LSST survey strategy, how it has started, how it may evolve

University of Delaware

Department of Physics and Astronomy

Biden School of Public Policy and Administration

Data Science Institute

Rubin Legacy Survey of Space and Time

LSST Survey Scientist

federica b. bianco

she/her

3,362.2 sq deg w > 1 visits
2,859.5 sq deg w > 2 visits
  814.1 sq deg w > 3 visits
  782.2 sq deg w > 4 visits
  105.8 sq deg w > 5 visits

survey optimization

LSST Science Book (2009)

is a word I am borrowing from Margaret Atwood to describe the fact that the future is us.

However loathsome or loving we are, so will we be.

Whereas utopias are the stuff of dream dystopias are the stuff of nightmares, ustopias are what we create together when we are wide awake

https://www.youtube.com/watch?v=QO3nY_u6hos

US-TOPIA

LSST survey strategy optimization

a pioneering process of community-centric design

Rubin LSST survey design

https://www.lsst.org/content/charge-survey-cadence-optimization-committee-scoc

Rubin has involved the community to an unprecedented level in survey design this is a uniquely "democratic" process!

Survey Cadence Optimization Committee

IMPORTANT: This process will contine throughout LSST!

The SCOC is tasked to deliver an annual recommendation for adjustments to the survey during operations

while we have made a recommendation for how to start LSST, we intend to continue to renew recommendations as needed throughout the first year of LSST as we improve our understanding of the evolving system performance and how to adapt to it to preserve our scientific objectives.

As planned earlier, the SCOC is tasked to deliver an annual recommendation for adjustments to the survey, but in the early survey phases, we expect recommendations may happen much more rapidly.

We look forward to discuss our recommendation and the expectations for LSST at the Rubin Community Workshop 2026 on Tuesday July 2018.
To facilitate interactions, we have set up a Vevox (like Slido) where you can the SCOC questions ahead of the Rubin Community Workshop: https://vevox.app/#/m/430733856

Rubin LSST survey design

2017

80,000

Rubin LSST survey design

Rubin has involved the community to an unprecedented level in survey design this is a uniquely "democratic" process!

2019

80,000

Rubin LSST survey design

Rubin has involved the community to an unprecedented level in survey design this is a uniquely "democratic" process!

2023

80,000

Rubin LSST survey design

Rubin has involved the community to an unprecedented level in survey design this is a uniquely "democratic" process!

2024

80,000

Rubin LSST survey design

Rubin has involved the community to an unprecedented level in survey design this is a uniquely "democratic" process!

2024

2026

80,000

Rubin LSST survey design up to 2018

# pairs of observations (1e5)

time gaps (days)

Eric C. Bellm et al 2022 ApJS 258 13

Presto-Color, Bianco+ 2019

Introducing Rolling Cadence

Current plan: rolling 8 out of the 10 years

https://arxiv.org/pdf/2411.04793

Rubin ToO program

newer simulations ->

<-bad good ->

newer simulations ->

Kilonovae in LSST Wide Fast Deep

Andreoni+ 2022a

DESC

e.g. analysis of
u band exposure time
and filter balance

e.g. analysis of
u band exposure time
and filter balance

<-bad good ->

Real Observatory Performance??

PSTN-056.lsst.io January 2025

2018

2023

DESC

Challenges

newer simulations ->

<-bad good ->

Real Observatory Performance??

TDA is most sensitive to efficiency and cannot recover by just adding survey time

PSTN-056.lsst.io January 2025

DESC

newer simulations ->

<-bad good ->

10% fewer visits

TDA and 3.2pf FoM most sensitive to efficiency

TDA cannot recover by just adding survey time

PSTN-056

PSTN-057

DESC

DESC

DESC REPORT 2026

What's in v5.3

The SCOC had proposed the survey design for Y1 implemented in baseline_v4.0 and described in PSTN-056 (which itself builds on earlier SCOC recommendations Rubin’s Survey Cadence Optimization Committee et al. PSTN-053 and Rubin’s Survey Cadence Optimization Committee et al. PSTN-055) which includes the following survey strategy details:

for footprint (PSTN-056 recommendation vi and xxiii),
filter balance (i, ii, viii, ix, xx),
rolling (iii, and v), including not starting rolling in Y1 (xxi),
special surveys (DDF, xvi and xviii; ToO, x-xiv; Twilight microsurvey, xxii),
observing in single exposures rather than “snaps” (xv), and
cadence (with no recommended modifications from PSTN-055)

We now recommend the following changes, which have been implemented in baseline_v5.3:

We introduce a “template-tier” observing mode that is activated when sky conditions (atmospheric seeing) are conducive to the collection of good PSF images that can be used to build sky templates. This observing mode attempts to collect five images in each filter [...]
The template tier collects images in pairs (primarily in different filters)[...] Observing in pairs enables tracking of solar system objects even in the absence of templates, and provides colors for transients and variables observed in the images, although the color will only be measured retroactively once the templates are available [...]

DESC REPORT 2026

ALERTS HAVE STARTED!

Why not always? We deliver alerts on DDF+Virgo when we run the survey in survey mode (=not engineering tests)

Why not everywhere? Limited templates available - template foorprint increasing

~7M alerts per night

*Conservative ML-reliability scores for now because the infrastructure (Rubin and brokers) is still under development

classifications by ALeRce and Lasair

We now recommend the following changes, which have been implemented in baseline_v5.3 [cont'd]

Translational and rotational dithering between visits within a night in the Deep Drilling Fields (DDFs) is added to translational and rotational dithering between nights following the current recommendations from Data Management (DM), [...]
Finally, in PSTN-056 and baseline_v5.0 we introduce a specific DDF observing strategy (PSTN-056 recommendation xviii) dubbed “ocean” that observes DDFs in either “deep” or “ultradeep” seasons. Each field receives one ultradeep season (except COSMOS, which receives three), with long sequences of visits within a night. The deep seasons have 3 Survey Cadence Optimization Committee’s Survey Start Recommendations | PSTN-057 | Latest Revision 2026-07-17 shorter sequences (typically two or three visits in each filter). Both the deep and ultradeep seasons have high cadence, with nightly observations in alternating filters (so typically two-night cadence in any given filter), and this strategy is retained in the current recommendation.

NOTE: bug fixes after release of PSTN-057 after DESC report pointed out issues

DDFs: main drivers for observational strategy

Static Science (co-added): faint sources, photo-z calibration, etc.

reach at least 10x WFD depth (= u:520, g:630, r:1670, i:1670, z:1500, y:1480 visits)
AGN, Galaxies, DESC desire extra u-band depth (e.g., z ~ 3 dropout galaxies)
AGN want to hit specific coadded depths: (u:28.3, gri:28.5, z:27.9, y:26.5 AB mag)

Time-Domain Science: SN Ia cosmo, AGN reverb/monitoring, extragalactic transients

AGN want every night or every-other-night sampling in griz (and u/y as possible)
DESC want high nightly depth (many visits) seasons for high-redshift SN Ia (z >~ 0.7)

7% DDF budget, “best of both worlds” solution: ocean
high-cadence shallow (2 visits) observations (all years) +
individual high-cadence deep seasons (1 per field; 3 for COSMOS)

In deep seasons:

On even days: g:2 i:2 On odd days: r:2 z:2 Every few days: u:3 y:2

These sequences “expire” so won’t build up over multiple days if visits are missed, but u/y occasionally double up.

For EDFS the sequences are the same, but the cadence is slower to accommodate the double pointing.

In ultradeep seasons:

On even days: g:2 i:2 On odd days: r:2 z:2 Every few days: u:3 y:2 +

additional sequences of u:8 g:4 r:18 i:55 z:52 y:20 every ~other/few days

The Ocean DDF Strategy (brain child of the DDF task force led by Saurabh Jha)

What is under immediate consideration?

We now recommend the following changes, which have been implemented in baseline_v5.3: [cont'd]

Since depth is not the limiting factor that requires the use of > 3 exposures for templates, reduced exposure time for each image may enable the construction of templates that are sufficiently complete and deep to enable DIA processing and DIA-based science, while increasing the efficiency of construction of the incremental templates. The SCOC favorably reviews the scientific impact of collecting 20 (28) seconds exposures for 𝑔𝑟𝑖𝑧𝑦 (𝑢) template exposures in the template tier observing mode.
However, a feasibility study is pending, to ensure the shorter exposure (1) does not impact data processing negatively, (2) truly leads to a higher efficiency in delivering templates (e.g. are the image cycles correspondingly faster and the image quality as good based on AOS performance), (3) does not negatively affect system calibration. Particular attention should be paid to the image quality expectations in 𝑢 band, where 25 seconds is close to the readout limit. Retaining the standard 38 sec exposure time in 𝑢 while reducing exposure time in other filters may be the optimal solution, and we expect the feasibility study to deliver an answer. [...]

DESC REPORT 2026

Footprint Task Force to be announced at RCW!

Includes from DESC Rachel Mandelbaum, Sylvie Dagoret-Campagne, Mariano Dominguez

also Rachel Street, Kathy Vivas and... a few more TBC

DESC REPORT 2026

Rubin Live-Status links for anyone (summit skycam too!)

ls.st/prenight what the lsst might look like tonight (24 hour ahead)

Rubin Schedule Viewer what the lsst might do next (currently only 24 hour ahead, but will hold more reliable nextVisit + next hour)

Nightly Digest what happened at the summit, including night report

ls.st/nightsum what visits were acquired during the night, survey context

survey-strategy.lsst.io/progress periodic survey progress

Times Square - many notebooks

The LSST

Science Collaborations

A community of practice funded on principles of Equity, Inclusivity, Cooperation

thank you!

University of Delaware

Department of Physics and Astronomy

Biden School of Public Policy and Administration

Data Science Institute

federica bianco

fbianco@udel.edu

https://slides.com/federicabianco/desc-2026

credit: AMNH Dark Universe

678 separate images taken in just over seven hours of observing time. Trifid nebula (top right) and the Lagoon nebula, which are several thousand light-years away from Earth. | NSF-DOE Vera C. Rubin Observatory