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A telescope with complementary primary mirrors






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:
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):






Eric Brasseur  -  April 9 till April 20 2010