Optical system and method for providing corrected optical images

An optical system includes a housing having an axis of elongation, and a non-spherical window affixed to the housing. An optical corrector, preferably in the form of an aspherical strip of transparent material, is positioned adjacent to the curved inner surface of the window. The optical corrector is mounted on an optical corrector support, which is rotatable about the axis of elongation. An optical rain is positioned such the the optical corrector lies between the window and the optical train. The optical train includes at least one optical element operable to alter an optical ray incident thereon, and a gimbal upon which the at least one optical element is mounted. The gimbal is pivotable about a transverse axis perpendicular to the axis of elongation. The optical train is mounted on an optical train support, which is movable independently of the optical corrector support. A sensor is positioned to receive the optical ray passing sequentially through the window, the optical corrector, and the optical train.

This invention relates to an optical system having a window therein, and in 
particular to such an optical system used with an aspheric window. 
An optical sensor receives radiated energy from a scene and converts it to 
an electrical signal. The electrical signal is provided to a display or 
further processed for pattern recognition or the like. Optical sensors are 
available in a variety of types and for wavelengths ranging from the 
ultraviolet, through the visible, and into the infrared In some 
applications the optical sensors are fixed in orientation, and in others 
the optical sensors are movable by pivoting and/or rotational motions to 
allow sensing over a wide angular field of regard. 
The optical sensors generally employ a photosensitive material that faces 
the scene and produces an electrical output responsive to the incident 
energy. The photosensitive material and remainder of the sensor structure 
are rather fragile, and are easily damaged by dirt, erosion, chemicals, or 
high air velocity. In service, the sensor is placed behind a window 
through which it views the scene and which protects the sensor from such 
external effects. The window must be transparent to the radiation of the 
operating wavelength of the sensor and resist attack from the external 
forces. The window must also permit the sensor to view the scene over the 
specified field of regard. 
The window would ideally introduce no wavefront aberration at the center of 
the field of view, other than possibly spherical aberration, particularly 
if the sensor is an imaging sensor. The thicker and more highly curved is 
the window, the more likely is the introduction of significant wavefront 
aberration. A wide variety of sensor windows have been used in various 
aircraft applications. In many cases such as low-speed commercial 
helicopters, flat windows are acceptable. Windows that are shaped as 
segments of spheres are used in aircraft and missile applications, but for 
these windows the wavefront aberration tends to be high if the gimbal 
location is not at the spherical center of the window. In all of these 
window types, if the window must be wide or must project a substantial 
distance into an airflow to permit a large field of regard, the 
aerodynamic drag introduced by the window is large. 
For applications involving aircraft and missiles operating at high speeds, 
the window should be relatively aerodynamic such that the presence of the 
window extending into the a does not introduce unacceptably high and/or 
asymmetric aerodynamic drag to the vehicle. A nonspherical or conformal 
window is therefore beneficial to reducing drag and increasing the speed 
and range of the aircraft. However, available conformal windows introduce 
large wavefront aberrations into the sensor beam, particularly for high 
azimuthal pointing angles of the sensor. 
The wavefront aberration may be corrected computationally, but the amount 
of processing may be great. To reduce the amount of computation or 
eliminate the need for computation, the wavefront aberration of the image 
may be minimized optically, either in the optical processing components or 
by providing a particular shape in the window. Available approaches have 
not been fully successful in accomplishing this type of correction. 
Accordingly, there is a need for an improved approach to providing a 
corrected image in an optical system viewing a scene through an aspheric 
window. The present invention fulfills this need, and further provides 
related advantages. 
SUMMARY OF THE INVENTION 
The present invention provides an optical system and a method for providing 
optical images using the optical system. The optical system is used with 
many types of aspheric windows. It may be tailored to provide minimal 
wavefront aberration over a wide range of azimuthal pointing angles of the 
sensor of the optical system. 
In accordance with the invention, an optical system comprises a window 
having a curved outer surface and a curved inner surface, an optical 
corrector adjacent to the curved inner surface of the window and 
comprising an aspheric transparent body, and a movable optical corrector 
support upon which the optical corrector is mounted. The system further 
includes an optical train positioned such that the optical corrector lies 
between the curved window and the optical train. The optical train 
includes at least one optical element operable to alter an optical ray 
incident thereon. There is a movable optical train support upon which the 
optical train is mounted, and a sensor disposed to receive the optical ray 
passing sequentially through the window, the optical corrector, and the 
optical train. 
The window is preferably mounted in a housing having an axis of elongation. 
The optical corrector support which preferably comprises a strip of 
transparent material having an axial component extending along the axis of 
elongation and a radial component extending outwardly from the axis of 
elongation, is preferably rotatable about the axis of elongation. The 
optical corrector support and the optical train sport are also desirably 
movable parallel to the axis of elongation, with each movement independent 
of the other. 
The optical system thus includes the aspheric window, which introduces an 
aberration into the optical ray tat is dependent upon the pointing angle 
of the sensor through the window, and two separately adjustable optical 
component which can partially or totally negate the introduced aberration. 
The optical corrector functions as a corrective lens whose position may be 
rotated about the axis of elongation and/or moved parallel to the axis of 
elongation. The position of the optical train may also be adjusted along 
the axis of elongation. These optical components and their adjustability 
serve to reduce the aberration introduced by the passage of the optical 
ray through the window. The design of these two optical components, taken 
together with their movability feature, permits the aberration correction 
to be custom selected according to the nature of the window. The positions 
of the optical components which yield the best image as a function of the 
sensor pointing angle are stored in memory, and these positions are 
restored during service of the optical system as a function of the 
pointing angle. 
Other features and advantages of the present invention will be apparent 
from the following more detailed description of the preferred embodiment, 
taken in conjunction with the accompanying drawings, which illustrate, by 
way of example, the principles of the invention. The scope of the 
invention is not, however, limited to this preferred embodiment.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 depicts a flight vehicle, in this case a supersonic missile 20, 
having a fuselage 22 with a curved window 24 attached thereto. The window 
24 is a nose dome that protrudes at least partially into the air of the 
missile 20. The fuselage is elongated along an axis of elongation 25, and 
in a preferred application the window 24 is rotationally symmetric about 
the axis 25. The missile 20 with the nose-dome window 24 is the preferred 
application of the optical system of the invention, but it is applicable 
in other contexts as well such as other missile windows and windows on 
manned aircraft. 
The window 24 is part of an optical system 26, which is shown generally in 
FIG. 2. The optical system 26 includes the window 24 attached to the 
fuselage 22, which serves as a housing for the optical system 26. A curved 
inner surface 28 of the window 24 is the concave surface of the window 24 
that faces the inside of the fuselage 22. A curved outer surface 30 of the 
window 26 is the convex surface of the window 24 that faces outwardly and 
projects into the airstream as the missile 20 flies. The window 24 has a 
spatially dependent curvature. 
An optical corrector 32 is located adjacent to the inner surface 28 of the 
window 24. The optical corrector 32 is a curved piece of material 
transparent to the radiation being sensed by the optical system 26 and its 
sensor. For example, for a visible radiation optical system the optical 
corrector 32 may be glass. 
The optical corrector 32 is preferably formed as a piece of the transparent 
material whose shape has an axial component X.sub.z extending along the 
axis of elongation 25 (FIG. 2), a radial component X.sub.r extending 
outwardly from the axis of elongation 25 (FIG. 2), and a circumferential 
component X.sub.0 (FIG. 3B). FIGS. 3A-3C illustrative one form of the 
optical corrector 32. As shown in FIG. 3A, the optical corrector 32 lies 
adjacent to the inner surface 28 of the window 24, and therefore extends 
outwardly from the axis of elongation 25 (the X.sub.r component) and 
rearwardly from a vertex 34 (the X.sub.z component) of the optical 
corrector 32. The cross section of the optical corrector 32 may be 
circularly symmetric or nearly circularly symmetric about the axis of 
elongation 25 at a location near to the vertex 34, as shown in FIG. 3B. At 
locations further rearwardly from the vertex 34, the optical corrector 32 
is formed as at least one strip 32a of the transparent material and 
preferably two strips 32a as illustrated to balance the loading on its 
support In longitudinal section, FIG. 3A, the strips 32a generally follow 
the curvature of the window 24, but may deviate from that curvature to 
some extent In transverse section perpendicular to the axis of elongation 
25, FIG. 3C, each strip 32a is preferably two-fold symmetric about a 
corrector transverse axis of symmetry 35 and subtends a total arc A about 
the axis of elongation 25. The use of the strip form of the optical 
corrector 32 allows the optical corrector to have a curvature and 
thickness different from that of the window 24, when viewed transversely 
to the a of elongation 25, as in FIG. 3C. In the illustrated preferred 
case of FIG. 30, the transverse curvature and thickness variation of the 
strip 32 are different from the transverse curvature and thickness 
variation of the window 24. 
The optical corrector 32 functions as a lens to correct the aberrations 
introduced into an optical (light) ray passing through the window 28. 
Because the aberrations are spatially dependent upon the vector of the 
optical ray, the optical corrector 32 is formed so that its correction is 
spatially dependent as well. The aberrations introduced into the optical 
ray depend upon the exact shape of the window 24, and therefore no 
specific design may be set forth for the shape of the optical corrector 
32. However, some generalizations may be made. 
As shown in the longitudinal sectional view of FIG. 3A and the transverse 
sectional view of FIG. 3C, the optical corrective characteristics (i.e., 
curvature and/or thickness) of the optical corrector 32 are, in general, a 
functions of position. The optical corrective characteristics of the 
optical corrector 32 may vary as a function of location along the axis of 
elongation 25, as shown in FIG. 3A, and/or as a function of angle about 
the axis of elongation 25, as shown in FIG. 3C. The curvature and 
thickness, and hence the optical properties, of the optical corrector 32 
are selected to correct aberrations introduced when a light ray passes 
through the window 24 and thereafter through the optical corrector 32. 
The optical corrector 32 is mounted on an movable optical corrector support 
36, shown in FIG. 2. The optical corrector support 36 is preferably 
movable by rotation about the axis of elongation 25, as schematically 
indicated by arrow 38. The optical corrector support 36 may also be 
movable by linear movement parallel to the as of elongation 25, as 
schematically indicated by arrow 40. The rotational and linear movement 
are produced by conventional actuators, which are known for other 
purposes. 
The rotational movement of the optical corrector support 36, and thence of 
the optical corrector 32, allows the strip 32a of the optical corrector 32 
to be rotationally positioned according to the rotational angle of regard 
of the optical train to be discussed subsequently. That is, when the 
optical in is positioned to look downwardly, the optical corrector support 
36 would normally be rotationally positioned as shown in FIG. 3C, so that 
an optical ray entering the optical train must pass through the optical 
corrector 32. If the optical train is rotated by 90 degrees to look to the 
left or right, the optical corrector support 36 would normally also be 
rotated by 90 degrees from the position shown in FIG. 3C so that the 
incident optical ray must pass therethrough. 
The axial movement of the optical corrector support 36, and thence of the 
optical corrector 32, allows different portions of the optical corrector 
32 to be used to correct the aberration introduced by the window 24. 
An optical train 42 is positioned such that the optical corrector 32 lies 
between the window 24 and the optical train 42. The optical train 42 
includes at least one optical element operable to alter an optical ray 
incident thereon. In FIG. 2, the optical element is illustrated as a 
refractive lens 44, but it may also include a mirror, a prism, or any 
other operable optical element The optical element may also include a 
combination of such lenses, mirrors, and/or prisms. The detailed design of 
optical trains is known in the art, and the present invention is not 
concerned with such design specifics. 
The optical train 42 directs incident optical rays, which previously passed 
first through the window 24 and then through the optical corrector 32, 
into a sensor 46. The sensor 46 is illustrated as a focal plane array 
sensor, but may be of any operable type. The sensor 46 is selected 
according to the nature of the energy to be sensed, and is typically a 
sensor of visible light or infrared energy. The design of such sensors 46 
is known in the art. The sensor 46 provides its output as an electrical 
signal to processing electronics, which are not illustrated but which are 
known in the art. 
The optical train 42 is mounted on a movable optical train support 48. The 
movement characteristics of the optical train support 48 are selected to 
permit the optical train 42 to point in the desired directions, and also 
to take advantage of the corrective properties of the optical corrector 
32. To allow the optical train 42 to point in the desired directions, a 
roll/nod movement is illustrated in FIG. 2. The optical train support 48 
rotates about the axis of elongation 25, as indicated by arrow 50. A 
gimbal 52 produces a nodding movement indicated by arrow 54 about a 
traverse axis 56 that is perpendicular to the axis of elongation 25 (and 
thence the axis of rotation). The combination of movements 50 and 54 
allows the optical train 42 to be pointed in any desired rotational and 
azimuthal directions. In another approach within the scope of the present 
invention, the optical train may be mounted on an X-Y rotational gimbal 
support, which permits the optical train to move about two transverse 
axes, so that the rotational movement is not required. 
The entire optical an 42 may be moved forwardly or rearwardly parallel to 
the axis of elongation 25 by a linear axial movement, indicated by arrow 
58. The axial movement 58 of the optical train support 48 allows the 
optical train 42 to be positioned for optimal performance relative to the 
window 24 and to the optical corrector 32. The movements 50, 54, and 58 
are produced by conventional actuators which are known for other purposes. 
The movements 38 and 40 of the optical corrector 32, and the movements 50, 
54, and 58 of the optical train 42, may be rely independent of each other 
or may be mechanically and/or electrically linked. For example, the 
rotational movement 38 of the optical corrector 32 may be linked together 
with, or even accomplished by the same actuator as, the rotational 
movement 50 of the optical train 42. In that case, the optical in 42 looks 
through the same portion of the optical corrector 32 for all angles of 
rotation about the axis of elongation 25. Similar linkages are possible 
for the axial movements 40 and 58, for example. 
FIG. 4 depicts a preferred approach for designing, tailoring, and operating 
the optical system 26. The physical components of the optical system, as 
described previously, are provided, numeral 70. The optical corrector 32 
is designed and fabricated, and the movements 38,40,50,54, and 58 are 
interrelated and programed for subsequent service applications, using an 
iterative procedure, numerals 72, 74, 76, and 78. 
First, the optical characteristics of the window 24 are evaluated, numeral 
72. This evaluation establishes the nature of the aberration introduced 
into the wavefront of an incident optical ray as it passes through the 
window 24, for all relevant incident positions and angles. This evaluation 
may be performed using conventional optical ray analysis and the known 
and/or measured shape of the window 24. The shape of the window 24 is 
dictated to a degree by aerodynamic requirements, but it may also be 
fine-tuned according to optical requirements. 
The required shape and position of the optical corrector 32 are calculated 
as a function of its position and the incident optical ray positions and 
angles, using conventional optical ray analysis. The shape and positioning 
of the optical corrector 32 are chosen to establish selected optical 
characteristics of the optical beam after it has passed through the window 
24 and the optical corrector 32. Examples of such characteristics include 
deviation of the apparent angle to the target, optical power or focal 
length as a function of optical ray position and angle, and axially 
symmetric aberration. The designed shape of the optical corrector 32 is 
then changed to adjust for asymmetric aberrations such as coma and 
astigmatism. In this analysis, the symmetric aberrations are chosen to be 
constant as the elevation angle is changed, whereas the asymmetric 
aberrations that change with elevation angle are corrected to acceptably 
small values. The optics of the optical train may also be designed to 
correct symmetrical aberrations to acceptably small values. In the final 
stages of the design process the optical elements of the optical train 42 
are designed to correct all of the symmetrical aberrations to acceptably 
small values. These aberrations have been rendered nearly constant by the 
prior design steps. Based upon this designing process, the optical 
corrector is fabricated, numeral 74. 
The window 24, the optical corrector 32, and the optical train 42 are 
mounted on the fuselage 22, optical corrector support 36, and optical 
train support 48, respectively, numeral 76. Test optical signals received 
at the sensor 46 are evaluated during manufacturing. The associated values 
of the movements 38,40, 50, 54 and 58 that yield the optimal optical 
properties are determined and stored, numeral 78. If these received 
optical signal properties are acceptable and within specifications, the 
manufacturing and assembly process is complete. Errors and aberrations are 
also detained and stored, so that they may be accounted for by other 
processing. If the results achieved are not acceptable, the steps 72, 74, 
76, and 78 are repeated as necessary until acceptable results are 
obtained. Typically, the modification is achieved by reworking the optical 
corrector 32 until its properties are acceptable, by polishing, grinding, 
machining and other known working operations. 
The shape of the optical corrector 32 may not be stated in any general 
form, inasmuch as it depends upon the shape and optical characteristics of 
the window 24, and is determined in the above-described design process. 
However, in a typical case, as shown in FIGS. 2 and 3A, the optical 
corrector typically conforms to the shape of the window 24 fairly closely 
but not necessary exactly, when the window and the optical corrector are 
viewed in the longitudinal section of FIG. 3A. However, the optical 
corrector 32 typically does not conform to the shape of the window 24 when 
viewed in transverse section in the strip section of the optical 
corrector, as seen in FIG. 3C. 
Once the optical corrector 32 is fabricated and the positions of the 
movements 38, 40, 50, 54, and 58 yielding acceptable optical properties 
are known, the missile is placed into service, numeral 80. When the 
optical system 26 is to be used during service, the angular positions of 
the movements 50 and 54 are typically chosen in order to point the optical 
train 42 along a desired line of sight. The optimum angular positions of 
the other movements 38, 40, and 58 (collectively, the "support 
positions"), associated with those desired angular positions of the 
movements 50 and 54, are recalled from the memory established during the 
initial manufacturing and calibration operation, steps 72, 74, 76, and 78, 
and set using the respective actuators. The result is an optimum image 
reaching the sensor 46 for all desired viewing (pointing) angles of the 
optical train. 
Although a particular embodiment of the invention has been described in 
detail for purposes of illustration, various modifications and 
enhancements may be made without departing from the spirit and scope of 
the invention Accordingly, the invention is not to be limited except as by 
the appended claims.