Optical: systems and elements – Mirror – Including specified control or retention of the shape of a...
Reexamination Certificate
2002-05-29
2004-11-23
Robinson, Mark A. (Department: 2872)
Optical: systems and elements
Mirror
Including specified control or retention of the shape of a...
C359S851000, C359S868000
Reexamination Certificate
active
06820986
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates, generally, to the construction of telescope or imaging systems and, more specifically, to systems which use optical elements having shapes and curvatures formed by the bending or stretching of a membrane over an appropriate boundary wherein such membrane assumes a shape that can concentrate electromagnetic radiation.
2. Description of the Prior Art
Known telescope systems typically are formed either with only a single primary optical element or with a combination of a primary element along with other optical elements so as to improve the performance of the overall system. Indeed, if the field of view needs to be larger than that which is afforded by a single primary optical element, subsequent optics (a secondary and tertiary) can be used to correct any aberrations induced by the primary optical element. A number of designs for two and three mirror systems have been developed over the years resulting in systems which have large focal surfaces. See
Astronomical Optics
, Schroeder 1987. Such systems are typically “on-axis”, wherein the secondary and tertiary optics obstruct the primary optical element. However, the scattering and diffraction of the incident electromagnetic radiation by both the secondary optics and its support structure reduces the performance of the overall on-axis system. This is particularly problematic for the observation of low-contrast objects as well as in communications systems where cross-talk between nearby antennas is undesirable.
One solution to the aforementioned problem with on-axis designs is to use an off-axis design. Unfortunately, however, the field-of-view of such off-axis designs is generally limited unless steps are taken to control the new set of off-axis aberrations. One known solution to control such off-axis aberrations is to tip the secondary reflector with respect to the primary optical element. As a result, the aberrations induced by the tipped secondary reflector cancel those of the off-axis primary optical element, thus affording a performance which is substantially equal to that of an unobstructed on-axis reflector of the same aperture. Be that as it may, the field-of-view for such a system still has much to be desired. Thus, it can be appreciated that an off-axis system with a wide field-of-view would be desirable in either a telescope or imaging system.
Further, there are problems associated with the construction of precision reflectors. Such problem of constructing precision reflectors (initially for use in telescope mirrors) has a long history tracing back to Gregory (1663), Newton (1668) and Cassegrain (1672). The first successful reflectors using glass substrates with silvered reflecting surfaces were constructed in the late 1850's by von Steinheil in Germany and Foucault in France. The function of any glass or metal mirror is to act as a substrate providing support for a thin layer of high reflectivity material—the glass or metal being formable into shapes that have useful optical properties. By examining the areal densities of the reflecting layer and the substrate we find that the current state-of-the-art has much to be desired, wherein a factor of at least 10
7
in areal density exists between the reflecting surface and the supporting substrate.
The areal density of the reflecting layer is given by
&sgr;
m=&rgr;t
with t being the thickness of the reflecting layer, and p the density. The thickness of the reflecting layer of a high electrical conductivity metallic film can be determined, to good approximation for a specific reflecting material, by considering the skin depth
δ
=
1
πνμσ
e
,
where &sgr;
e
is the conductivity of the reflecting surface, v is the frequency of the electromagnetic radiation, and &mgr; is the permittivity of the reflecting surface. For a very good conductor like copper, &sgr;
e
=5.7×10
7
&OHgr;
−1
/m and &mgr;=1. If we consider a drop in intensity of 10
6
to be opaque, we find that t=7&dgr;. In the case of optical light (&lgr;=0.5 &mgr;m), the film only has to be 50 nanometers thick to reflect the incident light with little loss; for microwaves (&lgr;=1000 &mgr;m), a 1.7 &mgr;m thickness is required. For this example, &sgr;
m
~2×10
−3
g/m
2
in distinct contrast to the areal density of the substrate material, which can be many orders of magnitude greater.
Current technology millimetric telescopes have densities of order 10 kg/m
2
, about a factor of 10
7
between the reflecting layer's density and that of the support structure. For optical telescopes, the situation is much worse with the current state of the art having areal densities of order 150 kg/M
2
(the NASA 2.5 m HST and the Air Force Starfire 3.5 telescopes). By examining existing telescopes one finds that the mass density of the supporting substrate (generally some form of glass) is
&sgr;
m
∝(aperture)
0.5
.
This is independent of the technology used, or the epoch when the telescope was constructed.
By comparison, the areal density of a membrane reflector system scales differently and is straightforward to calculate. For the reflective membrane itself
&sgr;
m
=&rgr;
m
t.
For the supporting ring
σ
⊕
=
-
4
ρ
⊕
h
(
d
)
Δ
d
d
here h(d) is the functional dependence of the ring's height on the diameter of the ring, and &Dgr;d is the width of the ring. The total density is simply the sum
σ
=
σ
m
+
σ
⊕
=
ρ
m
t
+
4
ρ
r
h
(
d
)
Δ
d
d
.
It is instructive to note two cases, h(d)=h (a constant height ring), and h(d)h=
0
(d/d
0
)
1/3
(a constant stiffness ring). In the first case, the areal density decreases with aperture as it does for the constant stiffness case. Only if the ring has h(d)=h
0
(d/d
0
)
&agr;
with &agr;>1 does &sgr; grow with
σ
=
ρ
m
t
+
4
ρ
⊕
(
ho
do
)
(
d
do
)
α
-
1
Δ
d
This is in distinct contrast to the data for current mirrors, which have &sgr;∝d
0.5
. For larger diameters, the thickness of the membrane can be reduced, since for a given deflection the pressure can be lower. Thus, not only is a membrane reflector less massive to being with, but the areal density can actually decrease with larger apertures if the ring and membrane are chosen correctly.
Clearly, the areal density of a telescope or imaging system could be reduced by large orders of magnitude by constructing only the desired reflective surface and not the heavy supporting structure needed to control the gravitationally induced deformations. To date, however, such design has not been practically implemented. Further, it is not known to use a space curve as a boundary to produce off-axis surfaces that can represent a segment of a conic section. Moreover, the prior art does not provide for the figuring of such a surface by selectively distorting the associated boundary. In addition, there is nothing in the prior art which discusses the re-imaging of an off-axis optical element onto a deformable tertiary so as to correct for non-ideal primary surface shape.
SUMMARY OF THE INVENTION
Therefore, the present invention is directed to a method for constructing telescope systems, antenna systems, imaging systems, or other concentrators of electromagnetic radiation having optical elements whose shapes, orientations and locations are specifically chosen to achieve a diffraction limited optical system. The individual optical elements may be constructed by deforming a membrane such that non-symmetric aspherical low-mass surfaces are achieved.
Accordingly, in an embodiment of the present invention, an apparatus for electromagnetic radiation concentration is provided which includes: a peripherally-disposed boundary member, the boundary member having a shape defined as a space curve which closes upon itself and which
Bell Boyd & Lloyd LLC
Robinson Mark A.
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