Cadence Design
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.
Cadence
LSST will take data as pairs of back-to-back, 15-second exposures to aid in cosmic-ray rejection. We call this pair 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.
- Cosmological parameter estimation by many techniques requires uniform coverage of 20,000 square degrees of sky. Obtaining accurate photometric red-shifts 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 these goals facilitates analyses made while the survey is still in progress.
Proposals
Before each observation a series of functions, called proposals, rank potential visits according to criteria such as timing, sky background, seeing, air mass, and progress toward survey goals. These rankings are then merged, penalties are applied for slew and filter change times and other operational considerations, and ranked again. The best visit is then made, and the process repeats. We have found that four proposals are sufficient to ensure meeting all requirements:
Deep-Wide-Fast is designed to provide the deep, uniform coverage of the sky with uniform progress toward the specified number of visits over ten years. In times of good seeing and at low airmass, preference is given to r and i band observations. It provides most of the temporal sampling for discovering time variability and detecting moving solar system objects. It requires, as often as possible, that each field be observed twice with visits separated by 15 – 60 minutes to provide motion vectors to link moving object detections and fine time sampling for measuring short-period variability.
Northern Ecliptic extends Deep-Wide-Fast to 4,000 square degrees of the northern ecliptic beyond the airmass limit of the main survey.
Deep-Drilling is implemented because a small fraction of time spent employing different strategies can significantly enhance the overall science return. This proposal allocates ten minutes’ exposure per night to a small number of fields; the time is distributed among filters on a five-day cycle so as to provide high-quality type-Ia supernova light curves at redshifts to z~1.2. Many of the these fields are distributed across the ecliptic plane to enable deeper searches for KBO's and other denizens of the outer solar system.
Galactic Plane allocates thirty observations in each of six filters in a region of 1000 square degrees around the galactic center where the high stellar density leads to a confusion limit at much brighter magnitudes than those attained in the rest of the survey.
South Ecliptic Pole allocates thirty observations in each of six filters in a region of ~1700 square degrees around the south ecliptic pole to provide data on the Magellanic Clouds and transients in the southern sky. It is similar to the Galactic Plane proposal, but has more relaxed seeing and sky brightness limits to allow higher airmass observations.
Source: American Astronomical Society 213th Meeting, Poster Exhibit, "LSST: Cadence Design and Simulation", K.H. Cook et al., 460.04