Large angle reflective scanning system and method

A system and method for optically scanning a scene are disclosed. The system is rotated about a pitch axis and a roll axis to provide the desired scanning capability. The system consists of a primary mirror, a secondary mirror, a tertiary mirror and a fold mirror. The fold mirror is located between the primary and secondary mirrors and reflects the light from the field of view along the pitch axis of the system. The light is reflected by a series of plane fold mirrors to the roll axis and out through an exit aperture. Because the light is reflected along the pitch axis and roll axis, the system can be rotated about these axes while the optical output is maintained as required. Rotation about these axes through internal points between the primary and secondary mirrors minimizes the volume swept out by the system during rotation and maximizes the angles through which the system can be rotated.

BACKGROUND OF THE INVENTION 
A typical telescopic system collects light through an entrance aperture, 
passes the light through reflective optics and onto a plane for viewing. 
The light impinges upon a primary mirror and is reflected toward a 
secondary mirror. The light can then be reflected from the secondary 
mirror toward a focus and extends through an aperture in the primary 
mirror to a series of mirrors aft of the primary mirror. The light can be 
reflected by these mirrors out of the system to a camera or detector. 
It is often desirable to scan such a system across a wide field of view for 
pointing or scanning in a raster or rectilinear method. When scanning, the 
output image must be directed onto the detector to allow for continuous 
viewing, recording or analysis. To that end, the entire system, including 
the mirrors aft of the primary mirror and the detector, rotates as a unit. 
As the entire system rotates, a large volume is necessary to permit 
rotation. This can limit the usefulness of the system in the confined 
enclosures of some airborn and space applications. Also, mechanical and/or 
optical interference inherent in such systems limit the angular range 
through which the system can be rotated. 
A continuing need exists however for a more compact scanning system that 
provides accurate imaging at long focal lengths and over a broad range of 
wavelengths. 
SUMMARY OF THE INVENTION 
The present invention is an scanning optical system and a method of imaging 
objects at long focal lengths. Light enters the system through an entrance 
aperture and follows an optical path through the system. The light exits 
the system through an exit aperture. The system is rotatable about a pitch 
axis and a roll axis. A primary mirror receives the light entering the 
system and reflects it toward a secondary mirror. The secondary mirror 
receives the light from the primary mirror and reflects it toward a focus 
between the primary mirror and the secondary mirror. A tertiary mirror 
also reflects light along the optical path. A fold mirror is located 
between the primary and secondary mirrors. The fold mirror reflects the 
light along the pitch axis and couples the light to the exit aperture. The 
system also includes a pitch actuator and a roll actuator for rotating the 
system about the pitch axis and the roll axis, respectively. 
In one embodiment of the invention, the tertiary mirror is located behind 
or aft of the primary mirror. In that embodiment, the system optical axis 
runs through the primary, secondary, tertiary, and fold mirrors as well as 
the focus. Light reflected from the secondary mirror passes through an 
aperture in the fold mirror and then through an aperture in the primary 
mirror to the tertiary mirror. The light is reflected back to the fold 
mirror where it is reflected along the pitch axis. In another embodiment, 
the tertiary mirror is located along the pitch axis. In this embodiment, 
the light from the secondary mirror is reflected along the pitch axis by 
the fold mirror toward the tertiary mirror. The tertiary mirror reflects 
the light back along the pitch axis past the fold mirror. 
In one embodiment, the pitch axis runs through a point approximately at the 
geometric center of a volume occupied by the system. In another 
embodiment, the pitch axis and the roll axis intersect at a point 
approximately at the geometric center of the system volume. In another 
embodiment, the fold mirror is located at a point approximately at the 
geometric center of the volume at the intersection of the pitch axis and 
the roll axis. In another embodiment, the roll axis runs through the exit 
aperture of the system. 
The scanning system of the present invention provides distinct advantages 
over prior scanning systems. Reflecting the light along the pitch axis 
between the primary and secondary mirrors allows the system to be rotated 
about the pitch axis. Because the system is rotated about an axis through 
the interior of the system, it sweeps out a much smaller volume when it is 
rotated as compared to prior systems. In addition, the locations of the 
pitch axis and the roll axis eliminate mechanical and optical interference 
from other system components. The angular range of rotation of the present 
invention is much larger than that of prior systems. Also, the system is 
purely reflective. Because no refractive components are included, maximum 
spectral performance is achieved. 
The scanning system of the present invention also has a longer focal length 
than typical dual mirror Cassegrain systems of the prior art. The tertiary 
mirror of the present invention provides the extended focal length. The 
system also has a wider field of view that prior art systems and a flat 
image field. These also are provided by the tertiary mirror. The flat 
image field eliminates the need in prior systems for additional optical 
equipment to flatten the image. Thus, detection devices such as cameras 
are much more easily applicable to the present invention than to the prior 
art systems. Some prior optical systems have achieved some of these 
benefits with the use of refractive optics, with the resulting loss of 
spectral range. However, in the present invention, they are achieved with 
reflective optics, thereby maintaining the wide spectral range of the 
system. 
The physical configuration of the present invention provides the present 
invention with an accessible exit pupil. The accessible exit pupil 
provides certain advantages. Optical devices such as baffling devices and 
analgesic wavefront correction devices can be coupled to the exit pupil to 
correct the optical output. Also, because the fold mirror is located at or 
close to the focus between the primary and secondary mirrors, it can be 
small in size. This small size reduces blockage or vignetting of the 
image. 
The system of the present invention can be incorporated into aircraft and 
satellites to image and record objects at large focal lengths on or 
adjacent the earth's surface. These embodiments are referred to generally 
as aerial imaging for the purpose of this application.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 is a perspective view of a preferred embodiment of the reflective 
scanning system 10 of the present invention. Light 11 from scene 12 enters 
the system 10 through entrance aperture 14. The light impinges on primary 
mirror 16 and is reflected to secondary mirror 18. Both primary mirror 16 
and secondary mirror 18 are curved to direct the light from the secondary 
mirror 18 toward a focus located at point 20. A fold mirror 22 reflects 
the light toward a tertiary mirror 24. The tertiary mirror 24 reflects the 
light along an axis back toward the first of three plane fold mirrors 26, 
28, 30. The light is reflected by the three plane mirrors 26, 28, 30 in 
succession along a path to an exit aperture 32. The light leaves the 
device 10 through the exit aperture 32 and is detected by optical 
detection apparatus 34. 
The system 10 is rotatable about a pitch axis 36 and a roll axis 38. When 
scanning a scene 12, rotation about both of these axes 36, 38 is 
simultaneously controlled by a system shown in greater detail in FIG. 3 
that points the system in the desired direction. 
The light from secondary mirror 18 is reflected by the fold mirror 22 along 
the pitch axis 36 toward tertiary mirror 24. Tertiary mirror 24 reflects 
the light back along the pitch axis 36 to the plane fold mirror 26. 
When the system 10 is rotated about the pitch axis 36, mirrors 16, 18 and 
22 rotate as a unit in a fixed relation to each other. Mirrors 24, 26, 28 
and 30 remain stationary with respect to mirrors 16, 18 and 22. Thus, the 
light leaving third mirror 22 will continue to impinge on tertiary mirror 
24 and will be directed by mirrors 26, 28, 30 to exit aperture 32 and on 
to detection apparatus 34. When the system 10 is rotated about the roll 
axis 38, all of the mirrors 16, 18, 22, 24, 26, 28 and 30 rotate as a unit 
in fixed relation to each other. As a result, regardless of the 
orientation of the system about the roll axis 38, light is directed 
through the system 10 to the exit aperture 32. 
FIG. 2 is a top view of the embodiment of FIG. 1 looking into the entrance 
aperture 14. In the Figure it can be seen that the light is reflected by 
mirror 22 along the pitch axis 36 to tertiary mirror 24. From the tertiary 
mirror 24, the light is reflected to plane mirror 26 then to mirrors 28 
and 30 and out through the exit aperture 32 along the roll axis 38 to 
detection apparatus 34. The tertiary mirror 24 performs the multiple 
function of providing long focal length and flat extended field of view 
over a broad spectral range. 
FIG. 3 is a schematic depiction of the mechanical configuration of the 
reflective scanning system embodiment of FIGS. 1 and 2. The system 10 is 
mounted to a dual-axis rotation system 50. Yoke 52 of the system 50 is 
mounted to driving base 54 via bearings 56. The driving base 54 includes a 
controller that controls the rotation of the yoke 52 about the roll axis 
38. Encoders 58 sense the rotation and provide feedback to the driving 
base 54. 
Primary mirror 16, secondary mirror 18, fold mirror 22, and tertiary mirror 
24 are mounted as a unit 51 to yoke 52 of the dual axis system 50 via 
bearings 60. The mirrors 16, 18, 22, and 24 are held together by 
well-known bridge and truss members (not shown) used for supporting 
optical systems. The driving yoke 52 is coupled to the mirror unit 51 to 
rotate the unit 51 about pitch axis 36. Encoders 62 sense the rotation and 
provide feedback to the driving yoke 52. 
Mirrors 26, 28, and 30 are mounted to yoke 52. Thus, it can be seen that 
when pitch axis rotation is initiated, only mirrors 16, 18, 22, and 24 
rotate as a unit, and mirrors 26, 28, and 30 remain stationary on the yoke 
52. However, when roll axis rotation is initiated, all of the mirrors 
rotate together. Under both rotational conditions, the light entering the 
system 10 is directed out of the system through exit aperture 32 along 
roll axis 38 to detection apparatus 34 in fixed relation with the driving 
base 54. 
FIG. 4 depicts the mirror arrangement of another embodiment of the 
reflective scanning system of the present invention. In this embodiment, 
light 11 enters the system 100 through entrance aperture 14 and impinges 
on primary mirror 116. The light is then reflected to secondary mirror 18. 
From the secondary mirror 18, the light travels back toward a focus 
between the primary 116 and secondary 18 mirrors. It passes through an 
aperture 121 in fold mirror 122 and then through an aperture 117 in 
primary mirror 116. It then strikes tertiary mirror 124 aft of the primary 
mirror 116. The light is reflected back to fold mirror 122 which reflects 
it along pitch axis 36 to plane mirror 26. Plane mirrors 26, 28, and 30 
direct the light out of the system through the exit aperture 32 along roll 
axis 38 to detection apparatus 34. 
As in the previous embodiment, the embodiment shown in FIG. 4 is rotated by 
similar means about the pitch axis 36 and the roll axis 38. This rotation 
about axes through internal system points provides for smaller swept 
volumes and larger scanning angles. 
FIG. 5 schematically depicts an airborn system 200 containing the 
reflective scanning system 10 of the present invention. The airborn system 
200 passes over the scene 12 in the direction indicated by arrow 202. As 
the system 200 moves over the scene 12, it is rotated in both directions 
indicated by arrows 204 and 206. As a result, the system 200 collects 
light from the scene 12 along the path 208. Thus, the entire scene 12 is 
scanned by rotating the scanning system 10 along two axes as it moves by. 
While this invention has been particularly shown and described with 
references to preferred embodiments thereof, it will be understood by 
those skilled in the art that various changes in form and details may be 
made therein without departing from the spirit and scope of the invention 
as defined by the appended claims.