Large-Sized Telescopes (LSTs) Overview

• Large-Sized Telescopes (LSTs) are advanced ground-based instruments designed to detect low-energy gamma rays and submillimeter signals using expansive primary mirrors and fast repositioning capabilities.
• They incorporate innovative optical architectures, active mirror control, and high-speed camera systems to achieve precise imaging, efficient calibration, and real-time transient event processing.
• LSTs serve as crucial components in multi-telescope arrays, enhancing energy coverage and sensitivity while enabling groundbreaking astrophysical research and deep cosmological surveys.

Large-Sized Telescopes (LSTs) constitute a technological and scientific keystone in current and next-generation ground-based very-high-energy (VHE) gamma-ray and submillimeter observatories. LSTs, as implemented in the Cherenkov Telescope Array (CTA/CTAO) and the Large Submillimeter Telescope projects, are optimized for rapid, precise, and highly sensitive detection of faint astrophysical signals at energies or frequencies inaccessible to other instrumentation. This entry synthesizes the technical architectures, scientific rationales, instrumental performance, and future directions of LSTs, focusing primarily on the 23 m imaging atmospheric Cherenkov LSTs of CTA/CTAO and the conceptual 50 m Large Submillimeter Telescope.

1. Scientific Motivation and Role in Instrumental Arrays

LSTs in the Cherenkov Telescope Array serve as the central asset for detecting low-energy gamma-ray events (≳20 GeV), a regime critical for probing distant extragalactic sources, transient phenomena (gamma-ray bursts, flaring blazars), and fundamental physics scenarios (e.g., dark matter annihilation, Lorentz invariance violation) (Ambrosi et al., 2013, Vercellone, 2014). The combination of a large primary mirror (23 m diameter, providing ~400 m² geometric area) and a focal length of 28 m enables detection of the faint Cherenkov light generated by these low-energy gamma showers (Ambrosi et al., 2013, Cortina et al., 2015, Cortina, 2019).

Located at the core of both CTA North and South arrays (each with four LSTs at ~100 m separation), LSTs act synergistically with MSTs (Medium Size Telescopes, 12 m) and SSTs (Small Size Telescopes, 4–9 m) to achieve broad energy coverage (20 GeV up to 300 TeV) (Vercellone, 2014). LSTs’ fast repositioning (<20 s for 180° azimuthal movement) is essential for prompt response to transient events, while their sub-2 tonne low-inertia cameras enable high-speed data acquisition (>7.5 kHz target rates) (Cortina et al., 2015, Cortina, 2019). In submillimeter astronomy, single-dish LSTs (50-m class) are intended for wide-field, deep continuum and spectroscopic surveys, especially for cosmic structure and star-formation history at high redshift (Kawabe et al., 2017).

2. Optical and Mechanical Architecture

Cherenkov LSTs

A tessellated parabolic reflector (23 m diameter, 198 hexagonal facets, each ~2 m²) is realized using a lightweight composite structure: the mirror panels use a sandwich of soda-lime glass, 60 mm aluminum honeycomb, and rear glass sheet, yielding segments of ~47 kg with drainage to avoid environmental degradation (Hayashida et al., 2015, Mazin et al., 2016). A durable five-layer sputtering (Cr/Al/SiO₂/HfO₂/SiO₂) process achieves reflectivity ≳94% near 370 nm and >90% through 310–510 nm even after abrasion/exposure (Hayashida et al., 2015). Each facet is actively aligned using two motorized actuators (5 μm step), a universal joint, wireless control, and a dedicated CMOS camera that detects the position of a near-IR laser spot projected from the camera plane (the Active Mirror Control, AMC system) (Hayashida et al., 2015, Mazin et al., 2016). The alignment proceeds in coarse and fine steps via lookup tables and real-time feedback, typically on a timescale of minutes (Hayashida et al., 2015).

The camera support structure uses an elliptical arch of carbon-fiber-plastic-reinforced (CFPR) tubes (310 mm diameter, 14 mm wall thickness), symmetrically stabilized by sets of CFPR tensioned ropes (up to 2 tons at low elevation), meeting displacement and tilt constraints (e.g., camera lateral shift <17–22 mm, on-axis displacement <12 mm, tilt <0.25°) (Deleglise et al., 2013). The mass of the moving assembly is ~100–110 tons (Cortina et al., 2015, Mazin et al., 2016).

Large Submillimeter Telescope

The 50 m class LST adopts a Ritchey–Chrétien optical design (primary 50,000 mm, secondary ~6,600 mm, Cassegrain F/6), enabling a field of view up to 1° and supporting high pixel-count cameras at the Nasmyth focus (Kawabe et al., 2017). Active surface control with small, multi-actuated panels ensures a total surface error ≤45 μm at 420 GHz, with dynamic correction for gravity, thermal, and wind-load deformations (Kawabe et al., 2017).

3. Camera System, Photodetector Modules, and Light Collection

Camera plane instrumentation employs 1855–2500 closely packed photomultiplier tubes (PMTs), each equipped with a hexagonal light concentrator (“light guide”, LG) designed (via Bézier/ray-tracing optimization) as a frustum, 50 mm entrance, 25 mm exit, with collection efficiency 84–88% (Saito et al., 4 Feb 2025, Okumura et al., 2015). Multilayer specular reflector films (modified ESR) with reflectance ≳95% (over 300–700 nm, angles 20–70°) line LG walls, yielding an active area fraction ≈99.2% and a 5–10% efficiency gain relative to aluminum-coated concentrators (Okumura et al., 2015). Light guides also suppress stray light at large field angles.

Each PMT module contains seven units (PMTs, two types: R11920-100 for LST-1, R12992-100 for LST-2–4), an integrated Cockcroft–Walton HV circuit, and PACTA preamplifier cards with dual-gain channels (covering 0.25–2000 p.e., S/N > 4, linearity within 10%). Modules are assembled and QC-tested in batches of 19, with per-channel afterpulsing required <4×10⁻⁴ (for pulses ≥4 p.e.), pulse FWHM <3 ns (Saito et al., 4 Feb 2025, Sakurai et al., 2019). The camera system’s buffer depth (using DRS4 chips at ≥1 GSps) is ≥4 μs (Saito et al., 4 Feb 2025).

Silicon Photomultiplier (SiPM) developments, including replacement modules with sum readout of multiple 3×3 mm² (Hamamatsu S13360-3075CN-UVE-1) devices, are underway (Depaoli et al., 2023, Saito et al., 2023, Sakurai et al., 2019). SiPM modules are designed to replicate the timing response (FWHM ~ 4 ns), control overvoltage to ≤0.25% variation (for currents up to ~3 mA), and include temperature/gain stabilization circuits due to breakdown voltage dependence (0.053 V/°C) (Saito et al., 2023). Night sky background (NSB) management is critical, as NSB lowers PDE and charge resolution (by up to 50% at high NSB levels), and optimized blue-reflecting light guides are implemented to suppress red/NSB photons (Saito et al., 2023).

4. Triggering, Readout, and Real-Time Data Analysis

Each camera integrates a hardware trigger using a dual-level analog system (Level-0/Level-1) implemented in module trigger mezzanines, with analog sum and delayed coincidence logic for p.e. discrimination up to ~100 p.e. (Saito et al., 4 Feb 2025, Peñil et al., 2017). The Trigger Interface Board (TIB) consolidates internal and external triggers, applies inter-telescope coincidence criteria (hardware stereo: typically requiring at least two LSTs within a 50 ns window), and time-stamps events with sub-ns precision via the White Rabbit protocol, providing redundancy with an internal 100 ns resolution timestamp (Peñil et al., 2017). Event dead time and counting rates are continuously monitored, and the readout bandwidth target is >7.5 kHz (goal 15 kHz) (Cortina et al., 2015).

Real-time analysis (RTA) pipelines process uncalibrated data at ~15 kHz, using fast vectorized C++ code, event/event image cleaning (double-threshold, matrix-based memory), and Random Forest–based classification (“gammaness”), achieving the CTAO speed requirement (~1000 events/CPU/telescope) (Caroff et al., 2023). The RTA chains (already integrated into LST-1) enable sub-minute identification and follow-up of transient sources.

5. Instrument Performance and Calibration

The LST-1 prototype, installed at CTA North (La Palma), demonstrates key performance metrics:

• Energy threshold (analysis): 30 GeV (design goal: 20 GeV).
• Energy resolution: 30% at relevant energies.
• Angular resolution: 0.3° at 100 GeV.
• Integral sensitivity: 1.1% of Crab flux above 250 GeV over 50 h; 12.4% for 30 min (Morcuende et al., 2023).
• Pointing accuracy (post-calibration): 14 arcsec (Cortina et al., 2015, Mazin et al., 2016).

Critical calibration uses laser-based flat-fielding, PMT gain/linearity testing, and muon ring images to anchor absolute throughput, accounting for all atmospheric and instrumental losses (Sakurai et al., 2019, López-Coto et al., 2021, Morcuende et al., 2023). Monte Carlo chain validation is carried out using CORSIKA/sim_telarray, tuned with muon rings to nail down optical throughput and PSF, and Random Forest pipelines are trained on MC for “gammaness,” energy, and direction reconstruction (López-Coto et al., 2021, Morcuende et al., 2023). Cross-calibration with Fermi-LAT is achieved at low energies (Morcuende et al., 2023).

6. Scientific Outcomes and Future Prospects

LSTs already deliver high-sensitivity observations of canonical VHE objects (Crab Nebula and Pulsar, Markarian 421/501, blazar flares) into the sub-100 GeV regime (Mazin, 2021, Morcuende et al., 2023). The four-LST CTA-North sub-array, even at large zenith angles (ZAs) of 58–60°, reaches effective areas ≳9×10⁵ m² at 10 TeV, enabling the discrimination (via dN/dE = Φ₀(E/1 TeV)^–Γexp[–(E/E_c)^β]) between dark matter annihilation (β > 1), millisecond pulsars (β ~ 1), and hadronic (β ~ 0.6) models for the Galactic Center VHE excess, with ≥3σ after 120 h and ≥5σ after 500 h total exposure (Abe et al., 4 Jun 2025).

The next-generation LSTs will pivot to full SiPM-based cameras, mitigate NSB limitations, extend operational duty cycles, and implement advanced digital signal processing (Depaoli et al., 2023, Saito et al., 2023). In the submillimeter, the 50–m LST’s wide field and high frequency design will underpin wide, deep cosmological surveys, rapid time-domain follow-up (e.g., of electromagnetic gravitational-wave counterparts), and very-long-baseline interferometry studies of black hole environments (Kawabe et al., 2017).

7. Summary Table: Key Technical Specifications

Parameter	Cherenkov LST	Submm LST (Conceptual)
Dish diameter	23 m	50 m
Focal length	28 m	Cassegrain F/6
Mirror facets	198 hexagons	Segmented, small panels
Reflective area	~368–400 m²	Full aperture (1940 m²)
Pixel count	1855–2500 PMTs/SiPMs	up to ≥10⁴ detectors
Timing resolution	≤3 ns, >1 GSps	≤~50 μs (submm backend)
Calib. approaches	Laser, muon rings	Holography, TP/abs. cal.
Energy/Frequency range	20 GeV–few TeV	70–420 GHz (up to 1 THz)

LSTs, through structural, optical, and readout innovations, enable ground-based VHE and submm astrophysics with unprecedented speed, sensitivity, and scientific reach, and their modular design and future SiPM adoption will further advance the frontiers of high-energy astronomical instrumentation.