1. The LSST Scheduler from design to construction
      1. 1. INTRODUCTION
      6. 6. DESIGN
      10. 10. SUMMARY

The LSST Scheduler from design to construction
Francisco Delgado*
, Michael A. Reuter
LSST, 950 N Cherry Ave, Tucson, AZ USA
The Large Synoptic Survey Telescope (LSST) will be a highly robotic facility, demanding a very high efficiency during
its operation. To achieve this, the LSST Scheduler has been envisioned as an autonomous software component of the
Observatory Control System (OCS), that selects the sequence of targets in real time. The Scheduler will drive the survey
using optimization of a dynamic cost function of more than 200 parameters. Multiple science programs produce
thousands of candidate targets for each observation, and multiple telemetry measurements are received to evaluate the
external and the internal conditions of the observatory. The design of the LSST Scheduler started early in the project
supported by Model Based Systems Engineering, detailed prototyping and scientific validation of the survey capabilities
required. In order to build such a critical component, an agile development path in incremental releases is presented,
integrated to the development plan of the Operations Simulator (OpSim) to allow constant testing, integration and
validation in a simulated OCS environment. The final product is a Scheduler that is also capable of running 2000 times
faster than real time in simulation mode for survey studies and scientific validation during commissioning and
LSST, OCS, survey, scheduler
The Large Synoptic Survey Telescope (LSST)
is controlled by a hierarchical architecture. The control system is
organized in subsystems, and the high control entity that orchestrates these subsystems during operations is the
Observatory Control System (OCS)
. The Scheduler is the OCS component that produces the targets for the
observatory, implementing in fact the survey. Due to the variety of science goals and the operational constraints of the
LSST observatory, the Scheduler was envisioned as a fully automatic and dynamic component, that according to the
current conditions of the observatory and the environment, the past history of observations, and the multiple goals from
the science programs defined in the survey, determines the next target at real time.
The need for such a Scheduler for the scientific success of the LSST was identified early on the project. In addition, the
scale of the survey and the fact that the mission is about multiple sciences simultaneously, made the Scheduler a high
risk component. In order to mitigate this risk, prototypes of the Scheduler were developed since the beginning of the
R&D phase in the project, working in a highly detailed simulation environment. The result is a detailed design for the
, and a construction plan as a deliverable from the Telescope and Site team
, that incorporates the
simulation aspect for constant validation in coordination with Systems Engineering and the Simulations team.
Since early in the project, it became clear that current scheduling approaches in other astronomy observatories were not
designed to deal with the operational concepts of the LSST and its philosophy of a single survey that serves many
science cases. Space-based telescopes have elaborate schedulers, designed to execute detailed observing blocks, more
constrained in pointing but free from unforeseen changes in external conditions, and where interruptions are handled by
operators. Unlike for space-based telescopes, observing efficiency for a given target in ground-based facilities varies
greatly with time due to atmospheric absorption. The presence of weather, the short exposures in the LSST with the

frequent re-pointings and the sheer number of possible targets require that a scheduler must be able to re-plan
autonomously and constantly while keeping the balance in the goals of all the science programs.
In order to explore this concept, a tool was created, the Operations Simulator (OpSim). Version 1 (OpSim1), written in
IDL, consisted of a set of rules and priorities representing the LSST mission, and tested the ability of automatically
respond to unscheduled interruptions. The algorithm was based on the computation of the demand in time for each target
position on sky based on the requested observations by multiple science cases, and execute the demand depending on the
telescope, the constraints and the environment. The result is a simulation of the visited positions with their conditions
like airmass, seeing, etc., which was later used to evaluate the level of compliance with each science case.
OpSim version version 2 (OpSim2)
elaborated this concept in more detail and accuracy. It was written in Python and
the models for the observatory and the environment were more comprehensive and fully configurable. The science cases
were programmed into sophisticated science proposals evaluating thousands of targets against dozens of constraints and
ranking rules competing simultaneously for the observation time of the simulated telescope. A target is defined as a field
filter combination, and a visit is defined as a sequence of consecutive exposures for a single target, being a visit the
observation unit of the survey. The current baseline specifies a visit as 2 exposures of 15 seconds. A database with the
observation history captured all the simulated details for each visit, allowing the development of analysis tools to
evaluate the survey performance and to estimate the capabilities of the observatory as designed. The major tool for this
version was Simulation Survey Tools for Analysis and Reporting (SSTAR), which served assisting in the analysis of
hundreds of simulations. All these runs with OpSim2 and SSTAR were key in proving that a design of a field of view of
3.5 degrees was needed to achieve the mission of the survey in 10 years. Also, it helped shaping the set of science
proposals to achieve the high level goals of the Science Requirements Document (SRD)
, and validating the telescope
and dome specifications in terms of constraints and slew time. Simulations for 3 different candidate sites also supported
the decision of selecting Cerro Pachón as the LSST site.
OpSim version 3 (OpSim3)
was a step forward, exploring more scheduling strategies and adding more fidelity in the
telescope constraints, filter changer rate in the camera, the scheduled and unscheduled downtime, and more variations in
the science proposals
. The scheduler components were better separated from the simulation components. The analysis
tool was also upgraded, consisting now of the Metrics Analysis Framework (MAF)
, provided the needed extended
automation for parameter space exploration. The scheduling algorithms in OpSim3 also experimented with limited look-
ahead capabilities, which demonstrated the potential and the challenges of using deterministic future knowledge about
the target conditions to improve the survey efficiency.
All this effort gave the knowledge and the experience needed by the project to produce a solid design for the Scheduler
and an organization between the teams to face the challenge of building a critical component of the control system of the
observatory, that due to its combined set of goals, constraints and scale of the problem space, was a high risk element
which was dramatically mitigated.
Another clear conclusion from the prototyping and simulation campaign is the need for an OpSim version during
operations, in order to constantly assess the survey progress and to evaluate potential changes in priorities, parameters,
system constraints, or even new science cases. In order to achieve a valid simulation of the Scheduler in operations, the
operational version of OpSim must use the same scheduling algorithms. The number of factors affecting the final
schedule of observations is so high, that the challenge of producing faithful simulations sharing only the algorithms or
even libraries of code seems risky. On the other hand, developing a Scheduler exclusively in the OCS control
environment would not exercise all the possibilities and the scientific validation before commissioning would be limited.
The decision was then to develop the OCS Scheduler in a framework that allows to run it directly in a simulation
environment with the same code and the same interface used in the OCS control environment. To achieve this, the
relevant functionality and telemetry from the OCS and the observatory subsystems have to be simulated in enough detail
to reproduce the inputs needed by the Scheduler. This simulator is called Simulated Observatory Control System
. The combination of the Scheduler and the SOCS is then envisioned as OpSim version 4 (OpSim4).

Figure 1. The Scheduler in the Observatory Control System context.
Figure 2. The Scheduler in the Operations Simulator context.
The approach of a single Scheduler implementation capable of running in 2 different contexts imposes some additional
requirements. For the Scheduler, a complex set of algorithms and elaborated models previously planned to run in real
time, now has to be capable with the same code implementation to run at faster simulations speed. The reference real
time speed is the computation of the next visit during the current visit time, corresponding to a cycle of 1 visit in 39
seconds. However, other controllers in the observatory need a previous notification about the next visit or potential
sequence of visits, which led to the requirement of computing a full night of visits during a visit time. This is an average
of 700 visits in 39 seconds, corresponding to a cycle of 18 visits computed per second, 700 times faster than real time.
Current full survey simulations with OpSim 3 typically takes 40 hours, which is equivalent to this speed, but the goal for
the simulations in the future OpSim4 era is to achieve 2000 times faster than real time, simulating the full survey in less
than 14 hours allowing a turnaround of simulation and automatic report of 24 hours. This speed imposes on the
Scheduler the requirement of producing 50 visits per second interacting with SOCS, or 2000 times faster than real time.
For SOCS there is also an important implication of this strategy: the interface implementation. In OpSim3, the
simulation code and models were in direct communications with the scheduling algorithms in one single python process.
Now, the Scheduler is a standalone component in the OCS
, with a publish/subscribe Data Distribution Service
interface. SOCS must use this same interface in order to operate with the Scheduler. As simulations have to
operate at high speeds, initial benchmarks were performed with the interface to validate the feasibility of the
architecture, achieving cycles of 5000 visits per second in term of messages being exchanged between the initial
versions of SOCS and the Scheduler with no targets being processed nor telemetry being simulated, just exercising the
interface between both processes in the same computer, replicating the use case of OpSim4. This result demonstrated
that the DDS interface will not be a bottleneck for fast simulations.

Another decision related to this approach is the continuation of the Python language in the coding of the Scheduler.
While this software is closer to the control architecture, the uniformity with SOCS and the common models from the
simulations team made Python the chosen language. This is also supported by the OCS team where a version of the DDS
interface is also available for Python.
The Scheduler in the OCS context obeys all the rules of the OCS components. The interface between the OCS and the
other subsystems, Telescope Control System (TCS), Camera Control System (CCS) and Data Management Control
System (DMCS) is based on the publish/subscribe Data Distribution Service (DDS). The OCS team develops a Software
Abstraction Layer (SAL) that builds some additional services on top of DDS to make it available to the other
subsystems. SAL is available in C++, Java, LabVIEW and Python. The OCS uses SAL to communicate its components
internally as well, defining in this way the implementation of the Scheduler interface. With the Scheduler written in
Python, SAL/Python is the flavor utilized.
Figure 3. The Scheduler data flow in the Observatory Control System context.
The figure illustrates the context of the Scheduler as part of the OCS. The OCS Application controls the operational
modes, the Engineering and Facility Database (EFD) provides the history of previous observations during start-up, TCS
and CCS provide the telemetry for the current observatory conditions, TCS also provides telemetry about the
environmental conditions, and DMCS provides the feedback with measured image properties from past observations. All
this information is computed by the scheduling algorithms to produce the targets, main output from the Scheduler. the
Sequencer executes those targets, and the actual observations are then notified back to the Scheduler to register the visits
and adjust the priorities for the upcoming targets. Scheduler telemetry is also produced, with information regarding the
decisions processes for monitoring the algorithms.
Below are the descriptions for the inputs and outputs of the Scheduler in this control context.

4.1 Control
This input comes from OCS Application, and includes all the commands that control the operational mode of the
Scheduler, the configuration, and the handling of the downtime periods and degraded modes.
The configuration commands control the system parameters, scheduling parameters and the definition of the proposals to
execute during the survey. Some of these can be changed dynamically at any time, others only during daytime, and
others require a Scheduler restart.
4.2 History
During operations the Scheduler stores an annotated history of observations internally. However, in the case of a restart
with a partial survey in progress, for example after introducing significant changes in the science proposals, a
warm start
function is activated in which the Scheduler queries the OCS EFD to obtain the full history of previous observations and
with this list rebuild this internal annotated history which is fundamental to evaluate the partial progress of the science
cases being pursued. This input represent the EFD feeding the history of the survey in a restart of the Scheduler.
4.3 Telemetry
The telemetry input includes internal and the external conditions of the observatory. The internal conditions are the
positions and status of the telescope mount, rotator, dome, filter, used to evaluate the cost in time to new targets. The
external conditions include but are not limited to the seeing, all sky transparency, measured sky background brightness,
used to filter out targets that are outside the constraints defined.
4.4 Image Quality
The image quality feedback is a measure of seeing, sky brightness, m5 (the 5-sigma detection limit for point sources)
among other parameters, coming from the image as taken and computed by some local component of Data Management.
It is used to assess past observations and eventually re-evaluate the progress of the targets to modify the priorities of the
re visits.
4.5 Visits
This is the control feedback of what is actually happening in the observatory regarding the observations. The Scheduler
proposes targets constantly, but it's the OCS Sequencer who commands the observations; the actual visits performed by
the observatory with the as taken telemetry and conditions is notified back to the Scheduler through this input in order to
register the survey progress and to update the priorities for the future targets.
4.6 Targets
This is the main output from the Scheduler, and correspond to the sequence of targets, being updated constantly as
observations go. The first target in this list is the next target to acquire, and this information is available one full visit
ahead of time to give the Camera and Data Management enough time to prepare for that acquisition. The following
targets are a prediction, but if conditions change to Scheduler re-evaluates this list.
4.7 Scheduler Telemetry
This is still an undefined output, and it is a placeholder to publish internal information that will help understanding the
performance of the scheduling algorithms, like the alternate targets that were not selected and eventual intermediate
ranking calculations.

In the context of the simulation environment, as part of OpSim4, the Scheduler works in exactly the same way with the
same OCS Middleware interface. The design is such that the time signal controlling the status of the internal models and
the day and night cycles comes from the telemetry and control signals. In this way, SOCS can feed an accelerated time
signal altogether with the simulated telemetry and the Scheduler will operate in the accelerated simulation use case.
Figure 4. The Scheduler data flow in the Simulated Observatory Control System context.
In the figure 4, SOCS provides all the needed inputs to the Scheduler with simulated data and reacts with the observatory
behavior to simulate operations and produce a survey database for further analysis.
The design for the Scheduler summarizes all the lessons learned during the prototyping and simulations phase of the
project. It also complies with the standard interface framework for the OCS, and addresses with a modular design the
speed requirements for the simulations use case. All the components are initially deployed inside a single process, with a
design that allows for further parallelization of time consuming tasks. The architecture also makes possible to separate
components of the Scheduler in different processes and utilize DDS as a messaging mechanism between the processes.

Figure 5. The Internal Block Diagram for the Scheduler with basic data flows.
6.1 Main
The component
is first of all the entry point of the Scheduler code. It instantiates and initializes the framework
including the logger and the SAL, and starts the
with default configuration. Secondly,
is the
implementation of the interface, including the command processor and the state machine that follows the standard
defined for every OCS component. Every DDS message received or transmitted by the Scheduler is handled here, where
the translation between the DDS topics and the internal data structures is performed. Lastly, this component controls the
operation mode of the Scheduler, according to the control commands received from the
OCS Application
. Also, new
configuration can also be received, and depending on the operation mode this new configuration is transferred to the
6.2 Driver
is the component that actually conducts the survey. At start-up, it instantiates and configures all of the
remaining components. It keeps track of the night and day cycles triggering the corresponding actions, maintains the
state of
Observatory Model
aligned to the telemetry received, and orchestrates the
to obtain the targets and
rank their priorities. The cost functions are implemented in this component, where information about the targets' values
coming from the
is combined to the predicted slew time from the
Observatory Model
to build the
configurable ranking function. Currently, the only cost considered is the slew time, but nothing prevents the inclusion of
other considerations given a suitable model for those.
also implements the high level behavior of the Scheduler, everything that goes beyond the individual
and that requires coordination with other components, such as the filter swapping during new moon. In order
to implement this feature, the
follows the moon phase and when the configurable dark period is reached, it
evaluates the relative progress per filter across the
, and decides which of the unmountable filters will be
swapped from the camera for the
filter during new moon. This desired operation is then communicated to the

at day time and executed by the operations crew. At the end of the dark period the reverse operation is
requested by the Scheduler.
Dynamic configuration is also possible when new parameters are transferred from
(after receiving a configuration
command at the interface). The actions performed by the
to apply new parameters could be as simple as to update
scheduling parameters, or more elaborately, like restarting the
Observatory Model
with new kinematic parameters, or
starting a new
6.3 Proposals
block in the diagram is the representation of the multiple instances of science cases configured in the
Scheduler. There is a generic parent class called Proposal, with 3 specializations for Scripting, Area Distribution and
Time Distribution. Any number of proposals can be running simultaneously in the Scheduler, with no communications
between them. The
is the one in charge of preparing the common input data for them and handling their outputs.
Each Proposal is in charge of suggesting targets for the next visit. It receives the internal observatory state, the external
environmental conditions, and after analyzing the previous history of observations delivers a valued list of targets. This
process is repeated for each visit, providing the required dynamic nature of the Scheduler, adapting the list of targets to
the variant external conditions and to the result of the competition among the multiple Proposals.
A Proposal implements a set of constraints to select the candidate targets at any given moment, and a value function to
prioritize these targets. The specific value function depends on the class of Proposal, and the constraints are particular to
each instance. Each instance is configured to implement a specific science case, and the most important constraints to
select the targets are listed below:
Sky region
Airmass limit
Hour angle limits
Sky brightness limits per filter
Sky transparency
Seeing limit
Below are the descriptions of the three Proposal specializations.
Scripting proposal
This class of Proposal is configured with a fixed sequence targets that are delivered one by one in the pre-configured
Area Distribution proposal
The Area Distribution Proposal implements an even distribution of visits in a defined sky-region. The main parameter is
the number of goal visits per filter for all the fields in the region, and the value function depends on the relative progress
of each target to the overall advancement in the proposal. This simple approach proves to be effective in achieving the
desired distribution, however the time domain aspect is strongly dependant on the conditions and targets availability,
because it is not enforced in the value functions.
A variant of this proposal is going to be created in the future, when the look-ahead capability will be incorporated. In this
look-ahead variation, the value function will consider the future availability of that field-filter given a defined rolling
time window. The expected improvements are better efficiency in the area coverage and an improved distribution of
observations during the survey time for these proposals.

Time Distribution Proposal
The Time Distribution Proposal implements a sequence of observations for each field in the defined sky-region. The
time intervals between these observations are enforced with value functions following an algorithm based on time
windows. This is the most complex class of proposal, due to the many variations for the sequence of observations:
A sequence can be composed of multiple subsequences ranking simultaneously different filters at different
A sequence can include a nested subsequence, where the main sequence controls the intervals to restart the
A sequence can be a special deep-drilling case, in which instead of scheduling simple visits for a field, a series
of back-to-back visits in different filters is scheduled.
This set of algorithms has proven to be very effective in obtaining the desired time distribution for the visits in this type
of proposal. The internal competition of targets in the proposal with time windows at different stages achieves a high
level of completion.
A variant of this proposal with look-ahead capabilities will be incorporated in the future, where the value functions to
organize the competing sequences will consider the availability of the targets in the following intervals. The expected
improvement is a higher efficiency in the completion of sequences by avoiding interruptions due to unavailable targets in
the look-ahead window.
6.4 Observatory Model
The Observatory Model is a kinematic simulation of all the moving axes and actions involved in the positioning of the
observatory from one target to the next. It is a sophisticated and fully configurable simulation, which is also utilized in
SOCS to actually produce the simulated telemetry during the observations. In the Scheduler context, the Observatory
Model is used to estimate the slew delay time from the current position to each candidate target to evaluate the cost and
produce the ranking function.
The simulated elements are:
Mount Azimuth position with cable wrap
Mount Altitude position
Rotator position
Dome Azimuth position
Dome Altitude shutter position
Active Optics corrections delay
Camera filter change time
Camera filter change rates limit
Camera readout time
The axes are simulated with a second order kinematic model, constant acceleration and deceleration, in order to make
the code fast to evaluate the millions of candidate slews in a night. The code is modular enough to expand it to a jitter
limited model for increased fidelity if needed.
All the kinematic parameters, limits and times are configurable, including the definition of the actions that can be
performed simultaneously and the ones that require a sequence.

6.5 Sky Model
The Sky Model component incorporates the sky brightness model by Yoachim et al.
, and provides the background
surface brightness for the fields in every filter for the given time. This model can't be replaced by eventually available
telemetry, due to the pre-calculations needed by the scheduling algorithms in the Proposals, plus the look-ahead
capabilities that require deterministic information in the rolling time window such as the expected sky brightness for a
target at a specific point in the future.
This component also provides the position of the Sun for handling the night/day cycles and twilight, the Moon phase to
activate the dark period observation with different filters and the Moon position to determine targets too close to the
Moon to observe.
6.6 Pre-Calc Data
The pre-calculated data component is the container of deterministic information about the targets in a rolling look-ahead
window. It is implemented by a SQLite database and in memory data structures. It fulfills 4 important objectives:
Storage of non-variant data across the survey, for example, the field coordinates table.
Storage and access of common data to all the Proposals, to avoid duplication of effort and improve efficiency.
For example, current alt-az coordinates for fields and sky brightness.
Storage and access of look-ahead data. Deterministic attributes of the known targets during a configurable
rolling time window that can be used by the look-ahead features in the Proposals.
For survey simulations, storage of pre-calculated data that does not change between simulations, to speed up the
6.7 Observation History
The Observation History is the component that stores the record of the visits performed by the observatory, annotated
with the relevant information known specifically by the Scheduler and its science Proposals in particular, such as which
proposal requested each observation, what is the sequence number if the visit is part of a Time Distribution Proposal,
pairs counting, deep-drilling identification, etc.
The data in this component is built as the survey progresses, but it can also be built from scratch in the case of a warm-
start, where the Scheduler is restarted with a new set of science Proposals and the history of previous observations is
queried from the OCS Engineering & Facility Database, and that sequence of past visits is re-run against the Proposals to
account for eventual partial progress.
Another important feature in this component is the ability to receive image quality feedback from DM about a past
observation. The parameters sent by DM about a particular image are stored in the metadata of the Observation History
for the corresponding visit, and the corresponding science Proposal is notified in order to re-evaluate that target and
eventually adjust its value for future observations.
The construction plan for the Scheduler was laid out considering the opportunities that the project produced after the
design and development phase. First, a very detailed set of requirements for the Scheduler, with a well understood set of
features and mechanisms to implement the survey utilizing the designed observatory. Second, the existence of a
prototype that reduced the risk in terms of evaluating the feasibility of the challenge and the assessment of the
development cost. Last, the existence of a consolidated simulations team in Systems Engineering, with a construction
plan for the Operations Simulator version 4, that will constantly integrate, test and validate the Scheduler in a Simulated
OCS environment (SOCS).

The plan consists of 12 incremental releases, in coordination with an equivalent 12 incremental releases of SOCS. Each
new release of the Scheduler will incorporate a pre-established set of new features that will be evaluated using SOCS.
There are 4 types of evaluations for each release. First, the Scheduler code includes unit tests, which validates the
behavior of the code in the isolated Scheduler context. Second, the integration test is when the Scheduler is utilized by
SOCS to simulate survey operations, which validates that the code is performing as expected, implementing the desired
functionality. Third, the scientific validation is when the Scheduler is exercised by the simulations team to analyze in
detail the survey performance of the new algorithms. Last, each release is the integration to the OCS in the control
The releases for the Scheduler in the plan are as follows:
Scheduler Release v0.1
Code Framework
DDS-SAL/Python interface
Scheduler Release v0.2
Time handling
Observatory Model
Scripted Targets Proposals
Scheduler Release v0.3
Sky Model
Survey Driver
Area Distribution Proposals
Scheduler Release v1.0
Filter swaps
Time Distribution Proposals
Scheduler Release v1.1
Look ahead for area distribution
Scheduler Release v1.2
Look ahead for time distribution
Scheduler Release v1.3
Performance enhancements
Scheduler Release v1.4
Warm start
Image quality feedback
Scheduler Release v1.5
Spatial weather telemetry
Scheduler Release v2.0
Predicted schedule

Scheduler Release v2.1
Weather forecast handling
Scheduler Release v2.2
Additional scheduling optimizations
Figure 6. The Scheduler construction timeline.
The construction plan is on track, with the release v0.3 successfully tested. Version v1.0 is currently under development,
and it will constitute an important milestone because it reproduces the features known in OpSim3. It will be validated by
the simulations team to evaluate the performance in the baseline survey, and compared against the figures obtained in
previous simulations. Version 2.0 will be available for the commissioning camera (ComCam) first light with a sufficient
float of 8 months in the current plan, however version 1.5 will be good enough for conducting commissioning surveys.
Version 2.2 with the complete set of features and optimizations in the current plan is expected to be released one year
before the full system first light.
The analysis for the Scheduler design was done in SysML, Enterprise Architect, combining high level requirements flow
down, behavior description, structure elements allocation and interfaces identification. The Systems Engineering model
for the Scheduler is part of the integrated SysML model for the LSST
, and regular updates are applied to this model as
new information becomes available.
The detailed construction plan is described in JIRA, keeping track of individual tasks that mark the way to each release.
The JIRA plan, releases, milestones and tasks are used periodically to report progress to the Project Management Control
System (PMCS) and to organize the work of the development team.

The Scheduler code is kept in a Stash Git repository, following coding standards from Telescope & Site Software and
Systems Engineering Simulations about templates, interfaces, code and unit tests. Continuous Integration (CI) is also
implemented with Jenkins.
There is version control over the requirements document, high level OCS requirements and system level requirements
documents are generated from the SysML models in Enterprise Architect, stored in Docushare. Also version controlled
are the Scheduler code, the SOCS code, the simulation libraries, the system and survey parameters, and the baseline
simulated 10 years survey. The simulation parameters for the current production in OpSim3 are stored in GitHub
repository, and plans are in place to set a system for the soon to come OpSim4 simulations with the real Scheduler.
The construction plan is based in the knowledge about the science requirements and the system design available at that
moment. In the probable case of change in requirements coming from the science or the engineering, the project has a
formal system to handle those. A change request can be submitted to the Change Control Board (CCB), and in the
Systems Engineering team the request is analyzed and actions can be triggered to evaluate the modification and its
impact in the science, the budget or the delivery dates. Simulations can be requested to OpSim to support the studies, and
if accepted the documentation and the plans will be updated, closing the loop in the JIRA projects and the PMCS.
Integration and testing in the simulations environment is planned for each of the 12 releases with SOCS. These tests are
focused on the expected behavior of the units in the Scheduler, and analyzing the simulated survey to validate that the
algorithms are producing the expected results in terms of achieving the configured constraints and time domain
distribution of the observations.
From version 1.0, which reproduces the capabilities explored in the prototype contained in OpSim3, scientific validation
will be also performed. MAF and other analysis and reporting tools will be updated to assess that the generated survey is
complying with the scientific goals of the LSST, and also that the parameter space can cover a wide variety of science
cases to test the flexibility of the design in the Scheduler.
Figure 7. The Scheduler in the OpSim4 context for technical tests and scientific validation.
This analysis not only covers the validation of the scheduling algorithms and the impact on the science, it also is a
powerful process to explore survey variations and parameters optimizations. One important deliverable of this process is
the reference simulated survey
, which is version controlled by Systems Engineering. This reference survey is updated
when important changes are applied to the science proposals formulation or the system design.

In this paper we presented the history of the Scheduler design since its initial prototype in simulations and the evolution
towards a modular design capable of producing the targets for the observatory as part of the Observatory Control
System, and also capable of producing full simulated surveys as part of the Operations Simulator. The design was
exposed in both contexts, and the plan to produce the Scheduler have been laid out showing the coordination with the
development plan for the Simulated OCS with regards to continuous integration, test and validation. The projected result
is a software deliverable that will be capable of producing the survey during operations, produce simulations for
studying alternate surveys and assess potential updates to the parameters of the science cases and the systems for
constant optimization.
Financial support for LSST comes from the National Science Foundation (NSF) through Cooperative Agreement No.
1258333, the Department of Energy (DOE) Office of Science under Contract No. DE-AC02-76SF00515, and private
funding raised by the LSST Corporation. The NSF-funded LSST Project Office for construction was established as an
operating center under management of the Association of Universities for Research in Astronomy (AURA). The DOE-
funded effort to build the LSST camera is managed by the SLAC National Accelerator Laboratory (SLAC).
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