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Here is the email by which Prof. Willstrop granted CC-BY permission:

Dear Lou,

Thanks for your email, and my apologies for the delay in replying.

I agree to release the diagram of the three mirror telescope, under the conditions that you recommend, i.e., that anyone can modify the diagram, but they must attribute the original image to me on all copies or derivatives. I will trust you to do whatever is necessary.

Did you know that Karl Schwarzschild had described a system of TWO mirrors free of spherical aberration, coma, astigmatism and with a flat focal plane ? It has some disadvantages, such as a convex primary mirror and a secondary mirror that is larger than the primary, and a large central obstruction. Both mirrors must be perforated. See Mittheilungen Sternwarte zu Gottingen, Part 10 (1905): Untersuchungen zur geometrischen Optik II Theorie der Spiegeltelescope. This system is shown in Fig. 3 on p15.

Rumsey, N.J., Proc. Astron. Soc. Australia, Vol.2, p.22 (1971) "Pairs of spherical mirrors as field correctors for paraboloid mirrors" describes the Paul-Baker system and two other arrangements which unfortunately have larger central obstruction.

My three papers on the three-mirror telescope in the Monthly Notices of the Royal Astronomical Society were:

The Mersenne-Schmidt - a three mirror survey telescope. MNRAS 210, 597 - 609 (1984)

The flat-field Mersenne-Schmidt. MNRAS 216, 411 - 427 (1985) (This paper includes a photograph of a working model of 100 mm aperture. You might have to obtain the agreement of the RAS if you wanted to use this).

Atmospheric dispersion compensator for a wide-field three-mirror telescope. MNRAS 225, 187 - 198 (1987).

Good luck with the article for Wikipedia.

Regards,

Roderick. In reply to:

Hi, Prof. Willstrop!

I noticed that Wikipedia had no information on three-mirror anastigmat designs, so I started to write one ( http://en.wiki.x.io/wiki/Three-mirror_anastigmat ). It's quite basic at the moment, with references to the Paul and Baker papers, and a few uses of the design in JWST, LSST, and some others.

The article would definitely benefit from a nice picture explaining the idea. You have such a picture on your web page titled "Three Mirror Telescope" ( http://www.ast.cam.ac.uk/about/three-mirror.telescope ), and I was wondering if I could use it to illustrate the article.

To do this, you'd have to give permission. You could release it into the public domain completely (no restrictions) or under one of the creative common licenses ( https://creativecommons.org/licenses/ ). I'd personally recommend "CC BY" which lets anyone do anything they want with the image, but they must attribute the original image to you on all copies or derivatives.

If you are willing to do this, you can just reply to this email with your licensing preference and I'll do the rest, including uploading the image and writing a description. I will include your reply under the licensing section.

Of course, as always you are welcome to improve the article a well, since it's Wikipedia...

Thanks,

 Lou Scheffer

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