Optical systems having multiple simultaneous functions

An all-reflective optical system has a first (10) and second (12) reflecting assembly. The first reflecting assembly (10) includes an afocal three-mirror anastigmat (18, 20, 22) with one or more apertures (28 A-D) in the tertiary mirror (22) to enable light or energy to pass therethrough. Light or energy reflects from the second reflecting assembly (12) through the apertures (28 A-D) to provide simultaneous viewing of a scene by a plurality of instruments (34). The second reflecting assembly (12) includes a planar mirror which provides pointing and stabilization motions for all of the instruments simultaneously without degrading image quality or pupil registration.

BACKGROUND OF THE INVENTION 
1. Technical Field 
This invention relates to reflective telescope systems and, more 
particularly, to an all-reflective optical system which provides energy 
from a viewed scene simultaneously to multiple scientific or strategic 
surveillance instruments. 
2. Discussion 
In both scientific observation and strategic surveillance activities, it is 
desirable to have an optical system which provides energy or light to 
separate instrument packages, which are utilized for specialized purposes 
by the system. Preferably, it is desirable to have the instrument packages 
located behind a common light gathering optical system or foreoptics. It 
is also desirable to provide energy or light from a viewed scene 
simultaneously to each of the packages. While it is generally not required 
in such applications that all instruments share precisely a common line of 
sight, it is desirable that all instruments point substantially in the 
same direction and that light from a point object of interest within the 
scene be rapidly and accurately pointed or steered to any or all of the 
instruments. Additionally, it is desirable that the mechanism for this 
pointing or steering function also provide the function of line of sight 
stabilization during the time that an instrument or instruments receive 
light from the scene. 
In the past, the integration of several instruments behind a common 
telescope generally utilized an image forming two-mirror Cassegrain-like 
optical system as its foreoptics. The instrument packages interfaced with 
the aberrated image formed by the Cassegrain-like foreoptical system. 
Furthermore, these devices utilize the Cassegrain secondary mirror as a 
line of sight pointing and stabilization mirror even though the required 
tilt motions degraded the image quality of the optical system and caused 
excessive beam wander on certain optical elements. Further, these systems 
have many other disadvantages. Some of the disadvantages are the size and 
weight of the system, operational utility, and testing and integration of 
the separate instruments. 
A number of afocal three-mirror anastigmat telescopes which have different 
magnifications have been designed and implemented in the past. An example 
of this type of system is illustrated in U.S. Pat. No. 3,674,334, issued 
Jul. 4, 1972 to Offner, entitled "Catoptric Anastigmatic Afocal Optical 
System." 
SUMMARY OF THE INVENTION 
According to the teachings of the present invention, a system is provided 
which utilizes an all-reflective afocal optical system to provide energy 
from a viewed scene simultaneously to several instruments. The present 
invention provides a foreoptical system which may be utilized as a general 
telescope to provide energy from a viewed scene simultaneously to several 
instruments. The configuration of the afocal three-mirror anastigmat 
utilizes the offset and annular nature of the telescope field of view, the 
afocal magnification, and the location of the exit pupil. 
The telescope provides substantially large aperture and field of view 
capabilities. The present invention provides for correction of spherical 
aberration, coma, and anastigmatism and provides a flat field of view. 
Also, multiple scientific instruments such as infrared spectrometers, 
imaging photometers, infrared array cameras, and fine guidance sensors can 
simultaneously receive the energy from the afocal three-mirror anastigmat. 
In the preferred embodiment, the all-reflective afocal optical system 
includes an entrance pupil region, a first reflective assembly and a 
second reflective assembly. The first reflective assembly includes an 
opening or aperture to enable passage of energy or light therethrough. The 
second reflective assembly is positioned at the exit pupil of the first 
reflecting assembly such that the energy or light reflected by the second 
reflecting assembly passes through the opening or aperture in the first 
reflective assembly and into the various instruments. The second 
reflective assembly provides the internal pointing and stabilization 
functions for all of the instruments simultaneously, and without 
degradation to either image quality or pupil alignment.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIGS. 1 and 2, an afocal optical system is shown including a 
first 10 and second 12 reflecting assembly. The system also includes an 
entrance pupil region 14 and an exit pupil 16. The first reflecting 
assembly 10 is comprised of a primary 18, secondary 20, and tertiary 22 
mirror. Likewise, the second reflecting assembly 12 is comprised of a 
planar or flat mirror. 
The primary mirror 18 of the first reflecting assembly 10 includes a 
central axis 24 defining the system optical axis. The primary mirror 18 is 
fixably positioned on-axis with respect to the optical axis 24. The 
primary mirror 18 includes a central hole 26 to enable passage of light 
therethrough. The primary mirror 18 is a positive power mirror and may be 
a paraboloid conic or higher order aspheric mirror. 
The secondary mirror 20 is a negative power mirror. The secondary mirror is 
fixably positioned on-axis with respect to the optical axis 24. The 
secondary mirror 20 is positioned in front of the primary mirror 18 such 
that light reflecting from the secondary mirror 20 passes through the 
primary mirror's hole 26 to the tertiary mirror 22. The secondary mirror 
20 may be a hyperboloid conic or higher order aspheric mirror. 
The tertiary mirror 22 is a positive power mirror. The tertiary mirror 22 
is fixably positioned on-axis with respect to the optical axis 24. The 
tertiary mirror 22 may be an parabaloid conic or higher order aspheric 
mirror. Due to the rotational symmetry of the three mirrors which comprise 
the afocal telescope of the first reflective assembly, the field of view 
of this telescope exhibits annular symmetry. Furthermore, the field of 
view can be apportioned and utilized as shown in FIG. 3, where provisions 
are made for four separate instruments. The center of the telescope field 
40 is centered on the telescope optical axis 24, but the four portions of 
the field 42(a-d) used by the four separate instruments are the triangular 
regions contained in the annulus defined by field angles 44 and 46. The 
angular field radius 44 may have a typical value of 20 to 30 arc minutes 
and the angular field radius 46 may be 50 to 60 arc minutes. 
Referring again to FIGS. 1 and 2, the tertiary mirror 22 includes one or 
more openings or apertures 28(a-d). Preferably, the tertiary mirror 
includes four such apertures 28(a-d) positioned radially about the mirror 
22. The apertures 28(a-d) are positioned such that they are diametrically 
opposite of the position where the reflected beam is passed from the 
second mirror assembly 12 as seen in FIG. 2 which will be explained 
herein. 
The flat pointing and stabilization mirror 12 of the second reflecting 
assembly is positioned at the exit pupil 16 of the three-mirror anastigmat 
system 10. The stabilization mirror is a flat planar mirror and receives 
collimated light or energy from the tertiary mirror 22. The stabilization 
mirror 12 is positioned in front of the tertiary mirror 22 on-axis with 
respect to the optical axis 24 as seen in FIG. 1. The mirror 12 is 
tiltable on the optical axis 24. 
As light is reflected from the tertiary mirror 22 to the stabilization 
mirror 12, the light or energy from the stabilization mirror 12 is 
reflected through the apertures 28(a-d) in the tertiary mirror 22. The 
stabilization mirror 12 is positioned such that the light reflecting from 
the tertiary mirror reflective portion 32(a-d) passes through its 
corresponding aperture 28(a-d) positioned diametrically opposing the 
reflecting portion 32(a-d) of the tertiary mirror as seen in FIG. 2. 
The light that passes through the tertiary mirror 22 then passes into the 
regions 34(a-d) in the individual instrument volume spaces 36(a-d) behind 
the tertiary mirror 22. The instruments will be positioned at a desired 
position behind the apertures as illustrated in FIG. 1. As the light 
reflects from the tertiary mirror 22 to the stabilization mirror and back 
through apertures 28(a-d) the light energy is received by the instruments. 
The instruments receive the light or energy simultaneously from the viewed 
scene. Thus, the telescope would enable simultaneous operation on the 
reflected energy. Because the telescope has a flat field and has very low 
residual aberrations, and due to the reimaging characteristics of the 
telescope and the location of the pointing and stabilization mirror, the 
pointing and stabilization motions preserve both image quality and pupil 
alignment. 
The optical path of the beam passing through the optical system is such 
that the light diverges from the intermediate image 38 formed by the 
primary and secondary mirror to the tertiary mirror 22. The light is 
recollimated by the reflection from the tertiary mirror 22. The light is 
next reflected from the tertiary mirror 22 to the stabilization mirror 12 
which is located at the system exit pupil 16. The light reflects from the 
stabilization mirror and passes through a desired aperture 28 in the 
tertiary mirror 22. The energy passes into the instrument volume space 36 
which is generally behind and outside the tertiary mirror 22. 
Thus, with the pointing and stabilization mirror 12 at its nominal position 
and tilt, light from the field of view region 42a in FIG. 3 will be 
reflected from region 32a of the tertiary mirror as shown in FIG. 2. This 
light will then reflect from the pointing and stabilization mirror 12 and 
pass through the aperture 28a in the tertiary mirror and proceed into 
region 34a in the instrument volume 36a. 
It is important to note that light from a scene contained within any one of 
the field of view regions 42(a-d) can be directed into any one of the 
instruments in regions 36(a-d) by a simple tilt motion of the pointing and 
stabilization mirror 12. Thus, light from field of view region 42a that 
strikes tertiary mirror region 32a can be directed using pointing mirror 
12 into any of the opening or aperture regions 28(a-d) and into the 
corresponding instrument contained in volumes 36(a-d). This pointing or 
steering is accomplished without degradation to either image quality or 
pupil alignment. 
A specific prescription for the system in accordance with the present 
invention as illustrated in FIGS. 1, 2 and 3 is as follows. 
TABLE 1 
______________________________________ 
Optical Prescription of a Specific Embodiment 
of the Optical System of the present invention. 
Conic Ma- 
No. Description 
Radius Constant 
Thickness 
terial 
______________________________________ 
18 Primary -238.000 -1.0010 -97.2324 
Refl 
Mirror 
20 Secondary -52.8443 -2.0489 157.3919 
Refl 
Mirror 
22 Tertiary -67.6732 -0.97975 
-42.1352 
Refl 
Mirror 
12 Point/Stab 
.infin. -- 100.000 Refl 
Mirror 
______________________________________ 
(+) Distance are to the right along primary optical axis 
(+) Radii have centers to the right 
(+) Decenters are up 
(+) Tilts are counterclockwise, degrees 
CC Conic Constant = -(Eccentricity).sup.2 
Thicknesses are between mirror vertices. 
All dimensions are in inches unless specified otherwise. 
The present invention has several advantages over the prior art foreoptical 
systems. The present invention uses an all-reflective afocal system which 
provides simultaneous functioning of several sensing devices. The present 
invention enables a small compact all-reflective system to be utilized as 
a common foreoptic assembly. The present invention minimizes the space 
requirement for the foreoptic system. Also, the present invention 
eliminates refractive elements and provides desired characteristics. 
Additionally, the present invention allows for large angle pointing and 
stabilization motions without degradation to either image quality or pupil 
registration. While the prior art uses a Cassegrain-like foreoptics which 
dictates an aberrated and focal optical interface to the instruments, the 
present invention uses an afocal telescope with a collimated and 
aberration free optical interface. This collimated and aberration-free 
optical interface is less sensitive to misalignment between the 
instruments and the foreoptics, and greatly aids in the alignment, 
testing, and integration of the various instruments. 
It should be understood that while this invention has been described in 
connection with the particular examples hereof, that various 
modifications, alterations and variations of the disclosed preferred 
embodiment can be made after having the benefit of the study of the 
specification, drawings and subjoined claims.