---
title: 'SST-1M: Single-Mirror Small Size Telescope'
url: https://www.emergentmind.com/topics/single-mirror-small-size-telescope-sst-1m-687ceb45-8461-49ee-8e18-b2860df867a0
type: topic
---

# SST-1M: Single-Mirror Small Size Telescope

The **Single-Mirror Small Size Telescope (SST-1M)** is an imaging atmospheric Cherenkov telescope developed for very-high-energy gamma-ray astronomy and originally proposed for the Small-Sized Telescope component of the Cherenkov Telescope Array Observatory. It combines a 4 m Davies-Cotton reflector with a wide-field, fully digitizing silicon-photomultiplier camera, and two prototypes now operate as a stereoscopic system at Ondřejov Observatory in the Czech Republic for observations in the TeV and multi-TeV regime [2409.11310][2507.16498].

## 1. CTA context and project trajectory

SST-1M was conceived as one of the three Small-Size Telescope concepts for CTA, alongside the dual-mirror ASTRI and GCT designs. Within CTA, SSTs were intended to provide the highest-energy coverage of the array, and the relevant papers describe SST-1M’s target domain variously as **above about 1 TeV**, **above about 3 TeV**, or **5–300 TeV**, reflecting different phases of the project and different operational framings [2507.17451][2307.09799][1610.01867]. CTA planning documents cited in the SST-1M literature describe an SST southern sub-array of about **70 telescopes** deployed over several square kilometers to sustain sensitivity above a few TeV and into the multi-TeV to hundreds-of-TeV range [1508.06472].

The instrument was developed by a consortium of institutes in **Poland, Switzerland, and the Czech Republic**. The prototype structure was installed at the **Institute of Nuclear Physics PAS in Kraków** in **November 2013**, while camera development and assembly were centered at the **University of Geneva** [1508.06459]. The project subsequently evolved from a single-telescope prototype into a two-telescope operational system: the pair was transported to **Ondřejov Observatory** and installed there in **2022**, after which stereoscopic observations became the central mode of scientific validation [2409.18587][2409.11310].

An important historical point is that the two prototypes were reported as **fully compliant with CTA requirements** after the first Kraków commissioning phase, although **SST-1M was not selected as the final small-telescope design for CTA** [2307.09799]. In the later literature, SST-1M therefore occupies a dual role: it is both a CTA-derived technology demonstrator and an operational stereoscopic Cherenkov system with its own astrophysical program.

## 2. Optical concept and mechanical structure

SST-1M uses a **single-mirror Davies-Cotton optical design** with a **4 m** dish, **5.6 m** focal length, and **\(f/D = 1.4\)**. The reflector is composed of **18 hexagonal facets**, each **78 cm flat-to-flat**, with **11.2 m** radius of curvature, arranged in two concentric rings. The reported mirror area is **9.42 m²**, while the effective reflecting or collection area after shadowing and reflectance corrections is reported as **6.47 m²** [1610.01867][2409.11310]. The field of view is consistently described as about **9°**.

The optical performance targets were set by CTA-style SST requirements, notably a **point spread function \(\le 0.25^\circ\) at \(4^\circ\) off-axis** and sub-nanosecond to low-nanosecond timing spread. Simulation and measurement papers report an on-axis full-mirror optical performance of **\(D_{80}(f) = 0.81 \pm 0.05\) cm**, corresponding to **\(0.083 \pm 0.005^\circ\)**, while later system descriptions quote **0.08° on-axis** and **0.21° at \(4^\circ\) off-axis** at **80% photon inclusion** [1508.06459][2507.16498]. Commissioning-era optics summaries also report **0.09° on-axis**, **0.21° at \(4^\circ\) off-axis**, and optical time spread **< 0.84 ns** [2409.18587].

The mechanical structure was designed for low cost, repeatability, and field deployment. It is predominantly steel, weighs about **8.6 tons**, and can be transported in a standard **12 m shipping container** [2507.16498][2409.11310]. The drive system uses **IMO WD-H slew drives** with **twin worm gears** and **two servo motors** per axis; the telescope can slew to any sky position in **under one minute**, with reported **7 arcsec** pointing accuracy and **5 arcmin** tracking accuracy [1307.4189][2507.16498]. Structural analyses and prototype tests reported survival under **200 km/h** wind, compliance with Chilean seismic assumptions, and lowest structural eigenfrequencies above the CTA minimum requirement [1307.4189][1509.01824].

Mirror alignment was treated as a dedicated subsystem from the outset. The prototype literature describes **Bokeh alignment** for initial manual alignment, **NAMOD** as a future alignment strategy, and later a **dual-camera system** in which one CCD supports astrometric calibration and another monitors the PSF during alignment and operation [1610.01867][2507.16498].

## 3. Camera architecture, digitization, and trigger

The distinctive subsystem of SST-1M is the **DigiCam** camera. The photo-detection plane contains **1296 hexagonal SiPM pixels**, arranged in **108 modules** of **12 pixels** each, with an angular pixel size of about **0.24°** and a total camera field of view of about **9°** [1610.01867][2507.16498]. The pixel optics use **hollow light concentrators** with **24°** cut-off angle, coupling a large optical entrance to custom **hexagonal Hamamatsu SiPMs** that were developed because the required size and geometry were not commercially available [1508.06453][1607.03412].

The camera enclosure is compact and weather-protected. The literature reports a total camera mass **< 200 kg**, an **IP65**-class enclosure, and a **3.3 mm Borofloat** entrance window with anti-reflective coating and a **dichroic filter** suppressing wavelengths above **540 nm** or **550 nm**, depending on the implementation description [1610.01867][2409.11310]. Thermal management is correspondingly elaborate: the power budget is about **2 kW**, with water cooling, heat pipes, and conduction-cooling elements distributed between the photo-detection plane and the digital backend [1508.06453].

DigiCam is a **fully digital readout and trigger system**. Signals are digitized with **250 MHz** ADCs, corresponding to **4 ns** samples, and later system descriptions specify **12-bit** FADCs [1610.01867][2409.11310]. The backend is partitioned into **three microcrates**, each serving one logical sector of the camera; each microcrate contains **9 fast digitizer boards** and **1 trigger board** [1610.01867]. Trigger formation is performed on reduced-resolution digital images. A low-resolution copy is built by summing **3 adjacent pixels** into patches, and the trigger logic can evaluate **circular**, **elliptical**, and **ring** topologies, the last being especially associated with muon events [1610.01867][1709.03914]. Accepted events are transferred through **10 Gb** Ethernet or fiber links, and the readout is described as **almost deadtime free up to a few kHz** [2507.16498].

SiPM adoption was not merely a component substitution. Across the SST-1M papers, SiPMs are repeatedly linked to operation under **high night-sky background** and **high moonlight conditions**, to negligible or reduced ageing, and to increased duty cycle relative to conventional PMT cameras [1610.01867][1709.03914]. This choice shaped the entire frontend: DC-coupled preamplification permits baseline-based monitoring of night-sky background, while per-pixel bias control stabilizes the SiPM operating point against temperature and illumination changes [1508.06453].

## 4. Calibration strategy, timing, and reconstruction software

SST-1M calibration is built around continuous sensor-state monitoring and dedicated calibration event classes. Early design papers describe a chain based on **dark count**, **pedestal**, and **light flasher** events, supplemented by an online feedback system that stabilizes SiPM gain by adjusting the bias voltage according to the sensor temperature [1508.06397]. The slow-control hardware reads embedded **NTC temperature probes** and compensates gain variations through a **CAN bus** control loop [1610.01867].

Commissioning analyses at Ondřejov formalized this into a nightly workflow: **dark runs were taken at the beginning and end of each observing night** to monitor gain, crosstalk, dark count rate, and noise parameters [2409.18639]. One calibration formulation defines the effective ADC-to-photoelectron conversion as
\[
g^* = \frac{1-\mu}{g},
\]
where \(g\) is the SiPM gain and \(\mu\) the crosstalk probability in the generalized Poisson model used to fit dark spectra [2409.18639]. A related treatment in the Crab performance paper models the photoelectron spectrum using electronics and multi-photoelectron terms with Borel-distributed crosstalk contributions [2506.01733].

Muon rings provide the second major calibration channel. Simulation studies established single-muon images as an optical-throughput calibrator for SST-1M, reporting a ring-radius reconstruction bias of **less than about 1%** and, for muons above **10 GeV**, a trigger/selection efficiency of about **60%** across optical efficiencies from roughly **60% to 100%** for the tested low-threshold configuration [1509.00266]. The Ondřejov commissioning analysis then used real muon data to compare optical throughput and PSF metrics between data and Monte Carlo, finding that Monte Carlo optical efficiency was **10% to 20% higher** than in real data and deriving muon-based **\(D_{80}\)** values of **\(10.5 \pm 0.1\) mm** for **SST-1M-1** and **\(10.8 \pm 0.1\) mm** for **SST-1M-2** [2409.18639].

The science-analysis backbone is **sst1mpipe**. It is described as a dedicated SST-1M pipeline built on **ctapipe**, inspired by **lstchain** and **magic-cta-pipe**, and using parts of **digicampipe** for low-level calibration and event handling [2307.09799]. The chain calibrates raw waveforms, performs image cleaning and Hillas parametrization, reconstructs energy and direction, and outputs **DL3** data products in **GADF** format for high-level analysis with **Gammapy** [2409.18587]. Gamma/hadron separation, energy reconstruction, and direction reconstruction use **Random Forest** models trained on Monte Carlo. In mono mode, the direction reconstruction uses the **disp method**; in stereo mode, the reconstruction is described as **MARS-like**, combining telescope-wise estimates with image-intensity weighting [2307.09799]. Later source-analysis papers summarize the event selection in terms of **gammaness**, an event-level gamma-likeness score on which energy-dependent cuts are applied before spectra, light curves, and sky maps are produced [2507.18445].

## 5. Ondřejov stereoscopic system and measured performance

The operational SST-1M system consists of **two telescope prototypes** installed at **Ondřejov Observatory** in the Czech Republic in **2022**. The site is repeatedly described as low altitude for Cherenkov work, at **about 510 m a.s.l.**, and later performance papers emphasize that observations have been carried out in **harsh meteorological conditions** [2507.16498][2506.01733]. The inter-telescope spacing is reported as **152.5 m** in commissioning and performance papers and **155.2 m** in later source-analysis papers [2409.18587][2507.18445]. Event synchronization is provided by the **White Rabbit** timing system, with stereoscopic triggering coordinated by the **CTA Software Array Trigger (SWAT)** [2409.18587][2507.16408].

The most complete low-altitude performance validation derives from Crab observations. For zenith angles below **\(30^\circ\)**, the **analysis-level** energy threshold is reported as **1 TeV** in mono mode and **1.3 TeV** in stereo mode, while the **trigger-level** thresholds are about **0.6 TeV** and **0.7 TeV**, respectively [2506.01733]. The same paper reports energy and angular resolutions of approximately **20%** and **\(0.18^\circ\)** in mono mode, and **10%** and **\(0.10^\circ\)** in stereo mode, with stereo curves reaching about **\(0.07^\circ\)** at favorable energies [2506.01733]. Another source-results summary compresses this into a stereo angular resolution of about **\(0.07^\circ\)** and an energy resolution of about **10%** [2507.17451]. Stereo sensitivity is described as roughly a **factor of 2** better than mono sensitivity [2307.09799][2506.01733].

A recurrent strength of the system is its large-offset response. Simulations and Crab validation studies find that the gamma-ray acceptance is **nearly flat up to about \(2.5^\circ\) offset** and then drops by only about **10%**; at \(2.5^\circ\), the angular resolution worsens by about **5%** in mono and **11%** in stereo [2506.01733]. This behavior is repeatedly cited as central to SST-1M’s usefulness for extended sources and poorly localized transients [2507.16408].

Prototype-era simulations provide a useful design baseline against which these operational numbers can be read. For a single telescope, the effective area was defined as
\[
A_{\rm eff,\gamma}(E) = 2\pi \int_0^\infty P_\gamma(E,r)\, r\, dr,
\]
and the early performance study reported a **safe threshold** of **17.2 PE**, a **total gamma-ray trigger rate** of **0.27 Hz** for a Crab-like spectrum, and a single-telescope energy threshold of **0.3 TeV**, defined as the energy where the differential trigger rate reaches its maximum [1508.06459]. The later Ondřejov results therefore reflect not only the matured stereo system but also the penalties associated with the temporary low-altitude site and real atmospheric conditions.

## 6. Astrophysical observations and scientific use

The **Crab Nebula** has been SST-1M’s principal benchmark source. A commissioning paper based on the **winter 2023/2024** stereoscopic campaign reported **22.03 hours** of stereo data, a **176.1** gamma-ray excess, and **Li & Ma significance \(21.3\sigma\)**, with the excess \(\theta^2\) distribution consistent with Monte Carlo expectations [2409.18587]. The later low-altitude performance paper used **33 h** of stereo Crab data and fit the spectrum with
\[
\frac{d\phi}{dE} = \phi_0 \left(\frac{E}{E_0}\right)^{-\Gamma},
\]
with \(E_0 = 7\,\mathrm{TeV}\), obtaining for the stereo dataset **\(\phi_0 = 1.76 \pm 0.12_{\rm stat} \pm 0.40_{\rm syst}\)** and **\(\Gamma = 2.78 \pm 0.10_{\rm stat} \pm 0.08_{\rm syst}\)** [2506.01733]. That analysis also reported **integral sensitivity above threshold in stereo** of about **7% Crab Units in 50 h** and a reconstructed Crab position consistent with the expected source location to about **0.02°** [2506.01733]. A separate first-results paper summarized the commissioning outcome as a **\(5\sigma\)** Crab detection in **1.5 hours** of good stereo data [2507.17451].

The blazar **Markarian 421** has served as the principal extragalactic target. During commissioning, SST-1M detected enhanced emission on **13 March 2024** with **about 3.3 h** livetime, **\(6.8\sigma\)** significance, excess emission up to **7 TeV**, and a flux close to the **Crab level**; a follow-up on **17 March 2024** confirmed the elevated state [2409.18587]. A dedicated later study expanded this into a full **January to May 2024** campaign covering **23 observing days**, with raw exposures of **55.24 h** for telescope 1, **60.60 h** for telescope 2, and **51.10 h** in stereo, reduced after quality selection to **34.31 h**, **40.48 h**, and **32.92 h** respectively [2507.18445]. In stereo mode the source was detected in **32.9 h** with total significance **10.95** and an excess of **178 counts**, while the off-region local-significance distribution was fit by a Gaussian with mean **\(-0.14\)** and standard deviation **\(1.03\)**, consistent with an unbiased background estimate [2507.18445]. Nightly fits used
\[
\frac{dN}{dE} = A \left(\frac{E}{3\,{\rm TeV}}\right)^{-\alpha},
\]
and the paper reported a mean flux above **1 TeV** of
\[
F = [8.01 \pm 2.54] \times 10^{-14}\ {\rm TeV}^{-1}\,{\rm cm}^{-2}\,{\rm s}^{-1},
\]
with no flaring activity in that longer observation window [2507.18445]. The best-fit intrinsic power-law parameters for the stacked stereo spectrum were a **photon index \(3.24 \pm 0.26\)**, **amplitude \([1.06 \pm 0.12]\times 10^{-12}\ {\rm TeV}^{-1}\,{\rm cm}^{-2}\,{\rm s}^{-1}\)**, and **reference energy \(3.00 \pm 0.00\) TeV** [2507.18445]. The same study modeled the broadband spectral energy distribution with a **one-zone SSC** model implemented in **agnpy**, deriving, among other parameters, **\(\delta_D = 24.29 \pm 0.01\)**, **\(B = 24.58 \pm 1.01\) mG**, **\(R = 6.1\times10^{16}\) cm**, and a broken-power-law electron distribution [2507.18445].

SST-1M has also been used to test extended-source capability on **VER J2019+368** in the Cygnus region. The dedicated 2025 paper reports a first campaign from **April to November 2024**, with **150 h** of stereo data collected and **97 h** retained after run-quality selection [2507.16408]. Using \(1\text{–}300\) TeV events and extended-source integration, the main excess was detected with **\(8.1\sigma\)** pre-trial local significance and **\(6.5\sigma\)** post-trial significance in the extended-source search [2507.16408]. The morphology was fitted with an asymmetric Gaussian over **2.5–100 TeV**, yielding **\(\sigma_{\rm long} = 0.42^\circ \pm 0.06^\circ\)**, **\(\sigma_{\rm lat} = 0.16^\circ \pm 0.03^\circ\)**, and best-fit coordinates **\(\alpha = 304.81^\circ \pm 0.07^\circ\)**, **\(\delta = 36.71^\circ \pm 0.04^\circ\)** [2507.16408]. The spectrum was fitted with a power law of index **\(\Gamma = 2.44 \pm 0.13\)** and normalization **\(\phi_0 = (3.17 \pm 0.41)\times10^{-14}\ \mathrm{cm^{-2}\ s^{-1}\ TeV^{-1}}\)** at **\(E_0 = 7~\mathrm{TeV}\)** [2507.16408]. The same paper used **naima** to show that both a simple one-zone inverse-Compton scenario and a hadronic pion-decay scenario can fit the current data with plausible parameters [2507.16408].

Additional reported targets include **CTA 1**, for which **30 hours** of observations yielded **\(3.5\sigma\)** significance and upper limits, and the broader commissioning set summarized as “multiple detections” across a standard candle, a variable blazar, and a complex extended Galactic field [2507.16498][2507.17451].

## 7. Status, limitations, and prospective development

The current SST-1M system is explicitly presented in the recent literature as an **operational stereoscopic Cherenkov system** whose **calibration** and **simulation benchmark** remain active areas of work [2507.16498]. That status is important for interpreting the published science: the instrument is beyond pure prototype hardware testing, but several papers still label the analysis products as **preliminary** and describe **sst1mpipe** and the instrument response functions as being under intensive development [2409.18587].

A recurring limitation is the observing site. Ondřejov is repeatedly described as a **low-altitude** location with **harsh meteorological conditions**, and several performance papers explicitly connect the relatively high analysis thresholds to this environment [2506.01733][2507.16498]. For that reason, later system papers discuss the possibility of relocation to a higher site, naming the **Indian Astronomical Observatory** and the **Pierre Auger Observatory** as candidate locations [2507.16498]. This does not negate the significance of the Ondřejov results; rather, it frames them as a lower-bound operational demonstration of the system’s capabilities.

The most consistent forward-looking theme in the SST-1M corpus is the combination of wide field of view, favorable off-axis response, and stereo reconstruction. The dedicated VER J2019+368 study argues that these properties make SST-1M well suited to **extended Galactic sources**, **poorly localized transients**, and **candidate Galactic PeVatrons** [2507.16408]. A further extension discussed in the system paper is a hybrid **SST-1M + SWGO** configuration, for which studies are reported to suggest a **30% sensitivity improvement above 10 TeV** when using additional SWGO gamma/hadron discrimination parameters [2507.16498]. Taken together, the published record defines SST-1M not as a deployed CTA production telescope, but as a mature, wide-field, SiPM-based stereoscopic IACT platform with demonstrated capability on point-like, variable, and extended very-high-energy gamma-ray sources.

Source: https://www.emergentmind.com/topics/single-mirror-small-size-telescope-sst-1m-687ceb45-8461-49ee-8e18-b2860df867a0