🔭 astrophysics

Towards the Deployment of the First NectarCAM, a Medium-Sized-Telescope Camera for the Cherenkov Telescope Array Observatory

This paper presents the performance metrics, calibration techniques, and production status of NectarCAM, the Cherenkov camera designed for the Cherenkov Telescope Array's medium-sized telescopes, which is scheduled to be ready for shipment by Summer 2026 to equip a pathfinder telescope.

Original authors: Pablo Correa, CTAO NectarCAM Collaboration

Published 2026-05-19

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Pablo Correa, CTAO NectarCAM Collaboration

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe is a giant, dark ocean, and high-energy gamma rays are like rare, invisible fish swimming through it. To catch these fish, scientists are building a massive fishing fleet called the Cherenkov Telescope Array (CTAO). This fleet isn't made of one big boat, but three different sizes: huge ones, medium ones, and small ones.

This paper is about the NectarCAM, a specialized "net" designed specifically for the medium-sized boats in this fleet. Here is a simple breakdown of what the paper says:

1. The Camera: A High-Speed Eye

The NectarCAM is essentially a giant digital eye made of 1,855 tiny pixels (like the dots on a TV screen, but much more sensitive).

The Job: It's designed to catch gamma rays with energies between 80 billion and 50 trillion electron volts.
The Structure: Think of the camera as a honeycomb. It has 265 honeycomb cells (modules), and inside each cell are 7 tiny eyes (pixels).
The Heart: Each eye is connected to a super-fast computer chip called NECTAr3. This chip is the brain of the operation.

2. The "Ping-Pong" Trick: Why It's So Fast

The most impressive part of the NectarCAM is how fast it can take a picture and get ready for the next one.

The Problem: Usually, when a camera takes a photo, it has to "think" for a moment before it can take another. This pause is called "dead time."
The Solution: The NECTAr3 chip uses a clever trick called "ping-pong" mode. Imagine a tennis player hitting a ball back and forth. While one side of the chip is busy writing down the data from a flash of light (the "ping"), the other side is already getting ready for the next flash (the "pong").
The Result: This allows the camera to reset in just 0.7 microseconds (that's less than a millionth of a second). It's so fast that it barely pauses between catching gamma rays.

3. How Clear is the Picture? (Performance)

The paper tests how well this camera sees two things: Time and Charge (the amount of light).

Time Resolution (Timing): The camera can tell exactly when a flash of light happened with incredible precision. If you shine a light on it, it can pinpoint the moment within less than 1 nanosecond (a billionth of a second). This is like a stopwatch that is accurate enough to time a race where the runners are moving at the speed of light.
Charge Resolution (Brightness): The camera is also very good at measuring how bright a flash is. It is so sensitive that its accuracy is limited only by the natural "fuzziness" of light itself (like the grain in a photo), not by the camera's own mistakes.

4. Cleaning the Lens: Calibration

Even the best camera needs to be calibrated to ensure the pictures are true. The paper describes three steps to "clean the lens" of the NectarCAM:

Pedestal (The Blank Canvas): Before measuring light, the camera needs to know what "zero" looks like. They subtract the background noise (like the static on an old TV) to find the true baseline.
Gain (The Volume Knob): The camera needs to know how much "volume" to assign to a single particle of light. They use two methods:
1. Flashers: They flash a bright light and measure how the signal changes.
2. White Target: They shine a light on a special white board that moves across the camera, ensuring every pixel gets an equal dose of light to check if they are all listening at the same volume.
Flat-Field (The Even Lighting): Sometimes, one part of the camera might be slightly brighter or dimmer than another, just like a lightbulb that isn't perfectly even. They use a special light source that covers the whole camera to create a map of these differences and correct them, making sure the whole image is perfectly balanced.

5. What's Next? (The Timeline)

The paper gives a clear schedule for getting these cameras into space (well, onto the telescopes on Earth):

NectarCAM1: This was the "practice model." It has been used for testing and will be fixed up later.
NectarCAM2: This is the first real production camera. It is currently being tested and is scheduled to be shipped in Summer 2026. It will be installed on one of the first medium-sized telescopes being built in Chile.
The Rest: The remaining 7 cameras will be built and shipped over the next few years, with the goal of having all 9 ready for the full fleet by 2028.

In short: The NectarCAM is a super-fast, super-sensitive camera with a "ping-pong" brain that is ready to help the Cherenkov Telescope Array catch the universe's most energetic light. It is being built, tested, and calibrated, with the first unit ready to go in 2026.

Technical Summary: Towards the Deployment of the First NectarCAM

Problem and Context
The Cherenkov Telescope Array Observatory (CTAO) is a next-generation instrument under construction designed to detect very-high-energy (VHE) gamma rays across an energy range from tens of GeV to hundreds of TeV. To achieve this, CTAO utilizes three types of imaging air-Cherenkov telescopes (IACTs), including Medium-Sized Telescopes (MSTs) which target the intermediate energy range of 0.08–50 TeV. The specific challenge addressed in this work is the development, characterization, and calibration of NectarCAM, the dedicated camera designed to equip nine MSTs at the CTAO-South site. The camera must operate with high time and charge resolution while managing the dynamic range of 0.1–2000 photoelectrons (p.e.) and minimizing readout deadtime to ensure efficient data acquisition.

Methodology
The NectarCAM employs a modular design comprising 265 focal plane modules (FPMs), totaling 1855 pixels. Each pixel consists of a photomultiplier tube (PMT) connected to a front-end board (FEB) via two gain channels. The core of the signal processing is the NECTAr3 chip, which features a 12-bit analog-to-digital converter (ADC) and a switched-capacitor array operating at a 1 GHz sampling rate. A critical methodological feature is the "ping-pong" mode of the NECTAr3 chip, where the 1024 ns analog memory is split; upon a trigger, one half is frozen for the readout of 60-ns waveforms while the other remains active, significantly reducing deadtime.

Performance evaluation was conducted using NectarCAM1, a qualification model extensively tested at CEA/Irfu. Time resolution was assessed using two complementary methods to determine the time of maximum (ToM) of signal pulses: a Gaussian fit and an interpolation method using scipy.signal.find_peaks. Charge resolution was analyzed as the integral of the signal pulse.

Calibration techniques were developed within the nectarchain software framework to convert raw waveforms ( $V_{raw}$ ) into calibrated waveforms ( $V_{calib}$ ) using the relation $V_{calib}(t) = (V_{raw}(t) - P(t)) / G \times F$ . The methodology involved three specific steps:

Pedestal Calibration: Subtracting the baseline noise ( $P$ ) caused by night-sky background and electronics, measured by forcing triggers at regular rates (e.g., 1 kHz) to average waveforms.
Gain Calibration: Determining the conversion factor ( $G$ ) from ADC counts to photoelectrons. Two algorithms were utilized: a photostatistic method analyzing charge variability at high illumination, and a single photoelectron (SPE) fit method using low-illumination flashers or a motorized white target.
Flat-Field Calibration: Correcting for pixel-to-pixel inhomogeneities ( $F$ ) using a dedicated flasher source. A novel method was implemented that fits a bivariate Gaussian amplitude distribution to the flasher light front, rather than modeling it as a simple plane wave, to achieve higher accuracy.

Key Contributions and Results
The paper presents the performance metrics and calibration status of the first NectarCAM unit:

Deadtime: The "ping-pong" operation of the NECTAr3 chip successfully reduces the typical readout deadtime to approximately 0.7 µs.
Time Resolution: For camera illumination above 5 p.e., the root mean square (RMS) of the ToM is less than 1 ns for both measurement methods, satisfying CTAO requirements.
Charge Resolution: The charge resolution generally reaches the statistical lower limit imposed by Poissonian fluctuations in photoelectron production, except at the highest charges where PMT saturation effects become dominant.
Calibration Implementation: The authors detail the implementation of the SPE fit method for gain calibration, which offers higher precision than the photostatistic method, and the bivariate Gaussian approach for flat-fielding, which describes the light front accurately down to the 2% level.

Significance and Outlook
This work establishes the technical readiness of the NectarCAM system for deployment. The successful characterization of NectarCAM1 validates the modular design and the NECTAr3 chip's performance. The paper outlines the production timeline for the full suite of nine cameras: NectarCAM2 (the first production-line unit) is scheduled for shipment by Summer 2026 to equip an MST pathfinder at CTAO-South, with subsequent units (NectarCAM3–9) following in the coming years. The development of robust calibration techniques ensures that the camera will meet the sensitivity and coverage requirements necessary for the CTAO's scientific objectives.

1. The Camera: A High-Speed Eye

2. The "Ping-Pong" Trick: Why It's So Fast

3. How Clear is the Picture? (Performance)

4. Cleaning the Lens: Calibration

5. What's Next? (The Timeline)

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