Overview of LSSTCam: Structure and Performance of the World's Largest Camera

language

eng

date

Jan 26, 2026

slug

lsstcam-overview-structure-performance

author

status

Public

Sensor Specifications

Parameter	e2v Sensor	ITL Sensor
Pixel Array	4,000 × 4,000 pixels	4,000 × 4,000 pixels
Pixel Size	10 µm × 10 µm	10 µm × 10 µm
Pixel Scale	0.2″ / pixel	0.2″ / pixel
Full Well Capacity (PTC turnoff)	103,000 e⁻	129,000 e⁻
Full Well Capacity (Average)	130,000 ± 10,000 e⁻	130,000 ± 10,000 e⁻
Read Noise (RMS)	5.40 e⁻	6.21 e⁻
Gain	1.51 e⁻ / ADU	1.68 e⁻ / ADU
Dark Current	0.023 e⁻ / pixel / sec	0.021 e⁻ / pixel / sec
CTE (Serial, Parallel)	7.3×10⁻⁶, 1.1×10⁻⁵	1.5×10⁻⁴, 1.2×10⁻⁶
Number of Amplifiers	16 channels	16 channels
Operating Temperature	−100 °C	−100 °C
Sensor Type	Fully-depleted, back-illuminated CCD	Fully-depleted, back-illuminated CCD

Raft configuration:

Nine CCDs are grouped into a 3×3 array to form a single raft, and these rafts are arranged in a 5×5 grid to construct the focal plane.

Mechanical and Optical Structure

World’s largest digital camera

LSSTCam features an effective resolution of approximately 3.2 gigapixels (≈60k × 60k).

Its physical size is comparable to a compact passenger car, with a total weight of about 3,000 kg.

Focal plane layout

The focal plane is a flat structure with a diameter of approximately 64 cm, populated by 189 CCDs, each with a resolution of 4,000 × 4,000 pixels.

These are organized into 21 raft modules (each consisting of a 3×3 CCD array).

The four corner rafts additionally contain three guide and wavefront-sensing CCDs each.

Pixel size and field of view

Each pixel has a physical size of 10 µm (flatness error ≤ 10 µm), corresponding to an angular scale of 0.2 arcseconds per pixel on the sky.

This configuration provides a 3.5° diameter field of view (approximately 9.6 square degrees) while maintaining ~0.2″ angular resolution.

Filter exchange and exposure

The camera supports six optical filters (u, g, r, i, z, y) via an automated filter exchange system and includes an electronic shutter.

The nominal exposure time is 15 seconds (minimum 1 second), with successive exposures taken at approximately 20-second intervals.

Full readout of the entire focal plane is completed within 2 seconds.

Readout architecture and sensitivity

Each CCD is equipped with 16 output amplifiers, enabling parallel readout across 3,024 video channels.

The system achieves a quantum efficiency (filling factor) exceeding 90%.

The sensors are fully-depleted, back-illuminated CCDs, operated at approximately −100 °C to minimize thermal noise.

Example Output

https://youtu.be/DTuq-vBsDJE

Flatness and Placement Accuracy

Sensor Packaging Tolerance Control

Post-packaging flatness controlled within ±5 µm.

A four-side buttable package design is used to keep inter-sensor gaps at the sub-millimeter level.

Epoxy underfill is applied between the sensor package and the PCB.

A variable three-point mounting mechanism allows compensation for sensor height mismatches, maintaining flatness even during cooling to −100 °C.

Raft Modules and SiC Grid-Based Precision Mounting

Rafts are mounted using kinematic three-point supports on a low-CTE, high-stiffness silicon carbide (SiC) grid.

Flatness tolerance is ±6.5 µm at the raft level, and within ±10 µm across the full 64 cm focal plane diameter.

Gravitational deflection is suppressed to < 1 µm.

Post-Assembly Metrology

Electrical and optical performance, flatness, and placement accuracy are verified under operating conditions (~−100 °C).

Based on metrology results, fine adjustments are made using the three-point sensor package adjustment and three-point raft mounting, ensuring all components remain within specified tolerances.

Real-Time Optical Correction

Wavefront sensors located in the corner rafts detect optical surface variations and provide real-time feedback to the telescope mirror control system.

Flat-Field Correction

Light from a flat-field projector is reflected off a large calibration screen to provide uniform illumination.

Images acquired from the twilight sky (just after sunset or before sunrise), where sunlight is scattered uniformly by the atmosphere.

Prior to each observing night, LED flat-field images are acquired for all six filters.

Exposure times range from 15 to 150 seconds, targeting SNR ≈ 1000, to generate flat sets correcting pixel-to-pixel sensitivity variations and dust contamination.

Four times per year, a tunable laser is used to scan each filter band in 1 nm steps, monitoring filter bandpass shifts and wavelength-dependent response.

Both background flats and reference flux flats are used in calibration.

After all other corrections, images are converted to electron units (e⁻) before flat-field correction is applied.

Residual Charge Images in e2v CCDs

Residual charge artifacts occurring after exposure to bright light sources.

These effects can be caused by bright stars or satellites, and are difficult to mitigate due to the unpredictability of trail locations.

Observed Artifacts

Figure 1. Example of residual charge images observed in e2v CCDs of LSSTCam.

The two images were obtained from the same 2000 × 512 pixel CCD amplifier segment.

The left image shows a circular bright spot (~80-pixel diameter) exposed at more than four times saturation.

The right image is the first subsequent averaged exposure (a 15-second dark image).

The location of the serial register is labeled for clarity.

Arrows indicate the parallel and serial transfer directions, and major residual image features are numbered.

Bright spot image

A larger halo is visible due to out-of-focus internal reflections; however, this reflected light is too weak to generate residual charge in subsequent images.

Bright trail following the spot in the same image

This trail is produced when charge flows back into pixels as they pass the spot location during parallel readout.

Residual image at the spot location in subsequent exposures

Charge becomes trapped at the CCD oxide surface during exposure and later leaks back into pixels during integration.

Residual charge trails in subsequent images

Root Cause and Mitigation

The exact root cause has not been conclusively identified. The following mitigations are currently applied:

Sensor voltage configuration (parallel clocking voltages)

Charge transfer timing adjustment (parallel clock overlap time)

Major Technical Challenges

Cryostat vacuum leaks

Dome crane failure and transportation risks during installation

Temporary cooling system failures (−100 °C operation)

Particle contamination and channel shorts within rafts (pixel loss)

Six non-operational CCDs