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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: 5, Informative) by hubie on Tuesday November 15 2016, @02:47PM
Because there haven't been any comments, I thought I'd give some background info as to why this is pretty neat. This telescope is a Cherenkov Telescope (CT). Just like how a supersonic jet that is travelling through the atmosphere faster than the sound speed creates a "sonic boom", a charged particle that is travelling faster than the speed of light in a medium will create a "light boom" (recall that though the speed of light in a vacuum is the ultimate speed limit, the speed of light in a medium slows down by a factor of the index of refraction (n), so even though light has to move slower than c in that medium, other things like charged particles can go faster than c/n but less than c). The light that is generated is called Cherenkov Radiation, and for particles in the atmosphere, it results in light that is in the UV or shorter wavelengths.
Cherenkov telescopes on the ground detect this light coming down from energetic particles plowing down into the atmosphere. These telescopes are designed to detect gamma rays, which makes the design of the telescope a challenge because you can't efficiently make a nice Cassegrain-style gamma ray telescope. Traditional CTs, known as Davies-Cotton telescopes [arxiv.org], use a single focussing mirror that focus light on to a set of detectors. The problem with using only one optic is that it is very hard to get good optical performance across any field-of-view, even for a single wavelength, which is why your best optical systems are composed of many lenses all doing their part to fix a specific issue. This Schwartzshild-Couder design [arxiv.org] uses a secondary mirror. It ends up with a HUGE field of view.
What I find very interesting is looking at the relative sizing of the mirror dimensions. Consider your standard Cassegrain design. Take Hubble [hubblesite.org]. It has a 2.4 meter diameter primary mirror with a 0.6 meter hole through which the 0.3 meter secondary mirror sends the light. This telescope has a 9.6 meter primary with a 4.4 meter central hole. The secondary mirror has a 5.6 meter diameter! It isn't the absolute sizes that I find interesting, but the relative sizes; much much different than a Cassegrain.
(Score: 2) by fishybell on Tuesday November 15 2016, @04:14PM
I wish you'd written the article. Thank you very much.
(Score: 2) by hubie on Tuesday November 15 2016, @05:06PM
I just realized that the mirror dimensions I mentioned are different from the ones in the article summary (I pulled mine from the ArXiv paper). The secondary for the ASTRI telescope is relatively smaller than what I pulled out of the paper, but it is still much larger than one normally sees for your run-of-the-mill visible telescopes.
(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.