Dual aperture multispectral Schmidt objective

A dual aperture, off-axis catadioptic Schmidt objective (40) is formed by symmetrically aligning two pairs of Schmidt objectives (12, 14) on opposite sides of a common plane (x, z). Each objective has a spherical primary mirror (16/18) with a spherical focal plane (44/46) and center of curvature (20/22) aligned along an optic axis (36/38) laterally spaced apart from the common plane. A multiprism beamsplitter (44/46) with buried dichroic layers (81-83) and a convex entrance (48) and concave exit (52a-52f) surfaces optically concentric to the center of curvature may be positioned at the focal plane. The primary mirrors of each objective may be connected rigidly together and may have equal or unequal focal lengths.

TECHNICAL FIELD 
The invention relates to catadioptic instruments generally and, more 
particularly, to Schmidt optical objectives for multispectral mapping 
systems. 
BACKGROUND ART 
High resolution multispectral mapping of the surface of the earth with air 
or spaceborne instruments requires optical objectives providing precise 
registration between counterpart detectors in the image planes of all 
spectral bands. Although there are numerous Schmidt optical systems for 
focusing multispectral images, those presently available systems which are 
sufficiently accurate to be used with multispectral instruments for 
mapping the surface of the earth must be optically linked to the mapping 
instrument by precision mechanical scanning mechanisms. The multispectral 
scanner (MSS) and the thematic mapper (TM) developed by the National 
Aeronautics and Space Administration are two examples of such instruments. 
Both use rapidly moving, mechanically driven, scanning mirrors to produce 
a whiskbroom scanning action of photodetectors upon ground resolution 
elements. Mechanical scanning mechanisms, however, inherently lose 
precision with continuous use, lack long-term reliability, generate 
undesired thrust forces which disturb other instruments, and have size 
limitations that limit the degree of resolution obtainable from a 
multispectral mapping instrument. 
A French optical system known as a "Spot" instrument, currently under 
development, avoids the problems of mechanical scanning systems by using a 
two on-axis catadioptic Schmidt objectives boresighted to scan different, 
but adjoining fields of view. Orbital motion of the spacecraft sweeps a 
detector array across the earth in a direction perpendicular to the long 
dimension of the swath of earth scanned, producing a scanning effect known 
as pushbroom scanning. Eight linear detector arrays produce a four 
spectral band mapping system. The on-axis structure of the Spot 
instrument, however, produces a central observation which causes a degree 
of diffraction precluding operation of the system in a high resolution, 
short wavelength infrared (SWIR) mode. Also, the on-axis structure limits 
the space available for incorporation of a beamsplitter into the system 
which restricts application of the system to three, relatively closely 
spaced bands in the visible spectrum. 
STATEMENT OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
improved Schmidt optical objective. 
It is another object to provide a Schmidt optical objective free of 
paraxial obscuration. 
It is still another object to provide a Schmidt optical objective suitable 
for high resolution in the near and intermediate infrared band. 
It is yet another object to provide a Schmidt optical objective permitting 
a higher signal-to-noise ratio. 
It is a further object to provide a Schmidt optical objective compatible 
with a multispectral beamsplitter functioning over widely separated bands 
in the visible and invisible spectra. 
It is a still further object to provide a Schmidt optical objective which 
can produce a greater number of precisely registered image planes. 
It is a yet further object to provide a Schmidt optical objective which can 
produce a greater number of image planes of precisely the same focal 
length and distortion. 
It is also an object to provide a Schmidt optical objective which can be 
optically linked to a multispectral mapping system by stationary devices. 
These and other objects are achieved with a dual aperture optical 
configuration having a pair of off-axis catadioptic Schmidt primary 
objectives. The configuration is formed by two rigidly connected, 
side-by-side, off-axis spherical reflectors symmetrically aligned with 
parallel optical axes along a common plane of symmetry with two 
side-by-side refractive corrector plates located as aperture stops 
positioned at the centers of curvature of the spherical reflectors. A pair 
of multispectral dichroic beamsplitters having entrance and exit faces 
concentric to their respective aperture steps are symmetrically positioned 
about the common plane at the spherical focal planes of the spherical 
reflectors.

DETAILED DESCRIPTION OF THE INVENTION 
FIGS. 1 and 2 illustrate the Schmidt optical system 10 of the invention in 
right angle planar views taken along the x, y and x, z planes, 
respectively. The system is formed of a pair of side-by-side catadioptic 
Schmidt objectives 12, 14. The primary elements of the objectives are a 
pair of spherical mirrors 16, 18 symmetrically positioned side-by-side on 
opposite sides of the central x, z common plane. As shown in FIG. 3, the 
curvatures of the spherical reflecting surfaces 17, 19 of spherical 
mirrors 16, 18 are oriented to diverge outward with their centers 20, 22, 
respectively, separated on opposite sides of the common x, y plane. The 
orientation of the curvatures of spherical mirrors 16, 18 causes incoming 
paraxial light rays 24, 26 to be divergingly reflected away from the 
common x, z plane as rays 28, 30, respectively. Refractor lens plates 32, 
34 are symmetrically positioned side-by-side on opposite sides of the 
common x, z plane, between the optical axes 36, 38, respectively, of the 
objectives to correct for spherical aberrations by introducing spherical 
aberrations into transmitted light rays equal and opposite to the total of 
the spherical aberrations caused by the respective spherical mirrors. 
Corrector lens 32, 34 form dual aperture stops. Refractive beamsplitters 
40, 42 are positioned along the optical axes 36, 38 at the respective 
focal planes 44, 46 of the objectives. Concavity of spherical mirrors 16, 
18 causes focal planes 44, 46 to be spherical and backward curving. Each 
beamsplitter accepts the image forming energy of a single optical 
objective 12, 14. To maintain the concentricity of the Schmidt system 10, 
the entrance 48, 50 and exit faces 52, 54 of the beamsplitters preferably 
are spherically concentric to the respective aperture stops of each 
objective 12, 14. 
The divergence of reflected central light rays 28, 30 caused by the 
outwardly directed orientation of the curvatures of the spherical mirrors 
16, 18 causes the focal planes to be located outside of the paths of the 
central rays. This allows the beamsplitter 40, 42 to occupy a larger space 
at the focal planes without affecting resolution of the images. FIGS. 4 
and 5 illustrate an assembly 70 of nine prisms 72-80 of conventional prism 
materials which may be used as one of the beamsplitters 40, 42. Prisms 72 
and 73, 72 and 74, 76 and 77 are cemented together along joints 81, 82 and 
83 respectively. Prism assembly 70 uses thin multilayer dielectric 
coatings 86-90 as dichroic elements between prisms 72 and 79, 72 and 76, 
74 and 75, 77 and 78, and 79 and 80, respectively, to split an image into 
six different spectral bands by reflecting the shorter wavelengths. In 
assembly 70, the shorter wavelength bands A through D are reflected while 
the longer wavelength bands E and F are transmitted by dichroic surface 
coating 86. The shorter wavelength bands A and B are again reflected by 
dichroic coating 87 while the longer wavelength bands C and D are 
transmitted. Dichroic coating 88 reflects band A and transmits band B 
while dichroic coating 89 reflects band C and transmits band D. Dichroic 
coating 90 reflects band E perpendicularly through concave exit face 52e 
and transmits longer wavelength band F. The rays of transmitted band F 
pass perpendicularly through concave exit face 52f. Prisms 74, 75, 77, and 
78 cause total internal reflection of the rays of bands A-D, respectively, 
directing those rays perpendicularly through concave exit faces 52a-52d, 
respectively. 
Beamsplitters 40, 42 may be used to provide redundant spectral images. In 
other applications, however, beamsplitters 40, 42 do not need to be 
identical. One beamsplitter may be dedicated to bands within the visible 
spectrum while the other is dedicated to the infrared spectrum. By using 
two different beamsplitters similar to prism assembly 70, optical system 
10 may be made to provide images in at least twelve different spectral 
bands. The beamsplitters may also be focused upon photodetector arrays 
maintained at different operating temperatures. The arrays of detectors 
focused on one beamsplitter may be cooled to a different temperature than 
those arrays focused on the other beamsplitter because their off-axis 
locations easily accommodate ancillary equipment without obstructing light 
rays directed toward the primary objectives. 
Lacking moving parts and servomechanisms, stationary dual aperture optical 
system 10 has good stray light rejection and is capable of simultaneously 
providing high spectral and high spatial resolution images, for example, 
to multispectral linear photodetector arrays in an instrument such as a 
multispectral terrestrial mapping system. The divergent, off-axis 
orientation of the image planes 40, 42 by spherical mirrors 16, 18 permits 
the use of large, dichroic beamsplitters 40, 42 to simultaneously split 
reflected light rays into a very large number of images in different bands 
over a very wide visible and invisible spectrum without introduction of 
diffraction effects in the paraxial regions and provides high resolution 
for long wavelengths. It is possible to obtain much wider spectral ranges 
with this system because each half of this dual aperture Schmidt may be 
separately corrected for spherical aberration and color. 
Several features may be incorporated into the optical system 10 to enhance 
its performance. The spherical mirrors 16, 18 may be connected rigidly 
together as for example, by a mirror bracket or an epoxy type bonding 
cement 56 applied between their parallel arcuate edges 58, 60. Rigid 
connection assures the maintenance of precise band-to-band alignment 
between the focal plane images of spherical mirrors 16, 18 even when the 
primaries are subjected to motion. Since the beamsplitter elements are 
positioned close to the focal planes 4, 6, their optical lever arm is 
quite short, thus maintaining good optical alignment even when subjected 
to some motion. When made with entrance and exit faces optically 
concentric to their respective aperture stops, all off-axis aberrations 
arising in the beamsplitter are eliminated. Consequently, beamsplitters 
40, 42 contribute no aberrations to the system except for small amounts of 
spherical aberration and longitudinal color. Both aberrations may be 
compensated for by achromatizing and modifying the spherical aberration 
correction of the corrector plates. Concentricity of the beamsplitters, 
focal planes, and primary mirrors make the effects of coatings upon their 
surfaces independent of position in the field of view, thereby increasing 
the uniformity of response. The performance of optical system 10 may be 
made, therefore, nearly aberration free, limited only by small amounts of 
off-axis aberrations of coma and astigmatism introduced by weak corrector 
plates 32, 34. When the optic axes 36, 38 of both primaries are axially 
aligned to point in the same direction and aximuth, photodetector arrays 
(not shown) positioned at the spherical focal planes 44, 46 will be 
conjugate to the same line along a terrestrial surface and there will be 
counterpart correspondence between individual detectors in all spectral 
bands. 
Other modifications may be made in the performance of optical system 10 by 
changing the characteristics of various of its elements. It is possible, 
for example, to configure refractor corrector plates 32, 34 for optimal 
chromatic correction in different spectral regions because the corrector 
plates are extremely weak in power. One or both of the corrector plates 
32, 34 may be removed and replaced with elements transmissive in different 
spectral regions as may be necessary, for example, to accommodate a 
thermal infrared photodetector array, without significantly affecting 
focal length or distrotion. Alternatively, in less demanding chromatic 
correction applications both corrector plates may be replaced with a 
single refractive corrector plate. Compound, all-spherical corrector 
plates may be used in place of aspheric corrector plates in other 
applications. 
Ideally, the primary spherical mirrors are assured of precise equality in 
focal lengths by being cut from a single, faster parent spherical mirror. 
Should the photodetectors fed by one of the objectives 12, 14 be of a 
different size than those fed by the other objective, two off-axis Schmidt 
objectives of different focal lengths may be juxtaposed with their primary 
spherical mirrors bonded together. Although bonding of the primaries in 
such a configuration provides only partial compensation for misalignments 
due to movement of the primaries, the similarity between the two 
objectives (e.g., one is a scaled version of the other), will cause 
distortion effects along each band to be identical.