How many galaxies will LSST observe and how well can we estimate their redshifts from colors?

LSST aims to catalog roughly four to five billion galaxies with redshift information. The photometric redshift accuracy from color information across the six bands is around five percent, thanks to multi-band photometry and template fitting.

What is etendue and why is it important for LSST?

Etendue is the product of the telescope's light-collecting area and its field of view. LSST is designed to maximize etendue to enable wide, deep, and fast surveying simultaneously, which is central to achieving its science goals.

Where will LSST be built and what makes the site suitable?

The LSST site is Cerro Pachón in northern Chile, chosen for its exceptionally good observing conditions. The location supports a very wide, deep, and fast survey capability with a stable atmosphere and favorable weather patterns.

How much data will LSST generate, and will it be publicly available?

LSST will produce on the order of 15 terabytes of raw data per day, escalating to tens of terabytes when processed. The project envisions publicly accessible data products, with a catalog and tools that the world can use, rather than a proprietary dataset.

When will LSST start operations and when will the sky be surveyed?

After design and construction phases, the plan is to begin science operations around 2012–2013, with the full sky survey delivering its first deep, multi-band data in the early years of operation and improving over the decade.

What is 4D mass tomography and how does LSST measure dark matter?

4D tomography uses weak gravitational lensing to map the mass distribution (dark matter) in the universe as a function of redshift. By measuring how distant galaxies’ shapes are distorted by intervening mass and how this distortion evolves with distance, LSST can reconstruct the mass distribution over cosmic time.

The New Digital Sky

Google Talks

Education5 min read56 min video

Aug 22, 2012|768 views|5|1

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TL;DR

LSST will map the visible sky in Chile in 4D (space, time, distance) with a giant, fast, deep survey.

Key Insights

A new kind of telescope design enables wide, deep, and fast sky surveys (large etendue) far beyond existing facilities.

The camera is a 3.2 gigapixel focal plane with ~190 4k x 4k detectors, enabling 15-second exposures and rapid data flow.

An alt-azimuth mount with rotating pupil plane provides superb control of systematic errors and suppresses diffraction artifacts.

The project emphasizes real-time data processing and public data access, delivering alerts within seconds and petabytes of data annually.

Strong science drivers include mapping dark matter via weak lensing and probing dark energy with four-dimensional cosmology, plus a rich time-domain program (asteroids, supernovae).

The data system envisions a public, Google Sky–like interface, turning the sky into accessible, community-driven knowledge.

A NEW GENERATION SURVEY TELESCOPE

This project, launched in 2000, is designing a revolutionary telescope to digitize the entire sky visible from Chile with unprecedented depth. It blends three enabling breakthroughs: advanced microelectronics for the focal plane detectors, massive on-site computing to handle teraflops and data processing, and breakthroughs in large-aspherical optics fabrication. The idea is to move astronomy from single-purpose observations to a mission that simultaneously surveys wide areas, goes deep in brightness, and captures rapid time variation, all in a single instrument.

ETENDUE: WIDE, DEEP, AND FAST

Etendue, the product of light-collecting area and field of view, governs how much sky you can cover efficiently. LSST targets an enormous etendue, dwarfing existing facilities like Keck in this metric. With an eight-and-a-half-meter mirror and a field of view of about 10 square degrees, LSST is designed to survey large swaths of the sky quickly and to great depth, enabling both broad statistical studies and time-domain investigations across the visible universe.

THE THREE-MIRROR DESIGN AND FLAT FOCAL PLANE

Light travels through a carefully staged path: a large primary mirror, a secondary mirror, a tertiary mirror, and into a sophisticated camera system. This three-mirror setup enables a wide, crisp, wavelength-independent image across a huge field. A notable achievement is the flat focal plane, which—along with the optical design—supports uniform image quality over the large field and simplifies the complex data capture and calibration required for a survey of this scale.

THE FOCAL PLANE AND DETECTORS

The camera is a $100 million focal plane comprising roughly 190 detectors, each 4k by 4k, forming about 3.2 gigapixels across a 65 cm span. Exposures are about 15 seconds, and the system reads out in one to two seconds. To maximize information, the design uses 16-bit raw data that is immediately upgraded to 32-bit processing. The pixel scale (about 0.2 arcseconds per 10 μm pixel) is chosen to match the best site conditions and preserve sharpness in the images.

WAVEFRONT SENSING AND STRUCTURAL CONTROL

Wavefront sensing relies on precise hardware and software to know exactly where the focal plane sits relative to the detector. A deliberate dithering of the detectors provides extra samples for wavefront curvature, ensuring micron-level precision. This rigorous attention to optical alignment and atmospheric effects is essential for achieving the desired resolution and photometric accuracy across the entire field.

ALT-AZ MOUNT AND SYSTEMATIC CONTROL

LSST uses an alt-azimuth mount, which causes the pupil and focal planes to rotate differently as the sky moves. This rotation is not a nuisance—it becomes a powerful tool for controlling systematic errors. As the sky drifts, the orientation of diffraction spikes changes in a known way, allowing reconstruction algorithms to suppress artifacts and deliver cleaner images, especially around bright stars.

TIMING, SPECTRAL BANDS, AND REDSHIFT ESTIMATION

The survey covers six optical bands from the ultraviolet to near-infrared, revisiting every sky patch around 2,000 times. Each set of six bands helps estimate photometric redshifts to within about 5% accuracy, providing distance information without full spectroscopy. Exposures are short, so the survey can simultaneously map faint objects and monitor changing sources, creating a 'poor-man spectrograph' that informs a broad set of science goals.

END-TO-END SIMULATIONS AND SITE SELECTION

Before construction, extensive simulations validate capabilities: end-to-end models trace photons from the early universe through atmosphere and instrument to data products. Chile was chosen as the optimal site after global consideration of weather, transparency, and stability. The project is a public-private partnership supported by NSF, DOE, and other collaborators, with rigorous planning to ensure that the telescope will deliver the promised science.

SCIENCE DRIVERS: DARK MATTER, DARK ENERGY, AND 4D TOMOGRAPHY

A central motivation is unlocking the mystery of dark matter through weak gravitational lensing—mapping how mass distorts light from distant galaxies. By reconstructing the mass distribution as a function of cosmic time, LSST probes the evolution of structure in the universe and the properties of dark energy that appear to accelerate cosmic expansion. The telescope acts as a 4D probe, linking spatial structure with time and distance to test competing cosmological models.

WEAK LENSING AND COSMIC SHEAR AS COSMOLOGY TOOLS

The key observable is the coherent distortion (shear) of distant galaxies caused by intervening dark matter. By measuring this signal across billions of galaxies and in multiple redshift slices, LSST performs 3D mass tomography. The resulting maps constrain the growth of structure and the geometry of the universe, offering stringent tests of dark energy models and gravity on cosmic scales while leveraging the survey’s breadth in area and depth.

TIME DOMAIN SCIENCE: ASTEROIDS, SUPERNOVAE, AND TRANSIENTS

LSST will revolutionize time-domain astronomy. It will detect hundreds of thousands of asteroids, catalog a million or more supernovae, and monitor a multitude of transient events. With rapid, on-the-fly data processing and alert generation, researchers can coordinate follow-up observations in other wavelengths. The time-domain aspect adds a dynamic dimension to cosmology and solar system science, enabling discoveries of events we have yet to imagine.

PUBLIC ACCESS, DATA VOLUME, AND COMMUNITY IMPACT

A defining principle is open data: the results, software, and data streams are intended to be publicly accessible, akin to an open-source model. The data stream is enormous—petabytes per year—with sophisticated catalogs describing object properties and time evolution. A Google Sky–style interface will help the world explore the sky, turning raw observations into usable knowledge and enabling education, citizen science, and cross-disciplinary research.

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LSST in Practice: Quick Do's and Don'ts

Practical takeaways from this episode

Do This

Consider etendue (A * Ω) as a unifying metric for wide-field survey capability.

Plan for end-to-end simulations to validate survey performance before construction.

Design for rapid data processing and real-time transient alerts (seconds to minutes).

Prepare for public data access and a scalable data catalog to enable broad use.

Avoid This

Don’t assume small telescopes can easily deliver wide, deep, and fast surveys all at once.

Don’t rely on long single-exposure deep fields with slow cadences if you want time-domain science.

Common Questions

LSST is the Large Synoptic Survey Telescope project. Its goal is to digitize and map most of the sky visible from Chile with a huge field of view, visiting each patch thousands of times across six bands to build a 4D (three spatial dimensions plus time) view of the universe. It enables deep imaging, time-domain astronomy, and a 3D tomography of mass via weak lensing, with a publicly accessible data set.