In a ten-year survey, the LSST will take more than five million exposures, collecting over 50 petabytes of raw image data to produce a deep, time-dependent, multi-color movie of 30,000 square degrees of sky. The sequence, or cadence, with which these exposures are made is essential to achieving multiple scientific goals from a single survey, an important feature of the LSST concept.  Read the community white paper on the LSST Observing Strategy [1] in arxiv and contribute to the living document in GitHub [1].

[2]

LSST will take data as pairs of back-to-back, 15-second exposures to aid in cosmic-ray rejection. This pair is called a visit - a single observation of a ten-square-degree field through a given filter. Designing the LSST survey requires ordering these visits in time and allocating them among its six filters so as to maximize the return on scientific goals in a fixed survey duration. Synthesizing the requirements to accomplish the four primary science objectives of the LSST,

• Constraining Dark Energy & Dark Matter
• Taking an Inventory of the Solar System
• Exploring the Transient Optical Sky
• Mapping the Milky Way

results in the following constraints:

• Cosmological parameter estimation by many techniques requires uniform coverage of 18,000 square degrees of sky. Obtaining accurate photometric redshifts in every field requires a specified number of visits in each filter.
• Weak lensing shear measurements benefit from allocating times of best seeing to observations in the r and i bands. Maximizing signal-to-noise ratios requires choosing the next filter based upon the current sky background.
•  Supernova cosmology requires frequent, deep photometry in all bands, with z and y observations even during dark time.
• Detecting the motion of solar system objects and transients, characterizing variability on various timescales, and acquiring the best proper motions and parallaxes place further demands upon the distribution of revisit intervals and observation geometries to each point on the sky.

Finally, making uniform progress in time toward each of the scientific goals facilitates analyses made while the survey is still in progress.

In addition to LSST’s 18,000 deg² main survey via its “universal cadence,” up to approximately 10% of LSST observing time will be dedicated to other programs, including intensive observation of a set of Deep Drilling Fields. Deeper coverage and more frequent temporal sampling (in at least some of the LSST’s ugrizy filters) will be obtained for the Deep Drilling Fields than for typical points on the sky. The full Deep Drilling Field (DDF) program [3] will address a broad range of science topics, including Solar System, Galactic, and extragalactic studies. 

Science Book - Chapter 1: Introduction [4]- An overview of the LSST Project including its history, science goals, and system design.

Science Book - Chapter 2: LSST System Design [5] -  A description of the basic elements of the LSST system design, with particular emphasis on elements impacting the science objectives.

Science Book - Chapter 3: LSST System Performance [6] - A description of the essential characteristics of the LSST system performance.