CA1103498A - Wide annulus unit power optical system - Google Patents

Abstract

A B S T R A C T

This invention provides, in a restricted off-axis field optical system having a broad spectral range, which includes refracting elements, the improvement comprising:
an optical system constructed and arranged so that the Petzval sum is substantially zero and the refracting elements balance the effects of the variation in the Petzval sum due to variation in color by introducing axial chromatic aberration of the opposite sense so that the position of focus at the off-axis field remains substantially constant.

Description

BACKGROUND OF THE INVENTION
This invention relates to optical systems for forming an image of an object at unit magnification, and more particularly to such a system having a large spectral range. Optical systems constructed in accordance with the concepts of this invention are particularly adapted, among many other possible uses, for effecting the exposure of photoresist-coated semi-:
conductor wafers in the manufacture of integrated circuits.

The present invention is closely related to the annular projection system of the type disclosed in my prior U.S. Patent -`

~; No. 3/748,015 issued July 24, 1973 and assigned to the same Assignee as the instant application. My prior patent discloses ~ a catoptric system for forming in accurate micro detail an ; image of an object at unit magnification with high resolution, ; characterized by convex and concave spherical mirrors having their centers of curvature coinciding at a single point. The . ~ , mirrors are arranged to produce at least three reflections within the systemj and they are used in the system with their axial conjugates at said point and to provide two off axis conjugate areas at unit magnification in a plane which contains the center of curvature, the axis of the system being an axis normal to the latter plane and through said point. While this : .
optical system has many features and advantages, the present invention is directed to improvements thereover, which will become apparent as the description proceeds.
Other related patents in this field include U.S. Patent No. 3,821,763, issued June 28, 1974, U.S. Patent No. 3,951,546 issued April 20, 1976 and U.S. Patent No~ 4,011,011 issued March 8, 1977 . These patents are assigned to the same Assignee as the present invention.

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SUMM~RY OF THE INVENTION
In order to accomplish the ~esired results, the invention - provides, in one form thereof, a new and improved annular field optical system for use in apparatus for photographically exposiny an image receiving surface to a light image of an object. This ;~ optical systPm includes at least one convex and one concave mirror, which are substantially concentrically arranged along an optical . .
axis. The system is arranged to form conjugate planes normal to said axis for which the system is of unit power. The system further includes at least one pair of symmetrically disposed nearly . .:
concentric meniscus elements whose convex radii are larger than ., .
their con~ave radii and whose thickness is graater than the differ-ence between their convex and concave radii.
According to one aspect of the invention, color compensating means are interposed between the mirroxs and the object and image locations which, in one form thereof, is a substantially plane parallel plate mounted normal to the optical axis oE themmirrors.
~referably, one of the faces of the plane parallel plate is made .
~ aspheric, or the plate is a meniscus element mounted normal t~

..~ ao the optical axis o~ said mirrors.

- In accordance with another aspect of the invention, the con-: ..
cave and convex mirrors are supported with a distance betwePn their ; centers of curvature of less than about two percent of thellength ;~ of the shorter radius, and according to another aspect thereof, the , ~' mirrors are arranged so that there are three reflections from the concave mirror and two reflections from the convex mirror.
-~ ~ In one form thereof, the invention provides a new and improved i annular field optical system for use in apparatus for photographi-` cally exposing an image receiving surface to a light image of an .; .
object which includes: a first half and a second half with each ; half including a unit optical system having an optical axis and - having conjugate p~nes substantially normal to that axis ~or

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which the system is of unit power. These two halves are coaxially disposed in back-to~back relationship so that the conjugate planes are superposed on at least one side of the optical system to form an intermediate image locatioh~ and provision is made for providing spaced object and image locations on the other side of the optical system. According to one aspect of the invention, each half of the optical system includes a nearly concentric meniscus element whose convex radius is larger than its concave radius and whose thickness is greater ~han the difference between its convex and concave radiiJ~
and according to another aspect at least one color correcting ; element is provided in the system.
.....
In one embodiment each unit optical system includes a concave spherical mirror and a convex spherical mirror facing the concave mirror, said mirrors being supported with their centers o~ curva-ture substantially coincident. Means are provided to define a location for an object ~he image o which is a real image at a second location, with said convex mirror ~eing positioned to re-;~ ~ flect to the concave mirror light from the object location initially reflected to the convex mirror from the concave mirror ! where~y light from the object location will be reflected at least twice atthe concave mirror and at least once at the convex mirror before being focused at the second location. T~e small aberrations aris-- ing from the departure of the meniscus from concentricity are -; compensated for by introducing a small departure from concentricity in the mirror pair. That is, in each half of the optical system, the concave and convex mirrors are supported with a distance be-tween their centers of curvature of less than about two percent of the length of the shorter radius.
Also, preferably, the color correction element is a plane parallel plate mounted normal to the optical axis of the mirrors.

In some forms of the invention, the plane parallel color correct-ing plate is mounted at the intermediate image location, and in ., .
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other forms a color correcting plate is mounted in each half of the system.
In some embodiments of the invention, the two halves of the optical system are coaxially disposed in back-to-back relationship so that the conjugate planes are superposed on both sides of the system to form an intermediate image location on one side and a superposed object and image location of the other side~ Means, such as folding mirrors, are provided to form spaced object and image locations to make them accessible for practical installations.
In other embodiments, the two halves of the optical system are coaxially disposed in back-to-back relationship to form an inter-madiate image location on on0 side of the optical system and to form spaced o~ject and image locations on the other side. For this purpose, the intermediate image is spaced axially from the other conjugate in at least one-half of the system. In some embodiments, the distance from the two mirror components to the intermediate image is greater than the distance to the o~her conjugate location in at least one-half, thereby spacing said object and image loca-tions one from the other. In still further embodiments, the distance from the two mirror components to the intermediate image is less than the distance to the other conjugate location in at -. ~
least one-h~lf, thereby forming crossed object and image planes.

In this case, reflecting means are interposed between the object and image locations to make them physically accessible.

It will be appreciated that a necessary co~dition for the ~- absence of field curvature (i.e., for a flat image surface) in --- the resultant system is that the algehraic sum of the quantities , ;~ obtained by dividing the power of each surface by its index of~ ,., refraction be substantially zero, the index of refraction of a reflecting surface being defined as negative one for this compu-tation. The meniscus refracting elements, which are essential components of the optical systems in accordance with the invention, result in a chromatic variation of the above mentioned sum so that even though it is substantially zero, there is a resultant varia-tion of the field curvature with color. In accordance with the invention, the focal posi~ion is made constant for a broad spectral range by the introduction of a longitudinal chromatic difference of focus which compensates for the chromatic variation of field curvature in the annular field of the optical s~stem.
There has thus been outlined rather broadly the more impor-tant features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated~ ~here are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto. Those skilled in the ; art will appreciate that the conception upon which the disclosure is based may readily be utilized as a basis for the designing of , other systems for carrying out the several purposes of the invention. It is important, therefore, that the claims be regarded as including such equivalent systems as do not depart from the spirit and scope of the invention.
Specific embodiments of the invention have been chosen for purposes of illustration and description, and are shown in the accompanying drawings, forming a part of the speci~ication.
BRIEF DESCRIPTION OF THE~DRAWINGS
_____________________________ :
Fig. 1 is a schematic repr sentation of an optical system, constructed in accordance with the concepts of the present inven-tion;
Fig. 2 is a schematic representation of a double optical system, wherein two optical systems similar to the system of Fig. 1 are mounted in back-to-back relationship;
Fig. 3 is a schematic representation of an optical system similar to the system of Fig. 2, but simpliied by removing .

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the requirement that each half of the system be symmetrical, while keeping the system symmetrical as a whole;
; Fig. 4 is a schematic representation of an optical system similar to the system of Fig. 3, but simplified by combining the two color correcting plates into a single element o~ the same form and by elimina~ing the folding flats required to separate ` the object and final image by making the conjugate distances of the two-mirror components unequal;
Figs. 5 to 7 are schematic representations of ~ther embodi-.
ments, reepectively, of optical systems according to the invention;
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Fig. 8 is a schematic representation of an optical system similar to Fig. 1, but showing another embodiment of the invehtion;
and ,, .
-~ Fig. 9 is a graphic representation showing the variation of : .~
`` the focal position as a function of the distance from the axis and the wavelength of the image forming light.
DETAILED DESCRIPTION OF T~E PREFERRED EMBODIMENTS
In the embddiment of the invention shown in Fig. 1, the new and improved optical system, indieated generally at 10, comprises Z0 two spherical mirrors, a convex mirror 12 and a concave mirror 14, .~ .
; arranged to pro~ide three reflections within the system. The - mirrors are arranged with their centers of curvature along the ; ~ system axis SA and to have off a~is conjugate areas centered at - points O and I. The points O and I are each a distance H from the reference axis SA at opposite sides thereof. In the optical :, system illustrated in Fig. 2 of my prior Patent No~ 3,748,015, the concave m~rror forms an image of the object O at I; the convex mirror forms a virtual image of point I at the point O which is reimaged by the concave mirror at I. It is noted that the width of the corrected annulus attainable with the optical systems of said patent is limited by the fifth order astigmatism inherent in the design. The high order astigmatism results from the ... .

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sph~rical aberration of the principal rays in this system.
I~ will be appreciated that meniscus elements can be used to reduce or remove the spherical aberration of principal rays `~ parallel to the optical axis. If the meniscus elements are concentric with the mirrors, they introduce no third order aber-ration except field curvature when the conjugates are at the com-mon center of curvature.
As seen in Fig. 1, a pair of symmetrically disposed meniscus elements 16 are provided for reducing the spherical aberration of the principal rays. It is noted that the meniscus elements would also be effective for reducing the spherical ab~rration of the principal rays if they were mounted directly adjacent the convex mirror 12 so that the surface of the mirror 12 and the convex surface of the meniscus elements 16 are parts of the same spherical surface. It will be appreciated that the high order astigmatism has been greatly reduced with the result that the width of the corrected annulus is increased by an order of magnitude.
A necessary condition for the absence of field curvature, iOe., for a flat image surface, in the resultant system is that the algebraic sum of the quantities obtained by dividing the `` power of each surface by its index of refraction be substanitally ` zero, the index of refraction of a reflecting surface being defined as negative one for this computation. Since the index of refraction of the meniscus elements varies with the wavelength of the image forming light, it will be appreciated that the incoxporation of these elements in the optical system results in a variation of field curvature with wavelengthO This results in a variation of the focal position as a function of the distance from the axis and as a function of the wavelength of the image -forming light, as shown in Figure '9. In an annular fi~ld optical system, the variation with distance from the axis is effectively . , .

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removed by restricting the fi~ld to an annulus whose distance from the axis is constant. The variation of field curvature with wavelength in such a system becomes a variation of focal position ~ with wavelength and it can be balanced by the introduction of -- color aberration of the opposite sense. To accomplish this in ` accordance with the invention, the refracting meniscus departs ~i~ from exact concèntricity by having its convex radius of curvature shorter than the aum of its concave radius and its thickness.
That is, its thickness is greater than the difference between the radii of its convex and concave surfaces. The way in which this works can be explained as follows:
The variation of field curvature with wavelenyth introduced by a nearly concentric meniscus whose power is negative is such . .
that the back focus is greater for short wavelength than for long wavelengths. A concentric meniscus with conjugate at its center ` of curvature does not introduce any longitudinal color aberration.
~.' - The same is substantially true of such a meniscus with a conjugate near its center of curvature. The acldition of a positive lens to such a meniscus introduces longitudinal color of the sense required to balance the variation in focus with wavelength resulting from the variation of the field curvature (contributed by the meniscus) with wavelength. This can be accomplished by making the convex radius of the meniscus shorter than the sum of its concave radius and its thickness. The meniscus is then equiv-alent to two lenses, one being a fictitious concentric meniscus with convex radius equal to the sum of the concave radius and the thickness, while the second is a zero thicknessppositive meniscus whose concave radius is the convex radius of the fictitious menis-cus and whose convex radius is the convex radius of the actual meniscus. For a nearly concantric meniscus with concave radius Rl, convex radius R2, thickness t,'and refractive index N, the longitudinal color compensates for the change in focus due to the ' variation of field curv~ture with wavelength in an annulus of radius H when R~ ~ Rl and t~ R2 ~ Rl ~ (H2 / 2N2) (l/Rl - l/R2) (1) I have found that the introduction of a pair of menisci whose parameters substantially satisfy equa~ion (1) into an optical system of the type disclosed in my aforementioned U.S.
Patent No. 3,748,015, together with accompanying modifications which will be discussed more fully hereinafter, results in a reduction in the high order astigmatism over a wide spectral band.
The resultant system can be improved considerably by modifying the menisci so that their thicknesses are greater than .. . ..
the values giv~n by equation (1). This results in a variation of focus of an annular field system whose sense is such that it can be compensated for by the introduction of a plane parallel plate of appropriate thickness, as indicated at 18 in Fig. 1.
The extra degrees of freedom provided by the additional element makes possible a much greater degree of correction~
Further improvement can be obtained by modifying the plane parallel plates in one of two ways:

(l) One of the faces of the plane parallel plate may be - ` -made aspheric.
(2) The plane parallel plate may be "bent" resulting in a -~ meniscus element.
The high~st degree of correetion has been obtained with a system in which the thickness of the menisci is greater than the value given by equation (1~ and in which color compensation is obtained by adding plane parallel plates modified in accordance with one of the two ways described above. The small aberrations arising from the departure of the meniscus element 16 from con-centricity are compensated for by introducing a small departure from concentricity in the mirror pair 12 and 14. That is, the _g_ .;~ - .
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concave mirr~r 14 and the convex mirror 12 are supported with a : distance between their centers of curvature of less than about :
two percent of the length of the shorter radius.
Table l is an example, indicating the construction data, of the annular field optical system of Fig. l. As i5 well known in - the art, a plus sign is used to denote that a s~rface is convex ` to the object and that distance is measured from left to right - whereas a minus sign is used to den~te that a surface is concave to the object and that a distance is measured from right to left.
_ _TABLE 1 : RADIUS OF ANNULUS - 100 mm.
: SURFACE NO. RADIUS (mm) DISTANCE TO NEXT MATERIAL NOTE
.~ FROM OBJECTSURFACE 'Qmm) TO IMAGE
, ~ ~ ~__ __ _________ _______________ ____~____ __ , . (PLANE) 144.92 AIR OBJECT
1 ~144.96 11.03 FUSED SILICA
2 ~151.75 88.70 AIR
3 -~57.30 16.75 FUSED SILICA

4 -967.84 295.25 AIR

5 -551.15 -279.0'7 AIR MIRROR

6 -267.18 279.07 AIR MIRROR

7 -551.15 -295.25 AIR MIRROR

8 ~967.84 -16.75 FUSED SILICA

9 -957.30 -88.70 AIR .:
-151.75 -11.03 FUSED SILICA
11 -144.96 -144.9~ AIR
,. . .

- 12_ (PLANE) _ _ IMAGE_ Table II is a table of the computed performance of the annular ., field optical system of Table 1 over an extended spectral range `.;`. (2800 A to 5461 A) in terms of the r~s wave aberration at various - 30 annular radii. The width of the usable annulus is the difference be~ween the values of the upper and lower radii for which the p~Efo~mance is adequate for the application. It is noted that ,, .

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. ., a system is usually called "diffraction limited", or more precisely "aperture limited" when the rms wave aberration is less than ~.07.
For a scanning system, the rms wave aberration may be as high as 0.09 or 0.1 at the edges of the annulus.

TABLE II

N.A. = 0.17 AT OBJECT AND IMAGE
RADIUS OF RMS~WA~E ~ERRATION_(WAVELENGTH ~NITS) :-~ ANNULU~ (mm) ; WAVELENGTH (ANGSTROM UNITS) :.. ~ ~ _________________________ ~ 38~ 3200 3650 400~ 4358 5461 ___~__________________________ _______________ 105 .09 .12 .13 .13 .13 .12 104 .05 .08 .09 .09 .09 .08 103 .02 .04 .05 .06 .06 .05 ~00 .02 .01 .01 .01 .01 .01 97 .04 .01 .02 .02 .02 .03 ;
96 .06 .02 .02 .03 .03 .03 ` 95 .08 .04 .04 .04 .04 .04 ~ 94 .11 .06 .05 .05 .05 .05 - 93 .13 .08 .07 .07 .06 .06 ~: _ ~ ___ _ _ ______ ~________________~ ______ As indicated hereinbefore, the annular field optical system of the present invention provides at least one convex and dne concave mirror, which are substantially concentrically arranged along an optical axis to form conjugate planes normal to said axis for which the system is of unit power. Fig. 1 shows one suitable arrangement of the convex and concave mirrors, and other suitable arrangements of these mirrors are shown and described in my prior Patent No. 3,748,015. Fig. 8 of the present specification shows an arrangement which includes a convex mirror 12e and a concave mirror ; 14e arranged substantially concentrically along an optical axis SA in a manner utilizing a total of five reflections within the ` 30 system, there being three reflections from the concave mirror 14e and two from the convex mirror 12e. In this embodiment the alge-braic sum of the powers of the reflecting surfaces utilized is zero .~ , ,, , --1 1--''' ., :~ .

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when the radius of the convex mirrox 12e is two-thirds that of the `~ concave mirror 14e. In a manner similar to that described in connection with the embodiment of Fig. 1, the system of Fig. 8 - includes meniscus elements 16e and a color correcting plate 18e that function in the aforesaid manner.
It is noted that annular field optical systems of the type described are usually used in a scanning mode and, for this purpose, it is highly desirable that the orientation of the object and image be the same so that their physical supports can be maintained in fixed relation to each other while being moved relative to the optical system for scanning and so that the accuracy requirèments of the scanning motion are minimized. An arrangement that achieves this by incorporating three flat mirrors in the optical system was shown in the aforesaid U.S. Patent No. 3,951,546. I have discover-ed another means of achieving this effect, by using two optical systems 10 and 10', each being of the type shown in Fig. 1, dis-posed in back-to-back relationship so that the object and image planes are superposed, as illustrated in Fig. 2. Thus, the optical system 10 includes two spherical mirxors 12 and 14, a pair of -~ 20 meniscus elemen~s 16 and a color correcting plate 18, and the . .
- symmetrical optical system 10' includes two spherical mirrors 12' and 14', a pair of meniscus elements 16' and a color correcting plate 18'. The physical sq~aration between the object and final .:.
image required for a practical arrangement is obtained by the add-ition of folding mirrors 20 and 20' shown by broken lines in Fig. 2, ..:
to move the actual object and image to 0' and I', respactively. In this arrangement the separation between the folding mirrors must be ....

sufficient to provide clearance ~or scanning. It will, of course, ~ . . .
be appreciated that other arrangements of folding flats, which retain the relative orientation of the object and image, are within~

the scope of this invention.

In the optical system of Fig. 2, the intermediate image, in-dicated at 22, i5 highly corrected because it is formed by the optical system 10 of Fig. 1. In this arrangement, each half 10 and

10', of the system is longitudinally symmetrical and thus revers-ible. Since for most applications a high degree of correction at the intermediate image is not requir~, the system can be simpli-fied by removing the requirements that each half o -the system be symmetrical, while keeping the system or at least the re~ractive components thereof symmetrical as a ~hole, and thereby reduce the number of compensating menisci and correcting plates to two each, as illustrated in the embodiment of Fig. 3. Thus, a half of the optical system, indicated at lOa, comprises two spherical mirrors 12 and 14, a meniscus element 16a and a color correcting plate 18a disposed on the side of the intermediate image 22, all of said elements being symmetrically disposed about the optical a~is SA~
The other half of the optical system, indicated at lOa', comprises two spherical mirrors 12l and 14', a meniscus element 16a' and a color correcting plate 18a' disposed on the side of the int~r-mediate image 22, all of said elements being symmetrical about the optical axis SA. For the same reasons indicated hereinbefore, each half of the optical system is provided with a folding mirror ~; 20 and 20' shown by the broken lines in Fig. 3, to mo~e the actual object and final image to 01 and I', respectively, ; Referring nex~ to the embodiment of Fig. 4 t it will be appre-ciated that systems with an intermediate image, indicated at 22, -~ can be further simplified by combining the two color correcting ;; plates 18a and 18a' of the embodiment of Fig. 3 into a qingle ele-- ~ ment of the same form as indicated at 18b in Fig. 4. Symmetry is maintained by placing the single color corrector 18b a~ or closely adjacent the intermediate image 22. Further, the folding mirrors 20 and 20' of Fig. 3 can also be eliminated by making unequal the conjugate distance of at least one of the two-mirror components 12b, 14b, and 12b', 14b'. That is, the intermediate image distance to ' . ' ~ t ' .

.

i3~8 the two-mirror component is made greater than t~e object and/or image distances, to therebyjyspace the final image I from the object 0. In this system, the color correcting plate 18b at the intermediate image can be a true plane parallel.
;~ In a truly afocal system, the magnification is the same for all conjugate positions~ Howe~er, this desirable feature is not ; achieved in practical applications because real systems do not in general remain truly afocal for all field positions. In the unit magnification system of Fig. 4, for example, if the object 0 and image I are moved together longitudinally by lmm., the magni-fication of a 4mm. radial annulus varies from unity by + .00032.
This variation, which results in ~racking smear during scanning, can be reduced to + .00001 by aspherizing one of the faces of ths color correcting plate 18b.
A modification o~ the optical system of Fig. 4 is illustrated in Fig. 5, wherein the correcting menisci are moved rom the inter- -~ mediate image side of the system -to the object-image side thereof.
- In the embodiment of Fig. 5, one half of the system includes two spherical mirrors 12b and 14b and a meniscus element 16c disposed on the object-image side and symmetrically about the system axis `-- SA, and the other half of the system includes two spherical mir-: :
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j~ rors 12b' and 14b' and a meniscus element 16c' al50 disposed on ; the object-image side and symmetrically about the system axis SA.

A single color correcting plate 18b is disposed symmetrically ."
about the system axis at or closely adjacent the intermediate image 22. As in the embodiment of Fig. 4, one of the faces of : .:
this plate is aspherized. Further, as in the embodiment of Fig.

4, the intermediate image distances to the two-mirror components - are made greater than the object and image distances, to thereby :
space the final image I from the object 0 for scanning purposes.
Table III is an example, indicating the construction data, af the annular field optical system of Fig. 5.

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TABLE IIT
, ~ _____ ____ ___________________________________________________ RADIUS OF ANNULUS = 100 mm.
SURFACE NO. RADIUS (mm~ DIST~NCE TO NEXT MATERIAL NOTE
F~OM OBJECT SURF CE ~prnm) . TO IMAGE
:~ --_________ O(PLANE) 107.13 AIR OBJECT
1128.18 10.48 ~USED SILICA
.~ 2-135.29 378.48 AIR
~; 3-541~32 -273.56 AIR MIRROR
4-264.61 273.56 AIR MIRROR
5-541.32 -590.28 AIR MIRROR
, . .
: ~ 6-1772.58* -7.01 FUSED SILICA
~: ASPHERIC
7(PLANE) -587.26 AIR
. ~ 8541.32 273.56 AIR MIRROR
~; 9264.61 -273.56 AIR MIRROR
10541.32 378.48 AIR MIRROR
11135.29 10.48 FUSED SILICA
12128.18 107.13 AIR
; 13`,~PLANE) IMAGE
*ASPHERIC SURFACE SYMMETRICAL ABOUT OPTICAL AXIS.
, ~ 20 DEPARTURE ,X, FROM PLANE SURFACE AT DISTANCE r F~OM AXIS:
. ; X= -1772.58~ ~1772.58-r~ ~1.732xl0~8r4~4.210xl0 13r6 ~ 8.2-78-x-lo-l8r8_~oo78xlo-2lrlo - Table IV is a table for the computed performance of the m annular field optical system of Table III over an extencled '' spectral range (2800 A to 5461 A) in terms of the rms wave abber-; ation at various annular radii. The width of the usable annulus `' is the difference between the values of the upper and lower radii for which the performance is adequate for the appliaation.
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TABLE IV
:, :
~ N.A. = 0.17 AT OBJECT AND IMAGE
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, RADIUS OF RMS WAVE ABBERATION (WAVELENGTH UNITS) ANNULUS (mm) : WAVELENGTH (ANGSTROM UNITS) " _________ ________________ 280~ 3200 3650 ~000 4358 5461 ______ ___ ________ __________ ______ ________ 1~ .0~ .08 .08 .08 .08 .07 102 .06 .05 .05 .05 .04 .04 10~ .05 .04 .04 .03 .03 .03 98 .06 .04 .0~ .04 .0~ .05 . 10 97 .09 . 06 .05 .05 .05 .05 ~;~ 96 ___ .13 .09 _ .08 _.07_ .07 .07 An optical system in which the distances from the intermediate . . .
image 22 to the two-mirror components l2d-14d and 12d'-b4d' are ~ less than the distances from the object O and final image I is ;;; shown in ~ig. 6. In the emhodiment of Fig. 6, one half of the ` ~ system includes two spherical mirrors 12d and 14d and a meniscus element 16a disposed on the intermediate image side and symmetrically .. ~. . .
;~`` about the system axis SA, and the other half of the system includes ......
two spherical mirrors 12d' and 14d' and a meniscus element 16a' also disposed~on the intermediate image side and symmetrically about the system axis SA. A single color correcting plate 18b is dis-po9ed at or closely adjacent the intermediate image 22. Parallel folding flats, as in the systems of Figs. 2 and 3 are introduced .. ..
'!' between the two crossed conjugate positions ~ and I to make them accessible for scanning purposes. However, in the embodiment of S !:
Fig. 6, the two ~olding flats are the front and back surfaces o~
~` a plane parallel plate 20d, whose thickness is determined by mechanical considerations, in contrast to the arrangements of Figs. 2 and 3 in which other considerati~ns determinad the separ-ation between the reflecting surfaces. Thus, in the embodiment of Fig. 6, the front and back surfaces of the plate 20d serve to deflect the object 0 to~`70' and the final image I to I', thereby :
:

providing the spacing therebetween necessary for scanning.
Another embodiment of the invention utilizing crossed object and image planes i5 shown in Fig. 7, wherein the single color correcting plate 18b of Fig. 6 at the intermediate image has been replaced by two epaced, color correcting plates 18e and 18e' on the object-image side of the system. In this embodiment, the substantially plane parallel plates have been "bent" to form a meniscus element. The remainder of the system of Fig. 7 is similar to that of Fig. 6. That is, one half of the system includes two spherical mirrors 12d and 14d and a meniscus element 16a disposed on the intermediate image side, and the other half of the system includes two spherical mi~rors 12d' and 14d' and a meniscus element 16a' also disposed on the intermediate image side. As described hereinbefore in connection with the embodiment of Fig. 6, plate 20d having mirror front and back surfaces serves to move the object 0 to 0' and the final image I to I' to provide physical separation between the object and image as required for a practical arrangement.
Table V is an example, indicating the construction data, of the annular field optical system of Fig. 7.
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TABLE V _ _ RADIUS OF ANNULUS = 100mm.
SU~FACE NO. RADIUS (mm.) DISTANCE TO NEXT MATERIAL NOTE
SURFACE (mm ) ~ _____________ _ ____ _____ ~ ___~____ _____~_ ____ (PLANE)151.33 AIR OBJECT
1 -726.8928.69 FUSED SILICA
.2 -730.32410.82 AIR
.:
, ~ 3 -552.06-280.16 AIR MIRROR
.. 4 -267.~18280.16 AIR MIRROR
-552.06363.25 AIR ~RROR
~: 6 -160.78-24.03 FUSED SILICA
7 -145.19-272.77 AIR
.;
.. 8 145.19 -24.03 FUSED SILICA
~ , - 9 160.78-363.25 AIR
; 10 552.06280.16 AIR M~RROR
: 11 267.18-280.16 AIR MIRROR
; 12 552.06410.82 AIR MIRROR
13 730.3228.69 FUSED SILICA
~ 14 726.89151.33 AIR
:. 15 (~BaNE~ AGE
~~~ ~~--~~~ ~-`.: ; Table VI is a table for the computed performance of the -~:
': annular field op~ical system of Table V~over an extended spectral range (2800 A to 5461 A) in terms of the rms wave aberration at various annular rad.ii. The width of the usable annulus is the '~ difference between the values of the upper and lower radii for ~;
~: ~ which the performance is adequate for the application.

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_ __ _ TABLE VI __ _ _ __ ____ N ~ A . = 0 ~1 7 AT OBJECT AND IMAGE
RADIUS OF RMS WAVE ABERRATION (WAVELENGTH UNITS ) ANNULUS (mm.) W~VELENGTE tANGSTRoM UN TS) 2800 3200 3650 ~000 4358 5461 ___________________ ~ ~_________________________ ~
104 .08 .07 .09.10 .10 .11 103 .06 .04 .05.06 .07 .08 102 .05 .02 .03.04 .05 .06 100 .03 .02 .02.02 .03 .04 , ~:~ 10 98 .07 .04 .03.03 .02 .03 . 97 slO ~06 ~05.04 .04 .03 :; 96 .14 .09 .07_ .06__.06 .05 .r.~ : It is noted that with the arrangement of Fi~. 7 and with the : configuration of Fig. 5, the refracting or meniscus elements 18a, :. 18e', 16c and 16c' can be used as windows for sealing the portions of the optical ~ystem therebetween.
... It will thus be seen that the present invention does indeed provide a new and improved optical system for use in applica~ions .~ whose spatial relations must be reproduced with great accuracy, : ~ 20 and which can be corrected for both a wide annulus and an extended : ::
spectral ranger Although specific embodiments have been illustrated ~: and described, it will be obvious to those skilled in the art thatvarious modifications may be made without departing from the spirit and scope of the invention, which is to be limited solely by the . appended claims.
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Claims (69)

WHAT IS CLAIMED IS:

1. In a restricted off-axis field optical system having a broad spectral range, which includes refracting means, the improvement comprising:

said optical system being constructed and arranged so that the Petzval sum is substantially zero, and said refracting means including means for balancing the effects of the variation in said Petzval sum due to variation in color by introducing axial chromatic aberration of the opposite sense so that the position of focus at the off-axis field remains substantially constant.

2. A restricted off-axis field catadioptric optical system having a broad spectral range comprising, in combination:

at least one convex and one concave mirror, said mirrors being nearly concentrically arranged along an optical axis; and refracting means;

said optical system being constructed and arranged so that The Petzval sum is substantially zero, and said refracting means including means for balancing the effects of the variation in said Petzval sum due to variation in color by introducing axial chromatic aberration of the opposite sense so that the position of focus at the off-axis field remains substantially constant.

3. An optical system according to Claim 1 or Claim 2 wherein said restricted off-axis field is an annular field.

4. An optical system according to Claim 1 or Claim 2 wherein the refracting means is constructed of a single optical material.

5. An optical system according to Claim 1 or Claim 2 wherein said means for compensating for the variation in field curvature with color in the restricted off-axis field by introducing axial chromatic aberration of the opposite sense comprises at least one pair of symmetrically disposed nearly concentric meniscus elements whose convex radii are larger than their concave radii and whose axial thickness is greater than the difference between their convex and concave radii, respectively.

6. An optical system according to Claim 1 wherein said means for compensating for the variation in field curvature with color in the restricted off-axis field by introducing axial chromatic aberration of the opposite sense comprises one pair of symmetrically disposed nearly concentric meniscus elements whose convex radii are larger than their concave radii and whose axial thickness is greater than the difference between their convex and concave radii, respectively; and color trimming means.

7. An optical system according to Claim 2 wherein said means for compensating for the variation in field curvature with color in the restricted off-axis field by introducing axial chromatic aberration of the opposite sense comprises one pair of symmetrically disposed nearly concentric meniscus elements whose convex radii are larger than their concave radii and whose axial thickness is greater than the difference between their convex and concave radii, respectively; and color trimming means.

8. The optical system of Claim 6 or Claim 7 wherein all of the refracting elements are constructed of a single optical material.

9. An optical system according to Claim 2 wherein said mirrors are arranged so that there are three reflections from said concave mirror and two reflections from said convex mirror.

10. An annular field catadioptric optical system having a broad spectral range comprising, in combination:

at least one convex and one concave mirror, said mirrors being nearly concentrically arranged along an optical axis; and refracting means;

said optical system being constructed and arranged so that the Petzval sum is substantially zero, and said refracting means including means for balancing the effects of the variation in said Petzval sum due to variation in color by introducing axial chromatic aberration of the opposite sense so that the position of focus at the annular field remains substantially constant;

said means for balancing the effects of the variation in said Petzval sum due to variation in color comprising at least one pair of symmetrically disposed nearly concentric meniscus elements whose convex radii are larger than their concave radii and whose axial thickness is greater than the difference between their convex and concave radii, respectively;

and wherein the relationship between the annular radius of the system and the characteristics of the nearly concentric meniscus lens is defined by the formula:

R2 ? R1 and t ? R2 - R1 + (H2/2N2) (1/R1 - 1/R2) where:

H = the annular radius of the system, R1 = the concave radius of the meniscus lens, R2 = the convex radius of the meniscus lens, t = the thickness of the meniscus lens, and N = the index of refraction of the meniscus lens.

11. An annular field catadioptric optical system having a broad spectral range comprising, in combination:

at least one convex and one concave mirror, said mirrors being nearly concentrically arranged along an optical axis; and refracting means;

said optical system being constructed and arranged so that the Petzval sum is substantially zero, and said refracting means including means for balancing the effects of the variation in said Petzval sum due to variation in color by introducing axial chromatic aberration of the opposite sense so that the position of focus at the annular field remains substantially constant;

said means for balancing the effects of the variation in said Petzval sum due to variation in color comprising at least one pair of symmetrically disposed nearly concentric meniscus elements whose convex radii are larger than their concave radii and whose axial thickness is greater than the difference between their convex and concave radii, respectively; and color trimming means;

and wherein the relationship between the annular radius of the system and the characteristics of the nearly concentric meniscus lens is defined by the formula:
R ? R
and t ? R2 - R1 + (H2/2N2) 1/R1 - l/R2) where:

H = the annular radius of the system, R1 = the concave radius of the meniscus lens;
R2 = the convex radius of the meniscus lens, t - the thickness of the meniscus lens, and N = the index of refraction of the meniscus lens.

12, An annular field catadioptric optical system having a broad spectral range comprising, in combination:

at least one convex and one concave mirror, said mirrors being nearly concentrically arranged along an optical axis;

means defining a location for an object the image of which is a real image at a second location, said convex mirror being positioned to reflect to said concave mirror light from said object location initially reflected to said convex mirror from said concave mirror, whereby light from said object location will be reflected twice at said concave mirror and at least once at said convex mirror before being focused at said second location; and refracting means;

said optical system being constructed and arranged so that the Petzval sum is substantially zero, and said refracting means including means for balancing the effects of the variation in said Petzval sum due to variation in color by introducing axial chromatic aberration of the opposite sense so that the position of focus at the annular field remains substantially constant;

said means for balancing the variation in said Petzval sum due to variation in color including at least one pair of symmetrically disposed nearly concentric meniscus elements whose convex radii are larger than their concave radii and whose axial thickness is greater than the difference between their convex and concave radii, respectively; and color trimming means.

13. An annular field optical system according to Claim 12 wherein said color trimming means is a substantially plane parallel plate mounted normal to the optical axis of said mirrors.

14. An annular field optical system according to claim 12 wherein said color trimming means is a plate having an aspheric surface.

15. An annular field optical system according to Claim 12 wherein said color trimming means is a weak meniscus element mounted normal to the optical axis of said mirrors.

16. An annular field optical system according to Claim 12 wherein said color trimming means is interposed between said mirrors and said object and image locations.

17. An annular field optical system according to Claim 12 wherein said color trimming means is interposed between said meniscus elements and said mirrors.

18. An annular field optical system according to Claim 15 wherein said system is characterized by the following construction data:

19. An annular field optical system having a broad spectral range, which includes refracting means, said optical system comprising:

a first half and a second half, each half including an optical system having an optical axis and having conjugate planes substantially normal to said axis; the first half and the second half being coaxially disposed in back-to-back relationship so that the conjugate planes are superposed on at least one side of the optical system, and means for providing spaced object and final image locations on the other side of the optical system, said optical system being constructed and arranged so that the Petzval sum is substantially zero, and said refracting means including means for balancing the effects of the variation in said Petzval sum due to variation in color by introducing axial chromatic aberration of the opposite sense so that the positions of focus at the annular field portions of the conjugate planes remain substantially constant.

20. An annular field optical system according to Claim 19 wherein said means for balancing the effects of the variation in said Petzval sum due to variation in the color includes in said first half and in said second half a symmetrically disposed nearly concentric meniscus element whose convex radius is larger than its concave radius and whose thickness is greater than the difference between its convex and concave radii.

21. An annular field optical system according to Claim 20 wherein said first half and said second half each further include color trimming means.

22. An annular field optical system according to Claim 21 wherein said color trimming means is a plane parallel plate mounted substantially normal to the optical axis.

23. An annular field optical system according to Claim 21 wherein said color trimming means is a plate having an aspheric surface.

24. An annular field optical system according to Claim 21 wherein said color trimming means is a weak meniscus element mounted normal to the optical axis.

25. An annular field optical system according to any one of Claims 19, 20 and 22, wherein said first half and said second half each include a concave mirror and a convex mirror, said mirrors being supported with their centers of curvature substantially coincident.

26. An annular field optical system according to Claim 21 wherein said first half and said second half each include a concave mirror and a convex mirror, said mirrors being supported with their centers of curvature substantially coincident.

27. An annular field optical system according to Claim 21 or Claim 26 wherein said means for providing spaced object and final image locations comprises folding mirrors.

28. An annular field optical system according to Claim 21 or Claim 26 wherein said first half and said second half are substantially symmetrical with respect to an axis in said conjugate planes.

29. An annular field optical system according to any one of Claims 19, 20, and 21, wherein the optical system of the first half and the optical system of the second half are each substantially unit power optical systems.

30. An annular field optical system according to Claim 26 wherein said meniscus elements are interposed between said mirrors and said object and final image locations, respectively.

31. An annular field optical system according to Claim 26 wherein said meniscus elements are interposed between said mirrors and said intermediate image location, respectively.

32. An annular field optical system according to Claim 26 wherein said color trimming means includes two elements which are interposed between said mirrors and said intermediate image location, respectively.

33. An annular field optical system according to Claim 26 wherein said color trimming means includes two elements which are interposed between said mirrors and said object and final image locations, respectively.

34. An annular field optical system according to Claim 26 wherein said color trimming means is a single element disposed at said intermediate image location.

35. An annular field optical system according to Claim 19 wherein each half includes a concave mirror and a convex mirror facing said concave mirror and a convex mirror facing said concave mirror, said mirrors being substantially concentrically arranged along said axis;

and wherein on the other side of the optical system the distance from the conjugate location to the two mirror component is different from the distance to the intermediate image location in at least one half, to thereby provide the spaced object and final image locations.

36. An annular field optical system according to Claim 35 wherein said means for balancing the effects of the variation in said Petzval sum due to variation in color includes in said first half and in said second half a nearly concentric meniscus element whose convex radius is larger than its concave radius and whose thickness is greater than the difference between its convex and concave radii.

37. An annular field optical system according to Claim 36 wherein said meniscus elements are interposed between said mirrors and said object and final image locations, respectively.

38. An annular field optical system according to Claim 36 wherein said meniscus elements are interposed between said mirrors and said intermediate image location, respectively.

39. An annular field optical system according to Claim 36 or Claim 37 or Claim 38 wherein said optical system further includes a color trimming element disposed substantially at said intermediate image location.

40. An annular field optical system according to Claim 36 or Claim 37 or Claim 38 wherein said optical system further includes a plane parallel plate mounted substantially normal to the optical axis and substantially at said intermediate image location.

41. An annular field optical system according to Claim 36 or Claim 37 or Claim 38 wherein said optical system further includes a plane parallel plate mounted substantially normal to the optical axis and substantially at said intermediate image location and wherein one of the faces of said plane parallel plate is made aspheric.

42. An annular field optical system according to Claim 36 or Claim 37 or Claim 38 wherein said optical system further includes a weak meniscus element, mounted normal to the optical axis of said mirrors and substantially at said intermediate image location.

43. An annular field optical system according to any one of Claims 35, 36, or 37 wherein said first half and said second half are substantially symmetrical with respect to an axis through the intermediate image location.

44. An annular field optical system according to Claim 19 wherein each half includes a concave mirror and a convex mirror facing said concave mirror, said mirrors being substantially concentrically arranged along an optical axis;
and wherein the distance from the intermediate image to the two mirror component is greater than the distance from the other conjugate location to the two mirror component in at least one half to thereby space said object and final image locations one from the other, said means for balancing the effects of the variation in said Petzval sum due to variation in color includes in said first half and in said second half a meniscus element interposed between said mirrors and said intermediate image location, said meniscus element being nearly concentric but having a difference between the radii of its meniscus surfaces of less than its thickness so that it is not exactly concentric while its power is negative, and a color trimming element disposed substantially at said intermediate image location, said element being normal to the optical axis of said mirrors.

45. An annular field optical system according to Claim 44 wherein said system is characterized by the following construction data:
*ASPHERIC SURFACE SYMMETRICAL ABOUT OPTICAL AXIS, and the RADIUS OF ANNULUS = 100 mm.

and the DEPARTURE, X, FROM PLANE SURFACE AT DISTANCE r FROM AXIS.

X = -1772.58 + 1772.58 - r2 + 1.732 x 10-8r4+4.210 x 10-13r6 +8.278 x 10-18r8 - 4.078 x 10-21r10.

46. An annular field optical system according to Claim 19 wherein said intermediate image location is axially displaced from the other conjugate plane in at least one half so that said final image location and said object location are crossed, and reflecting means interposed between the object and final image locations to reposition said locations to make them physically accessible.

47. An annular field optical system according to Claim 46 wherein said first half and said second half each include a concave mirror and a convex mirror facing said concave mirror.

48. An annular field optical system according to Claim 47 wherein said means for balancing the effects of the variation in said Petzval sum due to variation in color includes in said first half and in said second half a nearly concentric meniscus element whose convex radius is larger than its concave radius and whose thickness is greater than the difference between its convex and concave radii.

49. An annular field optical system according to Claim 48 wherein said meniscus elements are interposed between said mirrors and said intermediate image location, respectively.

50. An annular field optical system according to Claim 49 wherein said optical system further includes a color trimming element disposed substantially at said intermediate image location.

51. An annular field optical system according to Claim 49 wherein said first half and said second half each further include a color trimming element.

52. An annular field optical system according to Claim 49 wherein said first half and said second half each have a color trimming element interposed between said mirrors and said object and final image locations respectively, said element being normal to the optical axis of said mirrors.

53. An annular field optical system according to Claim 52 wherein one of the faces of said color trimming element is made aspheric.

54. An annular field optical system according to Claim 47 wherein in each half of the system, said concave and convex mirrors are supported with a distance between their centers of curvature of less than about three percent of the length of the shorter radius.

55. An annular field optical system according to Claim 52 wherein said first half and said second half are substantially symmetrical with respect to an axis through the intermediate image location.

56. An annular field optical system according to Claim 52 wherein said system is characterized by the following construct-ion data:

and RADIUS OF ANNULUS = 100 mm.

57. An annular field optical system according to Claim 19 wherein each half includes a concave mirror and a convex mirror facing said concave mirror, said mirrors being supported with their centers of curvature substantially coincident;
and wherein the distance from the intermediate image to the two mirror component is greater than the distance from the other conjugate location to the two mirror component in at least one half, to thereby space said object and final image locations one from the other, said means for balancing the effects of the variation in said Petzval sum due to variation in color includes in said first half and in said second half a meniscus element interposed between said mirrors and said intermediate image location, said meniscus element being symmetrically disposed and nearly concentric having a convex radius larger than its concave radius and a thickness great-er than the difference between its convex and concave radii.

58. An annular field optical system according to Claim 57 wherein said first half and said second half each have a color trimming element interposed between said mirrors and said object and final image locations respectively, said element being normal to the optical axis of said mirrors.

59. An annular field optical system according to Claim 58 wherein one of the faces of said color trimming element is aspheric.

60. An annular field optical system according to Claim 58 wherein said color trimming element is a meniscus element.

61. An annular field optical system according to Claim 58 wherein, in each half of the system, said concave and convex mirrors are supported with a distance between their centers of curvature of less than about three percent of the length of the shorter radius.

62. An annular field optical system according to Claim 58 wherein said first half and said second half are substantially symmetrical with respect to an axis through the intermediate image location.

63. An annular field optical system according to Claim 58 wherein said system is characterized by the following construction data:
RADIUS OF ANNULUS = 100 mm.

64. An annular field catadioptric optical system comprising:

at least one concave and one convex mirror arranged around an optical axis in face-to-face relationship with their centers of curvature being substantially concentric and falling on said axis, the convex mirror being smaller than and having a smaller radius of curvature than the concave mirror;

means defining an object location and a conjugate real image location, said convex mirror being positioned to reflect to said concave mirror light from said object location initially reflected from said concave mirror whereby light from said object location will be reflected at least twice at said concave mirror and at least once at said convex mirror before being focused at the image location;

means for limiting the image field to an annular zone centered about the optical axis; and refracting means positioned in the light path between the object and image locations, said refracting means including means to reduce the spherical aberration of principal rays parallel to the optical axis, and means to introduce axial chromatic aberration of the opposite sense to balance out in said annular zone variations in focus resulting from curvature due to variations in wavelength.

65. The system of Claim 64 wherein the entirety of said refracting means is made from the same optical meterial.

66. The system of Claim 64 or 65 further provided with color trimming means to maximize aberration reduction in said annular zone.

67. An annular field optical system according to Claim 23 or Claim 24, wherein said first half and said second half each include a concave mirror and a convex mirror, said mirrors being supported with their centers of curvature substantially coincident.

68. An annular field optical system according to Claim 26, wherein the optical system of the first half and the optical system of the second half are each substantially unit power optical systems.

69. An annular field optical system according to Claim 38, wherein said first half and said second half are substantially symmetrical with respect to an axis through the intermediate image location.