Camera Subsystem

Camera | Telescope & Site | Data Management

The LSST camera will be the largest digital camera ever constructed. Its size of 1.6 meters by 3 meters is roughly equal to that of a small car and it will weigh 2800 kilograms. It is a large-aperture, wide-field optical (0.3-1 μm) imager designed to provide a 3.5° field of view with better than 0.2 arcsecond sampling. The image surface is flat with a diameter of approximately 64 cm. The detector format will be a mosaic of 16 Mpixel silicon detectors providing a total of approximately 3.2 Gpixels. The camera includes a filter changing mechanism and shutter. It is positioned in the middle of the telescope where cross sectional area is constrained by optical vignetting and heat dissipation must be controlled to limit thermal gradients in the optical beam. The camera will produce data of extremely high quality with minimal downtime and maintenance.

Cut-Away View of Camera Assembly

Camera Management Team

Project Manager, Nadine Kurita ,SLAC National Accelerator Laboratory, Kavli Institute for Particle Astrophysics and Cosmology

Lead Scientist, Steve Ritz ,SLAC National Accelerator Laboratory, Kavli Institute for Particle Astrophysics and Cosmology

System Scientist, Andy Rasmussen, SLAC National Accelerator Laboratory, Kavli Institute for Particle Astrophysics and Cosmology

Exposure Time Calculator
Historical Camera Documents
LSST Camera and Telescope Group Photo -(June 14, 2005)

The focal plane array operates at a temperature of approximately -100°C to achieve desired detector performance. The focal plane array is contained within an evacuated cryostat, which incorporates detector front-end electronics and thermal control. The cryostat lens serves as an entrance window and vacuum seal for the cryostat. Similarly, the camera body lens serves as an entrance window and gas seal for the camera housing, which is filled with dry nitrogen gas to provide the operating environment for the shutter and filter change mechanisms. The filter carousel can accommodate 5 filters, each 75 cm in diameter, for rapid exchange without external intervention.

The Camera subsystem for LSST has been organized as a DOE-funded effort with SLAC taking the lead role, with major participation by BNL, LLNL, Harvard, and UIUC, and with NSF-funded participation from NOAO and UC Davis. The effort includes the LSST camera, front-end data acquisition system, and aspects of the pipeline software systems. The focus of current effort is to develop the camera conceptual design in sufficient detail to establish system and subsystem requirements, define interfaces with the Telescope and Data Management subsystems, narrow the range of design issues to be resolved and optimized during the subsequent preliminary design phase, develop an acquisition plan for the camera elements, and establish a reliable cost range for future planning.

CCDs are the baseline science sensors but we are also pursuing hybrid CMOS detectors as an option for the guide sensors and expect early prototyping of both.

Optical Design

LSST Focal Plane

189 4k x 4K Detectors

Focal Plane Requirements

  • High QE to 1000nm
    • Thick silicon (> 75 µm)
  • PSF << 0.7 arcseconds
    • High internal field in the sensor
    • High resistivity silicon substrate (> 5 kohm∑cm)
    • High applied voltages (40 - 50 Volts)
    • Small pixel size (0.2 arcseconds = 10 µm)
  • Fast f/1.2 focal ratio
    • Sensor flatness < 5µm p-v
    • Package with piston, tip, tilt adj. to ~1µm
  • Wide Field of View
    • ~ 3200 square cm focal plane
    • > 189-sensor mosaic (~16 square cm each)
    • Industrialized production processes
  • High throughput
    • > 90% fill factor. 4-side buttable package, sub-mm gaps
  • Fast readout (2 sec)
    • Segmented sensors (3024 total output ports )
    • 150 connections per sensor
  • Low read noise
    • < 5 electrons

FPA Flatness Allocations Established

 

The LSST Sensor to Raft/Tower Configuration

  • FPA is 189 4K x 4K CCDs, each with 16 outputs
    • 3024 video channels/FPA total
  • Sensors organized into identical rafts of 3 x 3 sensors
  • Clocking of Science CCDs is synchronous and global throughout the FPA
    • 500 kpix/sec * 16 outputs/CCD * 189 CCDs = 3.2 Gpix/2sec
  • A raft is an autonomous object and can function as a complete camera
    • 144 channels/raft
    • readout electronics fit in "shadow" of sensors
  • Raft functions as a dumb slave to OCS
  • 16-bit dynamic range is handled by a single-gain readout

 

Desired Features:

  • Fill Factor must approach unity (which favors a fairly large area footprint ∼ 16 cm2.
  • Flatness requirements argue for bond pads only on periphery.
  • Segmentation for blooming control of very bright stars - no more than ca. 500 pixels in the parallel direction per segment.
  • Contiguous imaging area should be at least 500 pixels in the parallel direction.

 

Comparison of wide field camera controllers

Electronics/Thermal Configuration

LSST Data Rates

  • 3.2 billion pixels read out in 2 sec (15 sec integration)
  • 1 pixel = 2 Bytes (raw)
  • Over 3 GBytes/sec peak raw data from camera
  • Real-time processing and transient detection: < 10 sec
  • Dynamic Range: 4 Bytes / pixel
  • > 0.6 GB/sec average in pipeline
  • 5000 floating point operations per pixel
  • 2 TFlops/s average, 9 TFlops/s peak
  • ∼20-30 TBytes/night

 

Shutter

Filter Location/Diameter Study

 

Filter Motion Study

Changer/Carousel Design

Carousel holds 5 filters:g-r-i-z-y
Rail-type changer engages carousel for automatic changing
Manual load-lock changer for inserting a 6th filter (u-filter)

 

Changer/Carousel Operational Requirements
RequirementValue
Number of filters in carousel5
Maximum filter change time2 minutes
Approx. number of changes4 / night
Operational life2 x 104 cycles

 

LSST Ideal Filter Curves and Specifications

Camera Thermal and Vacuum System Zone Definitions