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from the things-are-finally-looking-up-for-an-old-design dept.
In October 2016, the ASTRI telescope prototype, (Image 1) a novel, dual-mirror Schwarzschild-Couder telescope design proposed for the Cherenkov Telescope Array (CTA), passed its biggest test yet by demonstrating a constant point-spread function of a few arc minutes over a large field of view of 10 degrees.
Three classes of telescope types are required to cover the full CTA very-high energy range (20 GeV to 300 TeV): Medium-size telescopes will cover CTA's core energy range (100 GeV to 10 TeV) while the large-size telescopes and small-size telescopes (SSTs) will extend the energy range below 100 GeV and above a few TeV, respectively.
The ASTRI telescope is one of three proposed SST designs being prototyped and tested for CTA's southern hemisphere array. The ASTRI telescope uses an innovative dual-mirror Schwarzschild-Couder configuration with a 4.3m diameter primary mirror and a 1.8 m monolithic secondary mirror. In 1905, the German physicist and astronomer Karl Schwarzschild proposed a design for a two-mirror telescope intended to eliminate much of the optical aberration across the field of view. This idea, elaborated in 1926 by André Couder, lay dormant for almost a century because it was considered too difficult and expensive to build. In 2007, a study by Vladimir Vassiliev and colleagues at the University of California Los Angeles (UCLA) demonstrated the design's usefulness for atmospheric Cherenkov telescopes.
The design is meant to correct optical aberration.
(Score: 1) by khallow on Tuesday November 15 2016, @07:54PM
because you can't efficiently make a nice Cassegrain-style gamma ray telescope
I don't get it. The telescope is imaging visible light phenomena. And the Cassegrain normally works just fine at that, including designs [wikipedia.org] that get rid of coma and spherical aberration. What is it about this application that doesn't work? The huge field of view?
(Score: 4, Informative) by hubie on Tuesday November 15 2016, @08:54PM
Cherenkov telescopes need large apertures and large field of views, both of which you can't get with Cassegrain, etc. The Davies and Cotton design, interestingly enough, was not for an optical telescope, but a huge solar furnace [uni-wuerzburg.de]. They get the large mirror by using spherical mirror segments, which all have the same focal length, but are arranged on the surface of a sphere or parabola. The beauty of using a bunch of spherical mirror segments is that they are very easy to align, but you get a lot of aberration off axis. They apparently knock down the aberrations by changing the focal lengths of the mirror segments, which is also done with the Schwartzschild-Couder, but now you also have a secondary to do this on as well.
(Score: 1) by khallow on Thursday November 17 2016, @06:16PM
Cherenkov telescopes need large apertures and large field of views, both of which you can't get with Cassegrain, etc.
You can get large apertures with the Cassegrain design. After all, it's not any harder to make a particular sized main mirror for a Cassegrain telescope than for the two designs mentioned since all three designs are closely related to each other (the main different being where the imaging takes place. But I see what you mean about field of view. 10 degrees is quite a lot.