Start with a thin spherical mirror resting in a flimsy square frame. Set a round post on top of the center of the mirror. Attach the top of the post to the four corners of the frame with wires. Tighten the wires. Now the force of the post pushing on the center of the mirror parabolizes it, while the wires stiffen the frame. You end up with an ultra-cheap mirror in an ultra-light mirror box. The center post has no effect on the image since it is in the shadow of the secondary. The wires create a small amount of diffraction similar to a spider.
I believe this could work since people have produced excellent "flex mirrors" by pulling on the back of spherical mirrors. The pulling systems have to be carefully designed but are not hard to build. I don't know whether anyone has tried a center-push instead of a center-pull system. Maybe it would be better to drill a hole in the center of the mirror and extend the post to the back. I believe it would be possible to build a back-support cell that would correctly support the mirror so it would approximate a paraboloid. Unlike a PLOP design, the support would be biased towards the center.
I came up with this maybe-not-crazy idea while thinking about ways to stiffen an ultralight mirror box. One just-plain-crazy idea was to build a pyramid in front of the mirror, which would create a lot of light blockage and diffraction.
For more about flex mirrors see https://www.cloudyni...nto-a-parabola/.
Edited by Dick Jacobson, 01 August 2020 - 11:04 AM.