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Uniform Rolling: An LSST Observing Cadence Offering Sufficient Survey Uniformity for Comprehensive Cosmological Analysis

Boris Leistedt, Matthew R. Becker, Humna Awan, Eric Gawiser, Qianjun Hang, Renée Hložek, Saurabh W. Jha, R. Lynne Jones, Arun Kannawadi, Michelle Lochner, Rachel Mandelbaum, Jeffrey A. Newman, I. Sevilla-Noarbe, Hiranya V. Peiris, Eli S. Rykoff, M. A. Troxel, Peter Yoachim

Abstract

The Legacy Survey of Space and Time (LSST) that will be carried out by the NSF-DOE Vera C. Rubin Observatory promises to be the defining survey of the next decade, supplying unprecedented access to the night sky to static science- and time-domain science-focused researchers alike. Maximizing the output of the broad remit of Rubin Observatory science requires a non-trivial survey strategy. For time-domain science, the most promising strategy designed so far is a rolling survey strategy, whereby a subset of the full LSST survey area is observed at higher rate compared with the nominal rate dictated by weather conditions and the observatory's technical constraints. This strategy is now the baseline approach for the LSST as a whole. Focusing on static science (galaxy clustering and weak lensing), we study how these time-domain-optimized rolling strategies affect the depth uniformity at intermediate years of the survey. We characterize the amount of survey area at high risk of being lost in static-science analyses of a baseline rolling LSST dataset due to an insufficient combination of survey contiguity and uniformity. At intermediate data releases, nearly half of the survey could be lost for static science, decreasing the Dark Energy figure of merit by approximately 40\%. We describe additional metrics focused on key analysis tasks, such as photometric redshifts and galaxy clustering. We propose a new strategy that returns the survey to uniformity at key release years, enabling use of the full survey area and restoring our metrics to the values they would have in a non-rolling cadence without loss of time domain data relative to a rolling survey with the same number of rolling cycles. This work has informed the third round of optimization of the survey strategy, and the new uniform rolling strategies have been incorporated into the baseline strategy.

Uniform Rolling: An LSST Observing Cadence Offering Sufficient Survey Uniformity for Comprehensive Cosmological Analysis

Abstract

The Legacy Survey of Space and Time (LSST) that will be carried out by the NSF-DOE Vera C. Rubin Observatory promises to be the defining survey of the next decade, supplying unprecedented access to the night sky to static science- and time-domain science-focused researchers alike. Maximizing the output of the broad remit of Rubin Observatory science requires a non-trivial survey strategy. For time-domain science, the most promising strategy designed so far is a rolling survey strategy, whereby a subset of the full LSST survey area is observed at higher rate compared with the nominal rate dictated by weather conditions and the observatory's technical constraints. This strategy is now the baseline approach for the LSST as a whole. Focusing on static science (galaxy clustering and weak lensing), we study how these time-domain-optimized rolling strategies affect the depth uniformity at intermediate years of the survey. We characterize the amount of survey area at high risk of being lost in static-science analyses of a baseline rolling LSST dataset due to an insufficient combination of survey contiguity and uniformity. At intermediate data releases, nearly half of the survey could be lost for static science, decreasing the Dark Energy figure of merit by approximately 40\%. We describe additional metrics focused on key analysis tasks, such as photometric redshifts and galaxy clustering. We propose a new strategy that returns the survey to uniformity at key release years, enabling use of the full survey area and restoring our metrics to the values they would have in a non-rolling cadence without loss of time domain data relative to a rolling survey with the same number of rolling cycles. This work has informed the third round of optimization of the survey strategy, and the new uniform rolling strategies have been incorporated into the baseline strategy.

Paper Structure

This paper contains 8 sections, 1 figure.

Table of Contents

  1. Introduction
  2. Survey Strategy Optimization for the Rubin Observatory
  3. Rubin Operations Simulator
  4. The Phase 2 (LSST v3.4 - baseline) survey strategy
  5. The need for survey uniformity
  6. Rolling cadence
  7. Uniform rolling
  8. Simulated observing strategies considered

Figures (1)

  • Figure 1: Difference in $r+i+z$ exposure time from median of the LSST v3.4 - no rolling survey in kiloseconds. Edges of the WFD survey with total exposure time less than an average of 700 seconds per year, arising due to the incomplete coverage at the edge of a survey with dithers between pointings, have been cut from the plot to improve clarity. The left column shows the LSST v3.4 - baseline survey, the middle column shows the LSST v3.4 - no rolling survey, and the right column shows the LSST v3.4 - uniform rolling survey. The rows show LSST at years one, four, seven, and ten. Unlike the Phase 2 baseline (LSST v3.4 - baseline) survey where rolling stripes are still apparent, the uniform rolling survey returns to similar performance as the LSST v3.4 - no rolling survey at years four, seven, and ten. Year one is similarly uniform in all three surveys by design, since rolling does not start until after that point.