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CTA: the american prototype, with the same technology as the Meudon model

18 February 2019

On Thursday 17th of January 2019, scientists from the Paris Observatory weer present at the Fred Lawrence Whipple Observatory (Arizona)for the inauguration of a prototype of the « Schwarzschild-Couder » type, in view of the upcoming international Cherenkov Telescope Array - CTA project.

Members of the upcoming international CTA (Cherenkov Telescope Array) project, two scientists from the Paris Observatory were invited to Arizona for the inauguration of a new American prototype.

Inauguration on January 17th 2019 at the Fred Lawrence Whipple Observatory (Arizona) of a third protopye Schwarzschild-Couderc telescope for the CTA project.
© Consortium CTA

Following the small French prototype developed on the Meudon site of the Paris Observatory, this new American model was created on the basis of the « Schwarzschild-Couder » two mirror technology; the difference is that it is of intermediate size.

Destined for the study of cosmic gamma radiation,, CTA is one of the most ambitious ground based projects of the XXIst century.

This upcoming observatory will in the end involve a flotilla of 118 telescopes on the Earth’s surface. Involving three different sizes: small, medium, large, the telescopes will be grouped in two networks: one will be in the northern hemisphere, and the other in the southern hemispherer.

Various kinds of technology are currently being tried, via a number of prototypes.

The recently inaugurated American S-C telescope seen at sunsetr.
© Consortium CTA

The advantages of a « Schwarzschild-Couder » two mirror type technology

The two mirror idea was proposed a in the last century by Karl Schwarzschild (1873-1916), and was subsequently optimized by André Couder (1897-1979) who was at that time an optical astronomer at the Paris Observatory.

Before the birth of the CTA project, the « Schwarzschild-Couder » two mirror technology had never been tried in astronomy. There were various reasons for this, and notably the technical difficulties of making aspherican and not conical mirrors. However, this technology is theoretically capable of reaching the theoretical limit of «gamma ray» astronomy.

Astronomers became interested in this technology for the CTA optical telescope system since it leads to a significant improvement in the quality of wide field images, compared to the single mirror telescopes used till now for ground based gamma ray astronomy.

In effect, the secondary mirror corrects the aberrations and reduces the scale of the sensitive surface in the focal plane. There is another advantage: it also enables one to use the new silicon photo-multipliers (SiPM), which are well adapted for this purpose and are cheaper than the traditional photo-mulipliers.

With the inauguration of this new prototype in the U.S.A., the number of Schwarzschild-Couder type telescopes installed in the world for the CTA has now increased to three: this includes the French prototype inaugurated at Meudon in 2015 as well as a 4m Italian prototype installed on the slopes of Etna.

The small French prototype
The 6 segment 4m diameter mirror has been operational since the end of 2015 at the Paris Observatory on its Meudon site. The small size telescopes will open the domain of extreme gamma ray astronomy, from a few TeV to 300 TeV (1 TeV = thousand billion electron-Volts).
© Consortium CTA
The intermediate size Amarican prototyp
The 9.7 metre main mirror, made up of 48 segments, was inaugurated in January 2019. The intermediate size telescopes are destined to cover the 0.08 TeV to 50 TeV energy range.
© Consortium CTA

The overall CTA project

The construction of the infrastructure in the Southern hemisphere, in Chile, will begin in 2019, after the official signatures have taken place end December 2018.
This southern section of the network will consist of 99 telescopes, out of a total of 118:

  • 4 large (23 m de diamètre) telescopes,
  • 25 intermediate size (9 to 12 m diametre) telescopes,
  • 70 small size (4 m diametre) size telescopes.
Artist’s view of the 99 telescopes, of three different sizes, set up in the Southern hemisphere.
© Gabriel Pérez Diaz, IAC / Marc-André Besel, CTAO

The rest will be installed in the northern hemisphere, in Spain and in the Canary Islands, on the La Palma site, and will include 19 telescopes :

  • 4 large size telescopes
  • 15 intermediate size telescopes.

Already the very first large size telescope has been officially selected for the southern hemisphere CTA and was set up in the Canary Islands in 2018.

31 institutes are contributing to the CTA, including over 200 institutes world-wide.