CTA

Artist's impression of CTA. [Credits: CTA/flickr]

Artist's impression of CTA by night. [Credits: CTA/flickr]

Artist's impression of the Large Size Telescopes of CTA. [Credits: CTA/flickr]

Models of the different telescope sizes for CTA. [Credits: CTA/flickr]

The Cherenkov Telescope Array (CTA) is a planned international project in the field of TeV astronomy. It will extend the detectable energy range to both higher and lower energies by one order of magnitude. In the energies detectable so far, the resolution will be 10 times higher than with the current generation of Cherenkov telescopes (H.E.S.S., VERITAS, MAGIC, etc.). Thus, CTA will not only be the largest, but also the most accurate ground-based instrument capable of detecting and imaging gamma radiation. In order to be able to observe the entire sky, the telescopes will be distributed over two locations: in the southern hemisphere in the Chilean Atacama Desert and in the northern hemisphere on the Canary Island of La Palma. To be able to detect energies from about 30 GeV to 300 TeV, CTA will use three telescopes of different sizes (Large-, Medium- and Small-Sized Telescope). Construction of the first prototype of a Large-Sized Telescope on-site in La Palma was finished in October 2018. By 2022, sufficient telescopes should have been set up at both locations to start observations; by 2025, both arrays should be completed.

Operating principle

CTA will observe the sky in the gamma-ray energy range. When this very energetic electromagnetic radiation hits the Earth's atmosphere, the photons interact with air molecules and so-called electromagnetic showers are generated. Because of their very high energies, these secondary particles can move faster than the speed of light in air, which is 0.03 percent slower than the speed of light in vacuum.

What happens then is similar to what can be observed when the speed of airplanes exceeds the speed of sound: a shock wave is generated, which produces the well-known sonic boom. For particle showers, the equivalent effect is the Cherenkov radiation, a flash of bluish light lasting for some billions of a second, too short for the human eye to detect.

However, the mirrors of CTA and the high speed cameras can observe this radiation. Even though it occurs relatively rarely, the unprecendented area covered by CTA is expected to increase the number of known sources (about 150) by a factor of 10.

Scientific goals

CTA will observe ultra-high energetic gamma-rays with unprecedented sensitivity. The main scientific goals set by the consortium are summarized in three so-called Key Science Projects:

Understanding the origin and role of Relativistic Cosmic Particles
Probing extreme environments
Exploring frontiers in physics

To study these topics, particular regions and objects of the universe will be explored, such as the center of our galaxy, which not only contains a supermassive black hole but also molecular clouds. In addition, many other regions will be observed, such as the Magellanic Clouds, the galactic plane, active galactic nuclei, clusters of galaxies and star forming regions. All these regions are sources of high energy gamma-rays. Further, CTA will be operated as an open observatory. Observing proposals from the wider scientific community will be peer-reviewed by international experts that will change on a regular basis. Also, tools for efficient data analysis will be provided.

You can learn more about the Key Science Projects and further scientific goals in the free available book "Science with the CTA" which is linked below.

The Galaxy Cluster Abell 1689. [Credits: NASA/CXC/MIT/E.-H. Peng]

Simulation of a galactic plane survey observed by CTA. [Credits: CTA]

The Galaxy Centaurus A which contains a supermassive black hole in its center which generates gigantic jets. [Credits: NASA/CXC]

IAAT Participation

The High Energy Astrophysics group of the Institute of Astronomy and Astrophysics Tübingen (IAAT) is a member of the CTA Collaboration and is involved in several working groups.

Current projects conducted in our group are the following:

Development of actuators for the Medium Sized Telescopes (MST)
Coating of CTA mirrors
Mirror testing facility for CTA (based on CCDs)
FlashCam electronics development and testing
FlashCam prototype data analysis and simulations

FlashCam

FlashCam is a camera candidate for CTA Medium-Sized Telescopes. Cherenkov cameras have to be able to record the very short (a few nano seconds) and faint light signals of extended airshowers. The most important design properties of FlashCam are digitization of the signals at the earliest possible stage, modularity for easy maintainbility and the utilization of non-linear preamplifiers to cover the whole signal range with only one channel.

To reduce the potentially huge amounts of data a trigger system is used to reduce the amount of data by only reading relevant signals. The digitized raw-data initially are saved in a ring-buffer (32 micro-seconds buffer time). While buffered, they are examinded for relevant signals. All data containing these are passed to the server then, while the rest will be overwritten when the buffer-time ends. Further, the ring-buffer allows the camera to operate without any dead-time. The sample-rate therefore is determined by the velocity of the analog-digital-converters (ADCs) which is 250 MS/s (Mega-Samples per seconds). This corresponds to one sample each 4 ns!

The IAAT was involved in the development of the firmware for the readout electronics as well as designing test environments for the camera prototypes. Presently (early 2019), the main work is calibrating the non-linear regime and testing the electronics-hardware.

FlashCam model. From left to right: Window & shutter; Photon detection plane; Readout & safety control electronics; Mechanical structure & thermal insulation. [Credits: CTA]

Testing of a FlashCam prototype mounted on a Medium Sized Telescope in Berlin-Adlershof, July 2015. [Credits: CTA]

Backside of a Flashcam prototype. [Credit: CTA]

Front side of a Flashcam prototype. [Credit: CTA]

Mirror coating

The Balzers BA k 550 vacuum coating chamber at the IAAT. [Credits: IAAT]

Another critical part for the imaging capabilities of Cherenkov Telescopes are the mirrors. The mirror properties are mainly governed by their shape and coating. The shape is set by the geometry of the telescope, especially its focal length.

There are strict requirements for the coating of Cherenkov Telescopes. Their reflectivity has to be around 90% in the wavelength range of Cherenkov light (from the UV into blue, approximately 300 nm to 600 nm) to be able to image the weak signals. The reflective surface is usually placed on a substrate and therefore directly exposed to environmental conditions.

To optimize the reflectivity and longevity of the mirrors, small mirror samples have been coated with different compounds in the institute's vacuum coating chamber and their properties have been examined.

Links & Sources

Last Update 02/2019: Eva Laplace, Fabian Leuschner, Inga Saathoff, Chris Tenzer