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|
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)|
0.7? median seeing limit|
0.2? pixel size
|Collecting area||35 square meters (376.7 sq ft)|
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 (syn "together") and ? (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 Base Facility is located about 100 kilometres (62 mi) away by road, in the town of La Serena.
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, unlike almost all previous large astronomical observatories, has committed to making all data public as soon as it is taken. In their words "By providing immediate public access to all the data it obtains, it will provide everyone, the professional and the "just curious" alike, a deep and frequent window on the entire sky."
LSST is the successor to a long tradition of sky surveys. These started as visually compiled catalogs 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.
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.
In May 2018, Congress surprisingly appropriated much more funding than the telescope had asked for, in hopes of speeding up construction and operation. Telescope management was thankful but unsure this would help, since at the late stage of construction they were not cash-limited.
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 earliest reflecting telescopes used spherical mirrors, which although easy to fabricate and test, suffer from spherical aberration; a very long focal length was needed to reduce spherical aberration to a tolerable level. Making the primary mirror parabolic removes spherical aberration on-axis, but the field of view is then limited by off-axis coma. 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, giving a wider useful field of view. Most 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 a three-mirror anastigmat: three non-spherical mirrors to cancel astigmatism as well, resulting in sharp images over a very wide field of view.
The LSST primary mirror (M1) is 8.4 meters (28 ft) in diameter, the secondary mirror (M2) is 3.4 meters (11.2 ft) in diameter, and the tertiary mirror (M3), located in a large hole in the primary, is 5.0 meters (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 4.2 m secondary c. 2024.) The large hole reduces the primary mirror's light-collecting area to 35 square meters (376.7 sq ft), equivalent to a 6.68-meter-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 designed as a single piece of glass, the "M1M3 monolith". Placing the two mirrors in the same location minimizes the overall length of the telescope, making it easier to reorient quickly. Making them out of the same piece of glass results in a stiffer structure than two separate mirrors, contributing to rapid settling after motion.
The optics includes three corrector lenses to reduce aberrations. These lenses, and the telescope's filters, are built into the camera assembly. 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.
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 15 second exposures are a compromise to allow spotting both faint and moving sources. Longer exposures would reduce the overhead of camera readout and telescope re-positioning. allowing deeper imaging, but then fast moving objects such as near-Earth objects would move significantly during an exposure. Each spot on the sky is imaged with two consecutive 15 second exposures, to efficiently reject cosmic ray hits on the CCDs.
The camera focal plane is flat, 64 cm in diameter. The main imaging is performed by a mosaic of 189 CCD detectors each of 16 megapixels. They are grouped into a 5×5 grid of "rafts", where the central 21 rafts contain 3×3 imaging sensors, while the four corner rafts contain only three CCDs each, for guiding and focus control. The CCDs provide better than 0.2 arcsecond sampling, and will be cooled to approx -100 °C. to help reduce noise.
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 330 to 1080 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. In 2010, the initial computer requirements were estimated at 100 teraflops of computing power and 15 petabytes of storage, rising as the project collects data. By 2018, estimates had risen to 250 teraflops and 100 petabytes of storage.
Once images are taken, they are processed according to three different timescales, prompt (within 60 seconds), daily, and annually.
The prompt products are alerts, issued within 60 seconds of observation, about objects that have changed brightness or position relative to archived images of that sky position. Transferring, processing, and differencing such large images within 60 seconds (previous methods took hours, on smaller images) is a significant software engineering problem by itself. Approximately 10 million alerts will be generated per night. Each alert will include the following::22
There is no proprietary period associated with alerts--they are available to the public immediately, since the goal is to quickly transmit nearly everything LSST knows about any given event, enabling downstream classification and decision making. LSST will generate an unprecedented rate of alerts, hundreds per second when the telescope is operating.[note 2] Most observers will be interested in only a tiny fraction of these events, so the alerts will be fed to "event brokers" which forward subsets to interested parties. LSST will provide a simple broker, and provide the full alert stream to external event brokers. The Zwicky Transient Facility will serve as a prototype of LSST alerts, generating "only" 1 million events per night.
Daily products, released within 24 hours of observation, comprise the images from that night, and the source catalogs derived from difference images. This includes orbital parameters for solar system objects. Images will be available in two forms: Raw Snaps, or data straight from the camera, and Single Visit Images, which have been processed and include instrumental signature removal (ISR), background estimation, source detection, deblending and measurements, point spread function estimation, and astrometric and photometric calibration.
Annual release data products will be made available once a year, by re-processing the entire science data set to date. These include:
LSST is reserving 10% of its computing power and disk space for user generated data products. These will be produced by running custom algorithms over the LSST data set for specialized purposes, using Application Program Interfaces (APIs) to access the data and store the results. This avoids the need to download, then upload, huge quantities of data by allowing users to use the LSST storage and computation capacity directly. It also allows academic groups to have different release policies than LSST as a whole.
An early version of the LSST image data processing software is being used by the Subaru telescope's Hyper Suprime-Cam instrument, a wide-field survey instrument with a sensitivity similar to LSST but one fifth the field of view: 1.8 square degrees versus the 9.6 square degrees of LSST.
LSST will cover about 18,000 deg2 of the southern sky with 6 filters in its main survey, with about 825 visits to each spot. The magnitude limits are expected to be r<24.5 in single images, and r<27.8 in the full stacked data.
The main survey will use about 90% of the observing time. The remaining 10% will be used to obtain improved coverage for specific goals and regions. This includes very deep (r ~ 26) observations, very short revisit times (roughly one minute), observations of "special" regions such as the Ecliptic, Galactic plane, and the Large and Small Magellanic Clouds, and areas covered in detail by multi-wavelength surveys such as COSMOS and the Chandra Deep Field South. Combined, these special programs will increase the total area to about 25,000 deg2.
Particular scientific goals of the LSST include:
It is also hoped that the vast volume of data produced will lead to additional serendipitous discoveries.
NASA has been tasked by the US Congress with detecting and cataloging 90% of the NEO population of size 140 meters or greater. LSST, by itself, is estimated to detect 62% of such objects, and according to the National Academy of Sciences, extending its survey would be the most cost-effective way of finishing the task.
LSST has a program of Education and Public Outreach (EPO). LSST EPO will serve four main categories of users: the general public, formal educators, citizen science principal investigators, and content developers at informal science education facilities. LSST will partner with Zooniverse for a number of their citizen science projects.
There have been many other optical sky surveys, some still on-going. For comparison, here are some of the main currently used optical surveys, with differences noted:
The Cerro Pachón site was selected in 2006. The main factors were the number of clear nights per year, seasonal weather patterns, and the quality of images as seen through the local atmosphere (seeing). The site also needed to have an existing observatory infrastructure, to minimize costs of construction, and access to fiber optic links, to accommodate the 30 terabytes of data LSST will produce each night.
As of February 2018, construction is well underway. The shell of the summit building is complete, and 2018 will see the installation of major equipment, including HVAC, the dome, mirror coating chamber, and the telescope mount assembly. It will also see the expansion of the AURA base facility in La Serena and the summit dormitory shared with other telescopes on the mountain.
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 project remains within budget, although the budget contingency is tight.
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 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 four seconds.[note 3] This requires a very stiff pier and telescope mount. The basic design is conventional: an altitude over azimuth mount made of steel, with hydrostatic bearings on both axes, mounted on a pier which is isolated from the dome foundations. However, the LSST pier is unusually large (16 m diameter) and robust (1.25 m thick walls), and mounted directly to virgin bedrock, where care was taken during site excavation to avoid using explosives that would crack it. Other unusual design features are linear motors on the main axes and a recessed floor on the mount. This allows the telescope to extend slightly below the azimuth bearings, giving it a very low center of gravity.
The contract for the Telescope Mount Assembly was signed in August 2014. The nearly-completed TMA was inspected in April 2018, and is scheduled to ship from its construction site in Spain to Chile in November.
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 data must be transported from the camera, to facilities at the summit, to the base facilities, and then to the LSST Data Facility at the National Center for Supercomputing Applications in the United States. This transfer must be very fast (100 Gbit/s or better) and reliable since NCSA is where the data will be processed into scientific data products, including real-time alerts of transient events. This transfer uses multiple fiber optic cables from the base facility in La Serena to Santiago, then via two redundant routes to Miami, where it connects to existing high speed infrastructure. These two redundant links were activated in March 2018 by the AmLight consortium.
Since the data transfer crosses international borders, many different groups are involved. These include the Association of Universities for Research in Astronomy (AURA, Chile and the USA), REUNA (Chile), Florida International University (USA), AmLightExP (USA), RNP (Brazil), and University of Illinois at Urbana-Champaign NCSA (USA), all of which participate in the LSST Network Engineering Team (NET). This collaboration designs and delivers end-to-end network performance across multiple network domains and providers.