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Employing a leading-edge
technology can fundamentally change the requirements on an
optical design. This is true for a range of emerging technologies,
including the curved electro-optical detector. Curved electro-optical
detectors will enable the development of new optical design
configurations that can be smaller than conventional flat-field
designs, thereby benefiting many aerospace applications.
Space
borne, aerospace optical systems often require a very wide
field of view, for example, in full-earth observation applications.
On a satellite in a typical orbit altitude, an objective lens
needs to cover about 120° to view horizon to horizon on
the earth. Achieving this wide field coverage typically requires
a complex and long lens. However, the cost of launching a satellite
requires the size of each subsystem to be kept to a minimum.
It is often preferable to cover the entire field of interest
simultaneously (i.e. in a “staring ” mode), instead
of in a scanning mode in which the field of view at any given
instant is a subset of the full field of interest. Scanning
systems also require moving parts such as rotating mirrors.
Moving parts should be kept to a minimum in a space-borne system
due to the risk of mission failure if the moving mechanism
jams or breaks down.
Traditional Design
Optical designs are available to cover wide angles of 90 °to
120 °or more, but traditionally consist of numerous optical
elements. An example of this is a conventional inverted telephoto
lens, which needs this complexity to minimize image imperfections,
or aberrations, across the field of view. The length, as a
multiple of the input beam diameter, is typically 15 or more
for an F/3-class lens, as in the example in Figure 1A. Disadvantages
of a design with a large number of elements include greater
sensitivity to manufacturing and alignment errors, cost, and
length.
The image-recording
surface (film or detector) in a camera is typically flat,
which is the easiest shape to manufacture. However, in most
image-forming lenses, the natural tendency is for the surface
of best focus to be curved, not flat. This is due to an aberration
known as Petzval curvature, which is the power of an element
divided by its refractive index summed over all elements
in the lens. The wider the field angle of a lens, the more
severe the image degradation is due to Petzval curvature.
Severity increases as the square of the field angle. Therefore,
to create a wide angle flat-field lens requires a complex
arrangement of positive and negative elements to keep the “Petzval
sum” (and other aberrations) adequately low. The complexity
and length of the lens also increases with the field angle.
This can be seen by comparing the design of Figure 1A (90° field
angle, 14 elements) with those of Figure 1B (28°, 6 elements)
and Figure 1C (6°, 2 elements).
Another factor in the complexity
of Figure 1A is the asymmetry of the lens. Certain aberrations
are exactly or nearly zero in a lens with front-to-back symmetry.
In a non-symmetric lens, these aberrations must be corrected
by the complexity of the configuration. The best-performing,
flat-field wide-angle lenses have the characteristic and non-symmetrical
layout of Figure 1A, specifically, a negative power front group
of elements, and a positive power group toward the image plane.
Monocentric Design
A general design form that can achieve wide-angle field coverage
with a simple design is known as the monocentric form, consisting
of surfaces all concentric about a common point. Such designs
have been accepted in various forms for some time. Monocentric
designs can address the issues of length and complexity in
a wide-angle staring lens. The following example achieves wide
field coverage by symmetry rather than complexity.
The enabling factor in this design is the use of a curved
focal surface. Curved photographic film has been used in specialized
applications for years, such as in the well-known Schmidt camera.
Modern aerospace systems predominantly use electro-optical
detectors like CCDs, rather than film. Curved focal surfaces,
especially using electro-optical detectors, are not nearly
as common as flat focal planes due to the manufacturing difficulty.
Electro-optical detectors are preferable to film in aerospace
applications because of the nearly instantaneous data transmission
(no need to retrieve film and develop it) and the continual
reusability of the detector. These are the same reasons digital
cameras are becoming more popular in the consumer market and
are competitive with film cameras. Likewise in the commercial
filmmaking industry, digital cameras are expected to largely
displace film cameras in the coming years.
When the constraint for a flat image is removed, this eliminates
the need to control Petzval curvature, and therefore, the need
for asymmetric layout (Figure 1A). This allows the potential
to correct the remaining aberrations with a dramatically simpler
layout.
This design consists of two spherical glass components: a
meniscus (convex-concave) first element, and a ball-shaped
second element. The ball-shaped second element may be two hemispherical
plano-convex elements bonded together, with an aperture stop
to limit the beam boundary deposited in the center, as shown
in Figure 2. All surfaces are concentric about the central
aperture stop. This concentricity results in the aberrations
of coma, distortion, and lateral color (variation in image
height with wavelength) to be identically zero. The other aberrations
can be controlled (if not exactly zero) by appropriate choice
of glass types and element thicknesses.
The length from the first surface of the lens to the focal
surface is 7.5 input beam diameters, half the length of typical
inverted telephoto lenses of the same field of view and f
number. The f number is the ratio of the lens focal length
to the input beam diameter. The design is less sensitive
to fabrication and alignment errors than the flat-field
design of Figure 1A, because the angles of incidence of
the rays on the surfaces are lower, and there are only two
components to align instead of ten or more.
The concentricity of the design causes the performance to
be nearly uniform across the field, since the ray angles on
the air-glass interfaces are the same at any field angle. Variation
in performance across the field is due only to the change in
beam angle at the aperture stop, causing a truncation of the
beam in one direction according to the cosine of the incidence
angle. The example in Figure 2 has a 120° total field,
and in principle this could be extended to nearly 180°,
although the cosine falloff of the beam area would preclude
a useful signal at angles at or near 180°. The variation
in performance across the field is gradual, as shown in Figure
3, which plots the spot size at the focal surface in microns
versus the field angle in degrees.
This article was authored by J. Michael
Rodgers, principal engineer of Optical Design, for Optical
Research Associates (Pasadena,CA). For more information,
contact the author at miker@opticalres.com. Visit Optical
Research Associates at www.opticalres.com. The monocentric
design example included in this article is courtesy of Raytheon
Company (Lexington, MA), U.S. Patent 6,320,703; C.W.Chen;
Raytheon, 2001. Images created using CODE V®.
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