The LSST Camera is the digital-imaging package planned for the Large Synoptic Survey Telescope under construction at Cerro Pachon in Chile.
According to its design team, this will be the largest "digital camera" ever constructed—albeit one highly specialized for astrophotography. The design as planned "will be the size of a small car and weigh more than 3 tons" (excluding the optics of the telescope), with a sensor array boasting 3,200 megapixels.
It is not possible to fabricate a monolithic silicon image sensor of the required size. Current plans call for an array of roughly 200 individual 42x42 mm square CCD chips, each encompassing 16 megapixels. These are to be butted edge to edge with a minimal loss of imaging area.
To maximize sensitivity to faint light, each CCD pixel is about 10x10 µm square (about 40% more area than the pixels of the Sony α7S, perhaps today's most low-light-capable consumer model). Furthermore the sensor will be cooled to -100°C within a vacuum chamber to minimize spurious thermal noise.
There is no color filter array over the sensor. Instead the camera has a system of five 75 cm-wide filters, each of which passes a different spectral band, which can be swapped into place over the entire sensor for sequential exposures.
Some of the technology planned for the LLST camera is already employed in the 570-megapixel Dark Energy Camera, installed in 2012 on the nearby Chilean peak Cerro Tololo. In August, 2015 the LSST camera design team at SLAC received “Critical Decision 3” approval from the US Department of Energy, which ensures funding to construct the camera. The LSST is not expected to be complete and operational before 2022.
The LSST itself is an unusual telescope, optimized for rapid, repeated wide-field imaging of the entire sky in order to capture transient phenomena. It essentially represents a massive f/1.2 telephoto lens with an effective focal length of 10,310 mm.
The telescope's design calls for a folded light path to allow for a shorter and more maneuverable telescope structure. The primary mirror M1 is 8.4 meters in diameter (over 27 feet), however the central 5 meters are figured to a deeper curvature to serve as the system's tertiary mirror (M3), and do not contribute to its light-gathering area. (An unobstructed 6.7m telescope would have equivalent light-gathering power.) During 2015, the M1-M3 mirror fabrication was completed, and it is being held in temporary storage while construction proceeds at the Chilean telescope site.
In addition to the three-bounce mirror elements, the telescope also includes corrector lenses to provide a flat field at the sensor plane. The largest of these is 1.55 m or ~5 feet in diameter.
|A mockup showing the size of the planned|
3,200-megapixel imaging array for the Large Synoptic
Survey Telescope. The moon photograph pasted overtop
illustrates the angular coverage of the telescope's field of view.
image by LSST Corporation (Image rights)
- "World’s Most Powerful Digital Camera Sees Construction Green Light" from SLAC National Accelerator Laboratory of Stanford University
- "Optical Design" (archived) from lsst.org
- " The effective focal length of the optical system is 10.31-m making the final f-number 1.2345," according to an LSST presentation poster.