LST

Large-Sized Telescope

»Four LSTs will be arranged at the centre of the northern hemisphere array to cover the unique low energy sensitivity of the CTAO between 20 and 150 GeV.«

The Large-Sized Telescope (LST) Collaboration consists of more than 400 scientists and engineers from eleven countries: Brazil, Bulgaria, Croatia, Czech Republic, France, Germany, Italy, Japan, Poland, Spain and Switzerland. Because gamma rays with low energies produce a small amount of Cherenkov light, telescopes with large collection areas (mirrors) are required to capture the images. Four LSTs will be arranged at the centre of the northern hemisphere array to cover the unique low energy sensitivity of the CTAO between 20 and 150 GeV. The LSTs will also have a very good sensitivity up to energies of several TeV, which is, however, covered by Medium-Sized Telescopes (MSTs) more efficiently. 

 

The LST is an alt-azimuth telescope. It has a 23 m diameter parabolic reflective surface, which is supported by a tubular structure made of reinforced carbon fibre and steel tubes. A reflective surface of 400 m2 collects and focuses the Cherenkov light into the camera, where photomultiplier tubes convert the light in electrical signals that can be processed by dedicated electronics. Although the LST will stand 45 m tall and weigh around 100 tonnes, it will be extremely nimble, with the ability to re-position between any two points in the sky within 20 seconds. 

 

An illustrative schematic of the telescope with the main assemblies:

>> Visit our Flickr page for more photos and renderings of the LST<<

Credit: G. Pérez, IAC, SMM

Both the re-positioning speed and the low energy threshold provided by the LSTs are critical for CTAO studies of galactic transient, high red-shift active galactic nuclei and gamma ray bursts. The LSTs will expand the science reach to cosmological distances and fainter sources with soft energy spectra.

»The camera has a total field of view of about 4.3 degrees and has been designed for maximum compactness and low weight, cost and power consumption while providing optimal performance at low energies.«

LST Camera

Credit: Daniel López / IAC

The LST Camera shares many elements with the NectarCAM for the MSTs. It has a weight of less than two tonnes, a total number of channels of 1,855 divided into 265 photomultiplier tube (PMT) modules that are easy to access and maintain. PMTs are used as photosensors to convert the light into electrical signals. To maximize the light throughput, each photosensor is equipped with an optical light concentrator, optimized for the field of view and geometry of the photosensor. 

 

The camera has a total field of view of about 4.3 degrees and has been designed for maximum compactness and low weight, cost and power consumption while providing optimal performance at low energies. Each pixel incorporates a photosensor and the corresponding readout electronics, in charge of recording the gamma-ray signals. These electronics are based on the Domino Ring Sampler Version 4 (DRS4) chip, which is developed at the Paul Scherrer Institute in Switzerland and currently used by several experiments; among them the MAGIC Cherenkov telescopes, one of the CTAO’s predecessors for ground-based gamma-ray astronomy. 

 

The camera trigger strategy, to distinguish gamma-ray from other types of events, is based on the shower topology and the temporal evolution of the Cherenkov signal produced in the camera. The analogue signals from the photosensors are conditioned and processed by dedicated algorithms that look for extremely short but compact light flashes. Furthermore, the LST cameras are interconnected in order to form an on-line coincidence trigger among the telescopes, which helps suppress accidental triggers by up to a factor of 100.

LST Status

The construction of a LST prototype, LST-1, was completed in October 2018 in La Palma, Canary Islands, Spain, on the site of the Roque de los Muchachos Observatory. The prototype is foreseen to become the first LST telescope of the CTAO, and, in fact, the first telescope to operate on a CTAO site. The LST passed its Critical Design Review (CDR) in 2020, and the team has been working toward closing out all major action items from the review before the CDR is formally closed and the telescope is accepted by the CTAO.

LST Contacts

Work Package Leader: Prof. Masahiro Teshima, MPI for physics, Munich and ICRR, University of Tokyo

Co-Work Package Leader: Dr. Juan Cortina, CIEMAT, Madrid

Work Package Manager: Dr. Daniel Mazin, MPI for physics, Munich and ICRR, University of Tokyo