Institutions all over the world are contributing to the assembly and operation of the Large Synoptic Survey Telescope (LSST), and Brookhaven Lab is responsible for constructing one of the most critical components: the digital sensor array. After all, what is a camera without film?
Photographing five billion galaxies spinning in the farthest reaches of space demands powerful, ultra-sensitive, and utterly unique digital “film.” The same technology used in consumer digital cameras, sensors that convert visible light into electric signals, must be expanded and enhanced to capture near-ultraviolet and infrared light as well.
Developing these sensors, which will provide more than three gigapixels of resolution when complete, is a unique challenge and opportunity for Brookhaven's Instrumentation and Super Conducting Magnet Divisions, as well as the Physics Department.
The team is experimenting with an entirely new generation of charge-coupled devices (CCDs) featuring greater sensitivity, better resolution, and faster readout time than those used in other astronomical cameras.
CCDs are light-sensitive electronic circuits that capture an image in such a way that each pixel on the device converts light from the subject into an electrical charge that represents a specific detail. The higher the pixel count, the greater the range of colors and details that can be uniquely converted into digital information, which in turn can be interpreted by a computer. With 3.2 gigapixels built into supersensitive CCDs aimed at the night sky, very little will escape the LSST’s vision.
The key to achieving the wide range and massive images that make the LSST so unique is modularity, which guides the design and assembly process. The LSST is a mosaic camera, so called because it will use 201 separate custom-built sensors operating simultaneously to stitch together a complete image – similar to the way light strikes the distinct tiles within a mosaic. While each of the contact points, or tiles on the mosaic, can be viewed individually, it is only when taken as a whole that the complete picture can be seen. All the sensor tiles of the LSST camera must act in flawless, precise concert.
The entire camera array will contain 189 CCD sensors to collect the scientific images, and an additional 12 dedicated to guiding and focusing the camera. Each of these square sensors, made from specially grown silicon crystals, is roughly 1.5 inches across. These in turn are combined into what the team calls submosaics, flat "rafts" of nine sensors arranged on a 3 x 3 grid.
The assembly is painstaking, allowing for no more than 250 microns (millionths of a meter) between CCDs in order to catch nearly every photon arriving from deep space. The team will complete 21 of these submosaics, each of them effectively operating as an independent digital camera. This modular design allows for thorough testing as the technology develops, and also easier repairs should the need arise in the future.
Any speck of dust can distort and damage the LSST images. The scientists working to build the sensors in the Instrumentation Division work in clean rooms, wearing full-body suits and masks to keep the CCD surfaces pristine. Any slight tilt along the submosaic array can also spoil the image, so the CCDs must lie almost perfectly flat, with no more than a 10-micron [ten millionths of a meter] tilt in any direction.
Most astronomical cameras require up to several minutes to read out an entire image, but the LSST will need only two seconds — that's more than an order of magnitude faster. This breakthrough speed comes from custom designing the electronics behind every sensor and splitting the information into thousands of discrete bits.
The signal from each CCD sensor is divided into 16 read-out segments that can be independently processed in parallel. On each of the modular rafts, the nine sensors provide 144 read-out points of dense photography; the entire imaging surface will produce more than 3,000 channels of data.
When operating, the camera will convert the various forms of light captured by the telescope into digital data at a rate of about 11 trillion bits per hour.
To form the complete camera, the Brookhaven-designed-and-built sensor units will be integrated with components built by collaborating institutions — lenses, filters, and systems for keeping all these high-tech electronics cool. Then the camera will undergo exhaustive testing before being shipped to the telescope site in Chile. Finally, atop a 2,700-meter peak called Cerro Pachon, the team will install all the elements and aim the telescope at the night sky.
The large collaboration plans to capture its first complete LSST image —
what telescope scientists call “first light” — in 2018.