Rendering of completed LSST
|Location(s)||Cerro Pachón, Coquimbo Region, Chile|
|Altitude||2,663 m (8,737 ft), top of pier|
|Telescope style||Three-mirror anastigmat, Paul-Baker / Mersenne-Schmidt wide-angle|
|Diameter||8.417 m (27.6 ft) physical
8.360 m (27.4 ft) optical
5.116 m (16.8 ft) inner
|Secondary diameter||3.420 m (1.800 m inner)|
|Tertiary diameter||5.016 m (1.100 m inner)|
|Angular resolution||0.7? median seeing limit
0.2? pixel size
|Collecting area||35 square metres (376.7 sq ft)|
|Focal length||10.31 m (f/1.23) overall
9.9175 m (f/1.186) primary
The Large Synoptic Survey Telescope (LSST) is a wide-field survey reflecting telescope with an 8.4-meter primary mirror, currently under construction, that will photograph the entire available sky every few nights. The telescope uses a novel 3-mirror design which delivers sharp images over a very wide 3.5-degree diameter field of view, feeding a 3.2-gigapixel CCD imaging camera, the largest digital camera ever constructed.
The telescope will be located on the El Peñón peak of Cerro Pachón, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes.
The LSST was the top-ranked large ground-based project in the 2010 Astrophysics Decadal Survey. The project officially began construction 1 August 2014 when the National Science Foundation (NSF) authorized the FY2014 portion ($27.5 million) of its construction budget. The ceremonial laying of the first stone was performed on 14 April 2015. Site construction began on April 14, 2015, with engineering first light anticipated in 2019, science first light in 2021, and full operations for a ten-year survey commencing in January 2022.
The LSST design is unique among large telescopes (8 m-class primary mirrors) in having a very wide field of view: 3.5 degrees in diameter, or 9.6 square degrees. For comparison, both the Sun and the Moon, as seen from Earth, are 0.5 degrees across, or 0.2 square degrees. Combined with its large aperture (and thus light-collecting ability), this will give it a spectacularly large etendue of 319 m2?degree2.
To achieve this very wide, undistorted field of view requires three mirrors, rather than the two used by most existing large telescopes: the primary mirror (M1) is 8.4 metres (28 ft) in diameter, the secondary mirror (M2) is 3.4 metres (11.2 ft) in diameter, and the tertiary mirror (M3), located in a large hole in the primary, is 5.0 metres (16 ft) in diameter. (The secondary mirror is expected to be the largest convex mirror in any operating telescope, until surpassed by the ELT's 4.2m secondary c.2024). The large hole reduces the primary mirror's light-collecting area to 35 square metres (376.7 sq ft), equivalent to a 6.68-metre-diameter (21.9 ft) circle. (Multiplying this by the field of view produces an etendue of 336 m2?degree2; the actual figure is reduced by vignetting.)
The primary and tertiary mirrors (M1 and M3) are being constructed as a single piece of glass, the "M1M3 monolith".
A 3.2-gigapixel prime focus[note 1] digital camera will take a 15-second exposure every 20 seconds. Repointing such a large telescope (including settling time) within 5 seconds requires an exceptionally short and stiff structure. This in turn implies a very small f-number, which requires very precise focusing of the camera.
The camera will include three corrector lenses to reduce aberrations. The first lens at 1.55 m diameter will be the largest lens ever built, and the third lens will form the vacuum window in front of the focal plane. The focal plane is flat, 64 cm in diameter, and will be a mosaic of 189 CCD detectors each of 16 megapixels, providing for better than 0.2 arcsecond sampling. The CCDs will be cooled to approx -100°C.
The camera includes a filter located between the second and third lenses, and an automatic filter-changing mechanism. Although the camera has six filters (UGRIZY) covering 400 to 1060 nm wavelengths, the camera's position in front of the mirror limits the size of its filter changer. It can only hold five of them at a time, and one of the six must therefore be chosen to be omitted each night.
Allowing for maintenance, bad weather and other contingencies, the camera is expected to take over 200,000 pictures (1.28 petabytes uncompressed) per year, far more than can be reviewed by humans. Managing and effectively data mining the enormous output of the telescope is expected to be the most technically difficult part of the project. Initial computer requirements are estimated at 100 teraflops of computing power and 15 petabytes of storage, rising as the project collects data.
Particular scientific goals of the LSST include:
It is also hoped that the vast volume of data produced will lead to additional serendipitous discoveries.
Synoptic is derived from the Greek words (syn "together") and ? (opsis "view"), and describes observations that give a broad view of a subject at a particular time.
LSST construction is underway, with the NSF funding authorized as of 1 August 2014.
Early development was funded by a number of small grants, with major contributions in January 2008 by software billionaires Charles Simonyi and Bill Gates of $20 and $10 million respectively. $7.5 million was included in the U.S. President's FY2013 NSF budget request. The Department of Energy is expected to fund construction of the digital camera component by the SLAC National Accelerator Laboratory, as part of its mission to understand dark energy.
The primary mirror, the most critical and time-consuming part of a large telescope's construction, was made over a 7-year period by the University of Arizona's Steward Observatory Mirror Lab. Construction of the mold began in November 2007, mirror casting was begun in March 2008, and the mirror blank was declared "perfect" at the beginning of September 2008. In January 2011, both M1 and M3 figures had completed generation and fine grinding, and polishing had begun on M3.
The mirror was completed in December 2014. The M3 portion especially suffered from tiny air bubbles which, when they broke the surface, caused "crow's feet" defects in the surface. The bubbles trapped grinding abrasive, which produced scratches a few mm long radiating out from the bubble. Left as-is, these would enlarge the telescope's point spread function, reducing the sensitivity by 3% (to 97% of nominal) and increase the portion of the sky obscured by bright stars from 4% to 4.8% of the survey area. As of January 2015 , the project was exploring ways to fill the holes and scratches and concluded no further polishing was necessary as the mirror surfaces exceeded the structure function requirements.
The secondary mirror was manufactured by Corning of ultra low expansion glass and coarse-ground to within 40 ?m of the desired shape. In November 2009, the blank was shipped to Harvard University for storage until funding to complete it was available. On October 21, 2014, the secondary mirror blank was delivered from Harvard to Exelis (now a subsidiary of Harris Corporation) for fine grinding.
Site excavation began in earnest March 8, 2011, and the site had been leveled by the end of 2011. Also during that time, the design continued to evolve, with significant improvements to the mirror support system, stray-light baffles, wind screen, and calibration screen.
In 2015, a large amount of broken rock and clay was found under the site of the support building adjacent to the telescope. This caused a 6-week construction delay while it was dug out and the space filled with concrete. This did not affect the telescope proper or its dome, whose much more important foundations were examined more thoroughly during site planning.
The contract for the Telescope Mount Assembly was signed in August 2014.
In August 2015, the LSST camera project, which is separately funded by the U.S. Department of Energy, passed its "critical decision 3" design review, with the review committee recommending DoE formally approve start of construction. On August 31, the approval was given, and construction is beginning at SLAC.