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I was browsing NASA featured items and came across this - Out With the Old, In With the New: Telescope Mirrors Get New Shape

Called freeform optics, this emerging mirror technology, brought about by advances in computer-controlled fabrication and testing, has triggered a sea change in optical engineering... the technology holds great promise for scientists who want to develop compact telescopes for CubeSat and other small satellites — an increasingly popular and cost-effective alternative to more traditional missions that are more expensive to build and launch.

Telescopes for cubesats?!

The article had this illustration, and I'm trying to figure out how that is converted into a clear image without distortions, and why they say it allows the telescope to be much more compact.

freeform optics mirror design

It was a short article that didn't even try to explain this, and when I went looking for other articles on the topic for non-experts I found zip. Perhaps this is very challenging to explain, but I thought I'd ask anyhow. I told an enthusiastic youth just a few weeks ago that it wasn't possible to launch a telescope capable of anything useful on a cubesat. Oops...

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  • $\begingroup$ You are specifically looking for an article for a general audience? I think that sentence should be singled out or some might think you are wanting a deeper explanation. $\endgroup$
    – spacer
    Nov 6 '15 at 11:50
  • $\begingroup$ A quick look at freeform eyewear, say for astigmatism, might help simplify your question to those on this board. $\endgroup$
    – spacer
    Nov 6 '15 at 12:10
  • $\begingroup$ @spacer well, maybe general audience is a strong term. What i mean is I didn't find anything except articles for people in the field. I understand the principles of lens shaping for astigmatism, but I can't get from there to how light reflected from a saddle-shaped mirror gets collected and saved as an undistorted image. $\endgroup$
    – kim holder
    Nov 6 '15 at 14:28
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    $\begingroup$ You get 100 mm aperture on a cubesat. With classic optics, you could "compress" it before launch, and have it open up in space and push the back of the instrument further away from the front. With an all-spherical design, like a spot-Maksutov-Cassegrain, collimation is not that critical, so I think collimation would survive launch and deploy. Fully deployed it would be a little over 300 mm long, not too bad. So now you have 100 mm of aperture above atmosphere, free of light pollution, capable of very long exposure times. I'd say it would be useful, you could do some research with it. $\endgroup$
    – Florin Andrei
    Nov 6 '15 at 20:18
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    $\begingroup$ Better yet, look at the principle for the Hyperstar add-on for SCTs. It gives you an f/2 focal ratio, tremendously fast. It only requires a primary mirror and the corrector in front of the camera. Combined with the absence of light pollution, and the very long exposure times enabled by it, the f/2 focal ratio could really achieve something interesting, from a research p.o.v. Not a whole lot of resolving power with 100 mm aperture, but r.p. is not everything. The f/2 instrument in a vacuum would be able to see very faint extended objects (like some nebulae) very well. $\endgroup$
    – Florin Andrei
    Nov 6 '15 at 20:33
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Freeform optics are a response to the specific challenge of cramming a telescope in a very limited space. A traditional instrument would have all optics symmetrical and aligned on the same axis. It would waste a lot of space within the cubesat. Also, traditional designs tend to be much longer than they are wider; they don't fit well in a cube; it is very hard to make classic instruments that are as short as they are wide.

But with freeform optics you could bounce light in a few directions within the cube. You'd still achieve a decent focal length, and you would use all the volume available to you.

enter image description here

Since light is reflected from mirrors at angles different from normal, you cannot use the traditional symmetric shapes such as parabolic, spherical, etc. You need to basically take a paraboloid and squish it in one direction so that it works about the same like a parabolic mirror (I'm simplifying), but at an angle of reflection of, say, 45 degrees.

In such an instrument you could have multiple "potato chip" mirrors, as in the diagram above. You have to design the instrument as a whole; computer simulations will adjust the shape of each mirror until the performance of the whole instrument is close to a classic straight design.

As far as I can tell, the manufacturing precision is such that freeform optics are only usable at long wavelengths such as infrared, where less precise optics can be used. But technology improves all the time. It also depends on how much aberration you can tolerate in your image.

For usage from ground level this is less useful, unless you absolutely need a telescope in a very small form factor for some reason. Classic optics are still preferred when space and shape are not restricted.

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  • $\begingroup$ This helps me out a bit, though I'm not even close to being able to imagine this. So the inner walls of the cubesat (which could be a 6U too - of 6 units, I don't know where the cut-off is) have these shaped mirrors, and they don't interfere with the aperature? Does the saddle-shaped one go in as a primary mirror? $\endgroup$
    – kim holder
    Nov 6 '15 at 20:54
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    $\begingroup$ I can't find a good diagram for the cubesat design. Anyway, the laws of optics require you to have "freeform" mirrors wherever the angle of reflection is far from normal. The idea is not entirely new, as usual in optics: google the Schiefspiegler design for a similar idea, although the reasons there are different; they use a toroidal mirror for off-axis reflection without aberrations. $\endgroup$
    – Florin Andrei
    Nov 6 '15 at 21:10

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