The key part of a telescope is a huge parabolic primary mirror:
Some
telescopes contain only that one mirror and observers look directly
into
it. Most telescopes use a small secondary mirror and some lenses to
focus the image towards the observer's eye (or the CCD captor). In some
layouts, the
primary mirror is not parabolic. It may for example be spherical,
because this is less expensive to produce. Then the secondary mirror
has the appropriate shape to correct the aberrations caused by the
primary mirror.
Can two simplified primary parabolic mirrors complement each other? The
idea would be to use two flat surfaces and bend them. Their combination
would equal a virtual parabolic mirror:
+
=
The light is reflected by the first primary surface towards the second
one. These two successive reflections act as a single reflection on a
parabolic mirror. In the drawing below, the second primary mirror has a
stronger curvature
because it is closer to the focus point:
If pellicle mirrors are used, then the two mirrors can be
superposed, at the cost of loosing a lot of light. There will be
many
images superposed on the captor but this can be computer-filtered. I
guess the surfaces need to be quite thick so that secondary reflections
appear wider apart. Moving at least one surface, or using two
telescopes with a difference, is probably helpful to help compute away
the reflections:
The supposed advantage of this system is that manufacturing and
assembling each mirror is easier or less expensive, maybe allowing for
wider surfaces. A flat mirror can be bent
towards a parabola. The actuators can even stay active to compensate
for the atmospheric turbulences:
The assembly drawn above would cause ripples in the mirror, because the
traction points are too wide apart. In order to keep a smooth surface,
maybe thousands or millions of nanoactuators must be used. Another way
round, or complementary, would be to rely on the natural tendency of
the surface to bend. Two opposite sides can be pushed inwards, the
gravity can bend the surface or a vacuum can be created... Maybe a
combination can be used like pushing the sides inwards and creating a
pressure below to correct the shape... Or pouches beneath the mirror
can be filled with water and yet other pouches can be pressurized or
depressurized...
There are some likenesses with liquid mirror tchnology:
The standard way to create perfectly flat surfaces of glass
(hence mirrors) is to let liquid glass cool down drifting on liquid
metal. (By the way, why are solid parabolic mirrors not made this way,
rotating the liquid metal?)
An assembly with a solid structure and rigid actuators can
perhaps be changed in elevation quicky. But an assembly like depicted
above, with the aim to use a really huge surface, would be fixed just
like liquid mirrors are. To the least, a small change in elevation
would require a
lengthy re-shaping of the mirrors by pulling on the strings.
The curvature of both mirrors can be tuned according to the
needs, just like liquid mirrors, whose curvature can be
changed by altering the speed of rotation of the container.
Another supposed advantage would be that the mirrors can be rolled
towards tubes, for example to be launched to orbit and assembled in a
space telescope. (By the way this can be done with liquid mirrors too,
rotating a container assembled in orbit and using a ion motor to get a
constant acceleration.) (Or using the gravitational pull and the
rotation at the pole of an asteroid...)
The rectangular periphery creates an inhomogeneous diffraction pattern.
This can be exploited to extract information around very bright
objects. For example searching for planets around a star, by rotating
the telescope.
Can the mirrors be made out of separate rectangles, either soldered
together or bent individually and placed side by side? In this second
case, they need not to form a continuous curve. They can be placed at
the same height, like elements of a Fresnel lens. Yet the smears caused
by the diffraction will be worse I suppose...
I don't know it the mirrors can be put aside of each other so that all
the light can be focused. There is a fair chance that a more conical
bending of the surfaces allows for this or that a secondary mirror or
lens system can compensate the aberration (maybe use a second pair of
bended flat surfaces to be the secondary mirror, with the ability to
make the corrections dynamically):
