Questions and Answers
Q: What is the LSST?
A: The LSST, or the Large Synoptic Survey Telescope, is a facility that will produce a 6-band (0.3-1.1 micron) wide-field deep astronomical survey of over 20,000 square degrees of the southern sky using an 8.4-meter ground-based telescope. Each patch of sky will be visited about 1000 times in ten years. The LSST leverages innovative technology in all subsystems: the camera (3200 Megapixels, which will be the world's largest digital camera), telescope (simultaneous casting of the primary and tertiary mirrors; two aspheric optical surfaces on one substrate), a 9.6 square degree field of view with excellent image quality, and data management (30 terabytes of data nightly, nearly instant alerts issued for objects that change in position or brightness). This innovation on all fronts has attracted many institutional members and hundreds of other scientists in ten science collaborations. Click here for a description of the LSST system and science reach, written for the scientific community.
Q: What will the survey coverage be?
A: LSST will repeatedly scan the sky south of +10 deg DEC accumulating ~1000 pairs of 15 second exposures through ugrizy filters, yielding a dataset that simultaneously satisfies the majority of the science goals. This concept, the so-called "universal cadence", will yield the main 18,000 square degree deep-wide-fast survey (typical single visit depth of r ~24.5) and use about 90% of the observing time. The remaining 10% of the time will be used to obtain improved coverage of parameter space such as ultra deep frequent observations, observations with very short revisit times (~1 minute), and observations of "special" regions such as the Ecliptic, Galactic plane, and the Large and Small Magellanic Clouds. For example, fifty selected 10 square degree "deep drilling" fields could be covered with 40 hour-long sequences of 200 exposures each. Each exposure in a sequence would have an equivalent 5-sigma depth of r~24, and each filter subsequence when coadded would be 2 magnitudes deeper than the main survey visits (r~26.5). When all 40 sequences and the main survey visits are coadded, they would extend the depth to r~28 AB mag.
Q: How will LSST's cadence be decided?
A: LSST's cadence on the sky during operations will ultimately be determined though a community input process involving the LSST Science Advisory Committee and the Science Collaborations. The detailed distribution of revisits and filters will be decided by optimizing the overall science return, using tools such as the Operations Simulator. A simple "baseline" cadence for the main 18,000 sq.deg survey, as described in the LSST Science Book (Chapter 3), satisfies the requirements laid out in the Science Requirements Document, but this cadence is not frozen. There will be different optimized cadences for the main survey, the selected deep drilling fields, and the plane of the Galaxy.
Q: What does "Synoptic" mean?
A: Our use of the word derives from the Greek word "Synopsis" and refers to looking at all aspects of something. In astronomy it often means looking at phenomena over time. The LSST is a synoptic survey in several ways: billions of objects will be imaged in six colors in an unprecedented large volume of our universe out beyond redshift 3 for galaxies. Several billion galaxies and ten billion stars will be cataloged. This survey over half the sky will also catalog changes in the intensity or color of these sources over a wide window from tens of seconds to years. Alerts will be produced within one minute of the event.
Q: Why do we need the LSST?
A: Until recently, most astronomical investigations have focused on small samples of cosmic sources or individual objects. This is because our largest telescope facilities typically had rather small fields of view, and those with large fields of view could not detect very faint sources. With all of our existing telescope facilities, we have still surveyed only a minute volume of the observable Universe. Over the past two decades, however, advances in technology have made it possible to move beyond the traditional observational paradigm and to undertake large-scale sky surveys. From its mountaintop site in Chile, the LSST will image the entire visible sky every few nights, thus capturing changes and opening up the time-domain window over an unprecedented range of timescales for billions of faint objects. Each sky patch will be visited 1000 times during the survey with a pair of exposures per visit. The LSST data will enable qualitatively new science. Billions of objects in our universe will be seen for the first time and monitored over time. Motivated by the evident scientific progress enabled by large sky surveys, multiple national reports have concluded that a dedicated ground-based wide-field imaging telescope with an effective aperture of 6-8 meters is a high priority for astronomy, physics, and planetary science over the next decade. With a thousand-fold increase in survey power in time-volume space over current facilities, LSST is likely to make unexpected discoveries.
Q: Where does LSST rank among the many proposed national scientific facilities?
A: In its 2010 report, the Astronomy and Astrophysics Decadal Survey ranked LSST as the highest priority ground-based facility. Over the past decade six national reports have ranked LSST as a high priority. This is because LSST is uniquely capable of attacking some of the greatest mysteries in astronomy and physics. National committees studying options for the next generation facility have recommended LSST for its capability to study many fundamental questions in astronomy and physics all at the same time. Rather than building separate facilities to study near—Earth asteroids, or the outer solar system, or how our galaxy was formed, or the nature of energetic explosions in the universe, or the mysterious dark matter and dark energy, LSST has a sufficient light grasp (throughput) to undertake all these scientific programs simultaneously from the same Wide—Fast—Deep survey.
Q: Why did you choose to build the telescope in Chile?
A: The decision to place LSST on Cerro Pachón in Chile was made by an international site selection committee based on a competitive process that is described here. In short, modern telescopes are located in sparsely populated places (to avoid light pollution), at high altitudes and in dry climates (to avoid cloud cover). In addition to those physical concerns there are infrastructure issues such as existing facilities, bandwidth cost, and technical personnel. The best ten candidate sites in both hemispheres wordwide were studied by the site selection committee. Site image quality at Cerro Pachón is competitive with the best world-wide. Cerro Pachón was the overall winner in terms of quality of the site for astronomical imaging and available infrastructure. The result will be superb deep images from the ultraviolet to near infrared over the vast panorama of the entire southern sky.
Q: What is the LSST schedule?
A: The LSST received its federal construction start in 2014 and will achieve engineering first light five years after that. Full science operations for the ten-year survey will begin two years after engineering first light. Significant milestones have already been reached, including the casting of the primary/tertiary mirror.
Q: Who is involved with LSST?
A: Click here to meet the team and see a list of current institutional members. Over 100 engineers and scientists are working on the LSST system. Many scientists are looking forward to LSST data. Some are already at work planning their research and interfacing with the LSST project scientists and engineers. Click here to see a list of the ten Science Collaborations. Over 250 scientists are involved in the LSST Science Collaborations currently, and the number is growing. Using the SDSS experience as a guide, it is expected that ultimately there will be thousands of papers published based on LSST data with over 5000 distinct authors.
Q: Why build an entirely new telescope for this task?
A: Because no existing telescope can do what the LSST will do. The LSST is different from other ground-based telescopes in that it is a large aperture wide-field survey telescope and camera that can move quickly around the sky tiling the sky with 15 second exposures: Wide-Fast-Deep. That combination is unique: wide field of view (10 square degrees), short exposures (pairs of 15-second exposures), and sensitive camera (24th magnitude single images, 27th magnitude stacked). LSST is far more than a telescope. With its 3200 Megapixel camera, supercomputer, and giant data processing, analysis, and distribution system, the LSST facility will produce an entirely new view of our universe enabling unforeseen explorations of discovery. When using telescope-camera combinations with smaller throughput, one must undertake different types of surveys in series. LSST will cross a threshold where a single deep-wide-fast survey produces data for a wide range of studies.
Q: Why an 8.4 meter mirror with a 3.5 degree field? Couldn't a smaller telescope or an array of smaller telescopes do the same science in a somewhat longer time?
A: Some of the science can't be done at all with a smaller telescope, or group of small telescopes. For instance, the near-Earth object (NEO) survey is looking for things that won't sit still for a long exposure. An exposure longer than 10 or 20 seconds becomes ineffective, and so finding the vast majority of NEOs which are small and faint requires a telescope that can collect a lot of light in 10 or 20 seconds. Similarly, longer exposures on a smaller telescope will not help characterize faint transient objects lasting only seconds. In an array of smaller telescopes, longer exposures would be required (to reach the sky-noise limit) as well as multiple Gigapixel cameras.
Some of the science can be done on a smaller telescope in a longer time, but consider the numbers: The speed with which you can survey an area of sky for objects of a given faintness is proportional to throughput (collecting area times field of view in meters squared degrees squared). The LSST enables totally new windows on the universe because it has such a high throughput, or "etendue." The etendue of LSST is 320 square meters square degrees. A primary mirror diameter of 8.4 m (effective aperture 6.7 m due to obscuration) is the minimum diameter that simultaneously satisfies the depth (24.5 mag depth per single visit and 27.5 mag for coadded depth) and cadence (revisit time of 3-4 days, with 30 seconds per visit) constraints. Above a throughput or "etendue" of 200-300 square meters square degrees, many different surveys can be done using the same wide-fast-deep survey data -- a large multiplex advantage.
Q: Why not a space mission?
A: There are several reasons why the science mission of LSST is most cost-effective with a ground facility. To probe the physics of dark energy, hemisphere coverage of the sky is necessary, as well as deep coverage. Some deep probes of the universe would benefit from the higher angular resolution available in space. But they also require huge samples of objects over a wide area of sky (large volume of the universe.) This Wide—Deep capability is hard to obtain in space. Another reason is surveying for fast events -- LSST will open this new window on the universe. In any realistic space mission the Wide-Fast-Deep capability would be lost: space telescopes have small collecting area compared to what can be built on the ground, leading to long exposures and loss of timing information. Since all LSST's science goals can be achieved from the ground, we must weigh the incremental benefit against the drawbacks. The science that drives the need for the LSST requires ultra deep and rapid wide—field imaging at optical wavelengths—a mission best achieved on the ground at a superb site.
Q: What are the technology challenges and scale of effort?
A: The realization of the LSST involves extraordinary engineering and technological challenges: the fabrication of large, high-precision aspheric optics; construction of a huge, highly-integrated array of sensitive, wide-band imaging sensors; and the operation of a data management facility handling tens of terabytes of data each day. The design and development effort includes structural, thermal, and optical analyses of all key hardware subsystems, vendor interactions to determine manufacturability, explicit prototyping of high-risk elements, prototyping and development of data management systems, and extensive systems engineering studies. To validate system performance, full end-to-end simulations are being done. Over 100 technical personnel at a range of institutions are currently engaged in this program.
Q: How does LSST probe the physics of dark energy?
A: Current observations suggest that most of the energy density of the universe is in some unknown form. Dark energy affects the cosmic history of both the Hubble expansion and mass clustering. If combined, different types of probes of the expansion history and structure history can lead to tight constraints dark energy equation of state and other cosmological parameters. These tight constraints arise because each technique depends on the cosmological parameters or errors in different ways. A unique aspect of LSST as a probe of dark energy and matter is the use of multiple cross-checking probes that reach unprecedented precision. These probes include weak gravitational lens (WL) cosmic shear, baryon acoustic oscillations (BAO), supernovae, and cluster counting -- all as a function of redshift. Using the cosmic microwave background as normalization, the combination of these probes can yield the needed precision to distinguish between models of dark energy. In addition, time-resolved strong galaxy and cluster lensing probes the physics of dark matter.
LSST data could lead to a hundred- to thousand-fold increase in precision over precursor experiments about to be undertaken. The power and accuracy of LSST dark energy and dark matter probes is derived from samples of several billion galaxies and tens of millions of Type-I supernovae. The nominal high-SNR sample defined by i<25 mag (SNR>25 for point sources) will include four billion galaxies (55 per suare arcminute) with the mean photometric redshift accuracy of 1-2% (relative error for 1+z), and with only 10% of the sample with errors larger than 4%. The median redshift for this sample will be z=1.2, with the third quartile at z=2. For a subsample of 2 billion galaxies further constrained by flux limits in the g and z bands, the photometric redshift errors will be two to three times smaller. By simultaneously measuring mass growth (via weak lensing and cluster counting) and curvature (via BAO and SN), LSST data will tell us whether the recent cosmic acceleration is due to dark energy or modified gravity.
Q: Why do you need such accurate measurements of the shear power spectrum?
A: Outside of the huge and rare mass overdensities of rich clusters of galaxies, the mass contrast is much lower, giving rise to shear values below one percent. Here, in the vast volume between the rich clusters, is where most of the dark matter and dark energy of the universe exists. Current observations suggest that most of the energy density of the universe is in some unknown form. Weak gravitational lens "cosmic shear" as a function of distance will measure both the geometry of the universe and the growth rate of structure—two related probes of the equation of state of this dark energy. It is necessary to measure the weak gravitational lens induced shear of billions of galaxies over a large area and control systematic shear errors at the 0.0001 level.
Q: Will it be possible to subscribe to real time alerts of LSST discoveries?
A: Yes. Web alert pages and auto email alert services enabled by our data centers will permit users to custom filter alerts based on a number of classification parameters. About 2 billion objects will be routinely monitored for photometric and astrometric changes, and any transient events will be posted in less than 60 seconds.
Q: Will the full resolution, full depth image data be available to download?
A: Yes. There will be a range of data products and download portals. The LSST data system is being designed to enable as wide a range of science as possible. Standard data products, including calibrated images and catalogs of detected objects and their attributes, will be provided both for individual exposures and the deep incremental data coaddition. For the "static" sky, there will be yearly database releases listing many attributes for billions of objects. This database will grow in size to about 30 PB and about 20 billion objects.
As in the SDSS, we expect a power law of user interactions with the data. At one end of this distribution are simple lookup queries or color jpeg cutout downloads. At the other end are huge statistical calculations over the entire database, and image operation scripts on billions of objects. The data management system is budgeted to handle most but not all of that distribution. Institutions joining LSST early, and members of the LSST Science Collaborations, will have the customary advantage of deep familiarity with the LSST system and survey.
Q: Will LSST imaging data be available world-wide for scientific use?
A:LSST alerts and education products will be available world-wide. science images and catalogs will be available to all US and Chilean astronomers as well as to international institutions who support operations.
Q: What are the data management challenges?
A: The rapid cadence of the LSST observing program will produce about 30 TB per night, leading to a total database over the ten years of operations of 60 PB for the raw data, and 30 PB for the catalog database. The total data volume after processing will be several hundred PB, processed using 100 TFlops of computing power. Processing such a large volume of data, converting the raw images into a faithful representation of the universe, automated data quality assessment, and archiving the results in useful form for a broad community of users is a major challenge. There will be both mountain summit and base computing facilities, as well as a central archive facility and multiple data access centers. The data will be sent over existing optical fiber links from South America to the U.S. Although the data processing center will have substantial computing power (about 100 TFlops), current trends suggest that it will not even qualify for the top 500 list by the time of first light in 2022. Hence, while LSST is making a novel use of advances in information technology, it is not taking the risk of pushing the expected technology to the limit.
Q: I have questions. I'd like to make a scientific contribution to this project. Who should I contact?
A: You can contact us by emailing to "contact" at"lsst.org". The LSST Project solicits your input on the technical and scientific choices that we face. Share your views with us! Please make specific reference to technical documents or presentations when appropriate, so we can be sure to route your comments to the relevant people. A good place to start is our overview paper. Consider also browsing our team page.