In a ten-year survey, the LSST will take more than five million exposures, collecting over 32 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.
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 one 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 20,000 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.
From its site on Cerro Pachon in northern Chile, the LSST can view sky regions with Declination (Dec.) of less than 33.5 degrees at an airmass of 2.2 or smaller - a limit that is used to define the LSST survey. This airmass results in a 0.6 mag loss of sensitivity at 500 nm compared to an observation at zenith (due to both seeing degradation and atmospheric absorption), and corresponds to an observable area of 31,000 square degrees.
Sky regions with -75 < Dec. < +15 can be observed at an airmass of 1.4 or smaller, providing especially good image quality for weak lensing and other science programs that require it. The total accessible area in this range, outside of the star-crowded parts of the galactic plane, exceeds 20,000 square degrees. The two dashed blue lines in the figure below outline the 24,000 square degree region for which the minimum airmass reaches values of less than 1.4.
For the Wide-Fast-Deep (WFD) observing program, we use 18,000 of the possible 24,000 square degrees to meet the Science Requirements Document (SRD) design goals. The WFD science program is designed to provide data for cosmology, transients and moving objects.
A summary of the observing constraints in equatorial (top panel) and galactic (bottom panel) coordinates. The two dashed blue lines outline the 24,000 square degree region for which the minimum airmass reaches values of less than 1.4. The galactic plane regions with the highest stellar density are demarked by solid red lines and enclose 1,000 square degrees.
These four plots illustrate the potential airmass distribution in R and I bandpasses for the Wide Fast Deep ('universal cadence') portion of the LSST survey. Each plot represents a different realization of the survey, as simulated by the Operations Simulator. The opsim 3.61 and 3.87 runs represent a WFD survey with a footprint covering the LSST stretch goals of 20,000 square degrees and 1030 visits per field, approximately 230 of which are in R band with another 230 in the I band. The opsim 2.93 and 4.262 runs represent a WFD survey covering the LSST design goals of 18,000 square degrees and 824 visits per field, approximately 184 of which are in each of the R and I bands. The figure insets provide 25/50/75th percentile values for the airmass distribution in each bandpass.