Survey Design
Constraints
Constraints on LSST's Etendue
Detailed consideration of LSST science drivers results in a requirement to obtain multi-band imaging of 20,000 sq.deg. to a depth of V=27.5 (5-sigma for unresolved sources, on either an AB or Vega based system). The primary depth drivers are the number of galaxies usable in weak lensing analysis and the ability to detect main sequence stars at 100 kpc. With the field of view area limited to 10 sq.deg. by achievable image quality, the time to complete such a survey scales with the square of the primary mirror's diameter. As illustrated in Figure 1, in order to complete the survey in 10 years, the chosen effective diameter of LSST's primary mirror is 6.7m (8.4m geometric diameter).
Constraints on Exposure Time
The total exposure time per field and for all six bandpasses is 8 hours. The weak lensing and other systematics are minimized by maximizing the number of realizations of the seeing. The minimum exposure time which maintains high survey efficiency is about 30 seconds and results in about 1000 exposures, each of which reaches a V magnitude of 24.5. At this pace, the 10,000 sq.deg. of sky visible at any given time can be tiled in two bands every three days. The total number of visits in each band after 10 years of surveying are listed in Table 1.
This combination of the depth, area and revisit time simultaneously addresses the needs of LSST's main science themes (see Figure 2). The revisit time of several days will result in well-sampled light curves for Type Ia supernovae, and will enable orbital linking of moving objects. Detection of moving objects will also benefit from short exposure time that prevents trailing losses. A per-visit depth of V=24.5 will allow LSST to fulfill the Congressional mandate to detect 90% of 140m NEOs, to detect RR Lyrae stars to 400 kpc, and to make geometric parallax measurements for a complete solar neighborhood sample down to the hydrogen-burning limit.
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Figure 1: The co-added 5-sigma depth for unresolved sources as a function of time (assuming 22% of time per band) and the effective primary mirror diameter. Compared to single visits, 3 mag of depth are gained after 10 years of surveying (using 200 visits per band). LSST will survey 20,000 sq.deg to this depth. |
Figure 2: The tradeoff between the revisit time and single-visit depth. The requirement on revisit time (whole sky in two bands every three nights to V>24.5) is equivalent to requirements for exposure time (30 sec), the number of visits (1000 in six bands) and survey efficiency. |
Source: American Astronomical Society 213th Meeting, Poster Exhibit, "LSST: from science drivers to reference design and anticipated data products", Z. Ivezic et al., 460.03