG. 5C, even when the off-axis parabolic mirror 10 is slightly separated from the objective lens 60 in the direction of the optical axis OA, the focused spot CS spreads almost uniformly in a circular shape. As shown in FIG. 5B, off-axis parabolic mirror 1
In a state where 0 is completely aligned with the objective lens 60, the condensed spot CS is almost dot-shaped.

As shown in FIG. 6A, when the optical axis of the off-axis parabolic mirror 10 is slightly displaced only in the x direction (off-axis direction), the converging spot CS becomes an ellipse from one point. Spread in shape. On the other hand, as shown in FIG. 6B, when the optical axis of the off-axis parabolic mirror 10 is slightly shifted only in the y direction, the condensed spot CS spreads in a triangular shape from one point.

FIGS. 7A to 7E show off-axis parabolic mirrors 1.
When the optical axis OA of 0 is slightly displaced in both the x and y directions, the change of the condensing spot CS with respect to the displacement of the optical axis OA with respect to the objective lens 60 is shown, and the state is slightly closer to the objective lens 60. (A) to (e) are shown in the order from (a) to (e).

As described above, FIGS. 5 to 7 theoretically show the state of the converging spot CS caused by the misalignment of the off-axis parabolic mirror 10. It can be seen from the situation of occurrence of coma of the spot CS that the direction and amount of the misalignment of the off-axis parabolic mirror 10 can be calculated to some extent.

[Second Embodiment] FIG. 8 is a view for explaining the structure of an optical axis adjusting device for an aspherical mirror according to a second embodiment. Since the second embodiment is a modification of the first embodiment, the same portions are denoted by the same reference numerals, and redundant description will be omitted.

The optical axis adjusting apparatus of the second embodiment is an apparatus for adjusting the optical axis of a parabolic mirror 110 of the axial center mirror type. For this reason, the plane mirror 170 is arranged around the optical axis OA corresponding to the shape of the parabolic mirror 110, and the optical axis OA
An opening 170a is formed in the center through which.

The optical isolator 82 receives the incident light I
B is made to enter almost the entire entrance pupil of the objective lens 60.
That is, the center of the optical path of the incident light IB reflected by the light beam splitter PBS is on the optical axis OA, and the parabolic mirror 110 of the axial center mirror type is uniformly illuminated about the optical axis OA.

In the optical axis adjusting device shown in FIG.
The relationship between the adjustment deviation of 10 and the state of the focusing spot CS is
Although different from those shown in FIGS. 5 to 7, the off-axis parabolic mirror 11 is set so that the focused spot CS is minimized.
There is no change in fine-tuning the arrangement of zeros.

[Third Embodiment] FIG. 9 is a view for explaining the structure of an optical axis adjusting device for an aspherical mirror according to a third embodiment. The third embodiment is a modification of the first embodiment.

The optical axis adjusting apparatus according to the third embodiment is an apparatus for adjusting the optical axis of the off-axis elliptical mirror 210. For this reason,
Instead of the plane mirror 70, an off-axis spherical mirror 270 is used. In this case, the objective lens 60 and the like are positioned and arranged in advance so that one of the focal points of the off-axis elliptical mirror 210, which is the adjusted mirror, coincides with the focal point CP of the objective lens 60. . Also, the spherical mirror 270
The off-axis elliptical mirror 210 is positioned and arranged in advance with respect to the objective lens 60 such that the other focal point of the elliptical mirror 210 coincides with the focal point of the spherical mirror 270. Also in the third embodiment, the incident light IB is made to enter a part of the entrance pupil of the objective lens 60 that is shifted from the optical axis OA. That is, the beam diameter of the incident light IB is narrowed, and the polarizing beam splitter PBS is moved in a direction away from the objective lens 60 from the position shown in the drawing, so that a part of the entrance pupil of the objective lens 60 shifted from the optical axis OA. The incident light IB is made incident only on.

The optical axis adjusting device according to the third embodiment can also be used as a device for adjusting the optical axis of the elliptical mirror 310 of the axial center mirror type. In this case, the center of the incident light IB is made coincident with the optical axis OA, and the spherical mirror 270 is arranged around the optical axis OA, or the off-axis elliptical mirror 310 is perpendicular to the optical axis OA at the other focal point. Replace with the placed plane mirror.

[Fourth Embodiment] FIG. 10 is a view for explaining the structure of an optical axis adjusting device for an aspherical mirror according to a fourth embodiment. The fourth embodiment is a modification of the first embodiment.

The optical axis adjusting apparatus according to the fourth embodiment is an apparatus for adjusting the optical axis of the off-axis hyperbolic mirror 410. For this reason, the spherical mirror 47 arranged off-axis instead of the plane mirror 70
0 is used. In this case, the objective lens 60 and the like are adjusted such that the point to be the real image side focal point of the off-axis hyperboloid mirror 410 as the adjusted mirror coincides with the focal point CP of the objective lens 60.
They are positioned and arranged in advance. Also, the spherical mirror 47
0, the objective lens 6 is set so that the point to be the virtual image side focal point of the off-axis hyperboloid mirror 410 coincides with the focal point of the spherical mirror 470.
0 and are positioned in advance. Also in the fourth embodiment, the incident light IB is incident on a part of the entrance pupil of the objective lens 60 that is shifted from the optical axis OA. That is, the beam diameter of the incident light IB is narrowed, and the polarizing beam splitter PBS is moved in a direction away from the objective lens 60 from the position shown in the drawing, so that a part of the entrance pupil of the objective lens 60 shifted from the optical axis OA. The incident light IB is made incident only on.

The optical axis adjusting device of the fourth embodiment can also be used as a device for adjusting the optical axis of the axial center mirror type hyperboloid mirror 510. In this case, the spherical mirror 47
0 is a spherical mirror 570 that is arranged around the optical axis OA and has an opening at the center.

Although the present invention has been described with reference to the embodiment, the present invention is not limited to the above embodiment. For example, polarizing beam splitter PBS and λ / 4
The optical isolator 82 can be configured by using a semi-transparent mirror instead of the plate 85.

If necessary, the image forming lens 87 is exchanged and the condensed spot CS projected on the CCD camera 88 is changed.
The magnification of the image can be changed.

[0040]

As described above, according to the optical axis adjusting device for an aspherical mirror of the first aspect, the light source and the light emitted from the light source side are once focused, then diverged, and adjusted three-dimensionally. An objective lens which is made incident on an aspherical adjustable mirror arranged so as to be capable of being inverted, and a light flux reflected by the adjusted mirror via the objective lens is inverted into a backward traveling light flux which travels in the opposite direction and is guided again to the adjusted mirror. A reflecting means, and an observation optical system which enables observation of, via an objective lens, a state in which the backward traveling light flux further reflected by the adjusted mirror after being reflected by the reflecting means is collected again by the adjusted mirror. By adjusting the three-dimensional arrangement of the adjusted mirror and observing, via the objective lens, the state in which the backward traveling light beam is again condensed by the adjusted mirror, precise optical axis adjustment of the adjusted mirror can be performed. Becomes possible. In this case, in the device of the first aspect, it is not necessary to provide a collimator device or the like in particular, so that the optical axis can be adjusted with high accuracy with respect to various aspherical mirrors by a simple device. Also, before the light from the light source reaches the observation optical system, the light is reflected twice in the adjusted mirror in the forward direction and the reverse direction, so that the change in the wavefront caused by the adjusted mirror is measured using a collimator or the like. Since it is twice as large as the optical axis adjustment, the adjustment accuracy is expected to be twice as large as that when such a collimator device or the like is used, and more precise adjustment is possible.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a structure of an optical axis adjusting device according to a first embodiment of the present invention.

FIGS. 2A and 2B are a stage diagram and a side view on which a mirror to be adjusted is set. FIGS.

FIG. 3 is a diagram illustrating an arrangement of an optical system of the optical axis adjusting device in FIG. 1;

FIG. 4 is a diagram illustrating an optical path of light passing through the optical system of FIG.

FIG. 5 is a diagram illustrating optical axis adjustment using the optical axis adjustment device of FIG. 1;

FIG. 6 is a diagram illustrating optical axis adjustment using the optical axis adjustment device of FIG. 1;

FIG. 7 is a diagram illustrating optical axis adjustment using the optical axis adjustment device of FIG. 1;

FIG. 8 is a diagram illustrating an optical system of an optical axis adjustment device according to a second embodiment.

FIG. 9 is a diagram illustrating an optical system of an optical axis adjusting device according to a third embodiment.

FIG. 10 is a diagram illustrating an optical system of an optical axis adjusting device according to a fourth embodiment.

FIG. 11 is a diagram illustrating a conventional optical axis adjustment method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Off-axis parabolic mirror 50 Light source part 60 Objective lens 70 Plane mirror 80 Observation optical system 81 Imaging device part 82 Optical isolator 87 Imaging lens 88 CCD camera 90 Base 98 Arrangement jig 100 Inspection device CS Focusing spot

Claims (5)

[Claims] A light source; an objective lens that once converges light emitted from the light source side, diverges the light, and makes the light incident on an aspherical type adjustable mirror arranged so as to be three-dimensionally adjustable; A reflecting unit that inverts the light flux reflected by the adjusted mirror through the objective lens into a backward traveling light flux that travels in the opposite direction and guides the reflected light back to the adjusted mirror; and An optical axis adjusting device for an aspherical mirror, further comprising: an observation optical system that enables observation of the state in which the reflected backward traveling light beam is collected again by the adjusted mirror through the objective lens. . 2. The observation optical system includes: an imaging device; and a branching unit that causes the emitted light from the light source to enter the objective lens and causes the backward traveling light flux passing through the objective lens to enter the imaging device. The optical axis adjusting device for an aspheric mirror according to claim 1, further comprising: 3. The optical axis adjustment for an aspherical mirror according to claim 1, wherein the observation optical system includes a magnification adjusting unit that adjusts an imaging magnification of the backward traveling light beam incident on the imaging device. apparatus. 4. The optical axis adjusting device for an aspherical mirror according to claim 1, wherein the imaging device includes a solid-state imaging device and a display device that displays an image signal from the solid-state imaging device. 5. The mirror to be adjusted is an off-axis aspherical mirror, and the observation optical system outputs light emitted from the light source side.
2. The optical axis adjusting device for an aspherical mirror according to claim 1, wherein the light is incident on a part of the entrance pupil of the objective lens which is shifted from the optical axis.