Large Synoptic Survey Telescope
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 word synoptic is derived from the Greek words σύν (syn "together") and ὄψις (opsis "view"), and describes observations that give a broad view of a subject at a particular time.
The telescope uses a novel 3-mirror design, a variant of three-mirror anastigmat, which allows a compact telescope to deliver sharp images over a very wide 3.5-degree diameter field of view. Images are recorded by a 3.2-gigapixel CCD imaging camera, the largest digital camera ever constructed. The telescope is 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 proposed in 2001, and construction of the mirror began (with private funds) in 2007. LSST then became the top-ranked large ground-based project in the 2010 Astrophysics Decadal Survey, and 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.
LSST is the successor to a long tradition of sky surveys. These started as visually compiled catologs in the mid 1700s, such as the Messier catalog. This was replaced in the late 1800s by photographic surveys, starting with the Harvard Plate Collection, the National Geographic Society – Palomar Observatory Sky Survey, and others. By about 2000 the first digital surveys, such as the Sloan Digital Sky Survey (SDSS), began to replace the photographic plates of the earlier surveys.
LSST evolved from the earlier concept of the Dark Matter Telescope, mentioned as early as 1996. The fifth decadal report, Astronomy and Astrophysics in the New Millennium, was released in 2001, and recommended the "Large-Aperture Synoptic Survey Telescope" as a major initiative. Even at this early stage the basic design and objectives were set:
The Large-aperture Synoptic Survey Telescope (LSST) is a 6.5-m-class optical telescope designed to survey the visible sky every week down to a much fainter level than that reached by existing surveys. It will catalog 90 percent of the near-Earth objects larger than 300 m and assess the threat they pose to life on Earth. It will find some 10,000 primitive objects in the Kuiper Belt, which contains a fossil record of the formation of the solar system. It will also contribute to the study of the structure of the universe by observing thousands of supernovae, both nearby and at large redshift, and by measuring the distribution of dark matter through gravitational lensing. All the data will be available through the National Virtual Observatory (see below under “Small Initiatives”), providing access for astronomers and the public to very deep images of the changing night sky.
By the 2010 survey, LSST was the highest ranked ground based initiative, and by 2014 the rest of the construction was funded. The lead organizations are:
- The SLAC National Accelerator Laboratory will design and construct the LSST camera
- The National Optical Astronomy Observatory will provide the telescope and site team
- The National Center for Supercomputing Applications will construct and test the archive and data access center
- The Association of Universities for Research in Astronomy is responsible for overseeing the LSST construction.
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. This is more than three times the entendue of best existing telescopes, the Subaru Telescope with its Hyper Suprime Camera, and Pan-STARRS, and more than an order of magnitude better than most large telescopes.
The LSST is the latest in a long line of improvements giving telescopes larger fields of view. The first reflecting telescopes used spherical mirrors, which although easy to fabricate and test, had very small fields of view due to spherical aberration. Making the primary mirror parabolic removes this aberration, increasing the field of view, which is then limited by coma and off-axis astigmatism. Such a parabolic primary, with either a prime or Cassegrain focus, was the most common optical design up through the Hale telescope in 1949. After that, telescopes used mostly the Ritchey–Chrétien design, which uses two hyperbolic mirrors to remove both spherical aberration and coma, leaving only astigmatism, again giving a wider field of view. Almost all large telescopes between Hale and LSST used this design - the Hubble and Keck telescopes are Ritchey–Chrétien, for example. LSST takes the next step, using 3 non-spherical mirrors to cancel astigmatism as well, resulting in a very wide, undistorted field of view.
The LSST 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) were 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 includes three corrector lenses to reduce aberrations. The first lens at 1.55 m diameter is the largest lens ever built, and the third lens forms 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.
Image data processing
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:
- Measuring weak gravitational lensing in the deep sky to detect signatures of dark energy and dark matter.
- Mapping small objects in the Solar System, particularly near-Earth asteroids and Kuiper belt objects.
- Detecting transient optical events such as novae and supernovae.
- Mapping the Milky Way.
It is also hoped that the vast volume of data produced will lead to additional serendipitous discoveries.
As of November 2017, the data pipeline is expected to produce:
- 10 million time domain events per night, transmitted within 60 seconds of image acquisition.
- A catalog of roughly 6 million solar systems objects, with their orbits
- A catalog of approximately 37 billion sky objects - 20 billion galaxies and 17 billion stars.
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 funding construction of the digital camera component by the SLAC National Accelerator Laboratory, as part of its mission to understand dark energy.
As of November 2016[update] the project critical path was the camera construction, integration and testing. By February 2018, the camera and telescope shared the critical path. The main risk was deemed to be whether sufficient time was allotted for system integration.
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[update], 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 μm 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.
Telescope Mount Assembly
The telescope mount, and the pier on which it sits, are substantial engineering projects in their own right. The main technical problem is that the telescope must slew 3.5 degrees to the adjacent field and settle within 5 seconds. This requires a very stiff pier and telescope mount. The final design is an altitude over azimuth mount made of steel, with hydrostatic bearings on both axes, mounted on on a unusually robust pier that is built upon bedrock and completely isolated from support of the building and dome.
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. As of September 2017, construction of the camera was 72% complete, with sufficient funding in place (including contingencies) to finish the project.
- The camera is actually at the tertiary focus, not the prime focus, but being located at a "trapped focus" in front of the primary mirror, the associated technical problems are similar to those of a conventional prime-focus survey camera.
- List of largest optical reflecting telescopes
- The Dark Energy Survey
- VISTA (Visible and Infrared Survey Telescope for Astronomy)
- VLT Survey Telescope
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|Wikimedia Commons has media related to Large Synoptic Survey Telescope.|
- Official home page
- LSST construction site webcams
- LSST reports and documentation
- New Scientist SPACE Article
- LSST Tutorials for Experimental Particle Physicists is a detailed explanation of LSST's design (as of February 2006) and weak lensing science goals that does not assume a lot of astronomy background.
- The New Digital Sky is a video of a July 25, 2006 presentation at Google about the LSST, particularly the data management issues.
- HULIQ Google participation announcement
- Ž. Ivezić; et al. (2008-05-15), LSST: From Science Drivers to Reference Design and Anticipated Data Products (v1.0), 0805, p. 2366, arXiv: , Bibcode:2008arXiv0805.2366I, retrieved 2015-08-05, this is a comprehensive overview of the LSST.
- LSST Science Collaborations; Abell, Paul A.; Allison, Julius; Anderson, Scott F.; Andrew, John R.; Angel, J. Roger P.; Armus, Lee; Arnett, David; Asztalos, S. J. (2009-10-16), LSST Science Book, Version 2.0, 0912, p. 201, arXiv: , Bibcode:2009arXiv0912.0201L, retrieved 2011-01-16, an updated and expanded overview.