Combined HST and JWST Operations

D. Taylor (dtaylor[at], F. Abney (abney[at], A. Roman (aroman[at], R. Shaw (shaw[at], and W. Workman (workman[at]

The good news is that Science Operations has been supporting HST at STScI for over 30 years, contributing significantly over the decades to HST's remarkable success. The really good news is that we will be doing similar (but subtly different) support functions for JWST soon after launch, currently scheduled for October 2021.  Here we describe the similarities and differences between HST and JWST Science Operations, focusing on the planning and preparation of observations, the scheduling of those observations on the telescope, the processing and archiving of science and engineering data, and supporting data retrieval from MAST. The lifecycle of an observing proposal is illustrated in Figure 1.

steps for the life of a proposal are shown graphically in multiple colors
Figure 1: The Life Cycle of a Hubble/Webb proposal, from conception, planning, scheduling, and execution on the spacecraft, through data processing, archiving, delivery to users, and the ingestion of community-contributed science products to MAST.​​​​​
Step 1: Selection
  • Issue Call for Proposals
  • Investigators submit proposals
  • Conduct peer review and program selection
Step 2: Implementation
  • Prepare observations
  • Build long-range plan
  • Schedule observations
Step 3: Archive
  • Receive data from MOC
  • Perform science data processing
  • Archive data products and notify proposer
Step 4: Delivery
  • User queries MAST for science data
  • MAST verifies user authorization and provides data
  • MAST accepts community-created high-level science products


The Program Implementation Team implements and assists in the planning of observing programs. Each team member can implement programs for either mission in their role as Program Coordinators (PCs). The PCs serve as the primary points of contact between STScI and investigators, helping them create their observations with the Astronomer's Proposal Tool, while ensuring that the specified observations are technically feasible and will execute as expected.

PCs implement programs for the full range of HST observing categories. For JWST, the PCs are currently working on Guaranteed Time Observer programs, Commissioning programs, the Director’s Discretionary Early Release Science programs, and programs used in rehearsals and testing exercises. General Observer programs for JWST will be assigned to the PCs in 2021 after they are approved. For both observatories, the PCs verify that each program contains only the targets, instrument configurations, scheduling constraints, and total observing times that were approved. The PCs then resolve any scheduling issues with the observations that may exist at the time of submittal, typically due to timing constraints. Additionally, PCs work with the Long-Range Planners and Short-Term Schedulers in the Science Planning and Scheduling Team (SPST) to resolve scheduling conflicts between programs, conferring with investigators when program changes are needed. 

The SPST uses observatory operating characteristics and constraints to create a year-plus observing plan and short-term schedules that optimize the use of JWST and HST. Spacecraft and science operations tasks for these missions are similar at a high level, but important differences do exist. While both observatories use reaction wheels to effect spacecraft motion (to slew from one target to the next), managing that motion in the low-earth orbit of HST is much different than at the L-2 orbit of JWST. HST uses the Earth's magnetic field to provide torque to manage momentum, while JWST must periodically use on-board thrusters. Thrusters are also used to counteract the solar wind, which imparts a torque on JWST because of its large solar shade, resulting in the telescope deviating from the desired path. HST does not have this issue, but it does suffer orbital drag from the earth's atmosphere, which causes its orbit to decay. The earth occults HST observing for most targets for ~40–50 minutes of each orbit, and HST observing must periodically halt during South Atlantic Anomaly passages to avoid excessive radiation interfering with the exposures. JWST does not have these constraints, but its observing Field of Regard is constrained by its solar shield to an annulus about the Sun/Earth/L2 line (see Fig. 2). All of these constraints are handled by SPST through the use of Proposal Planning Subsystem software, which helps the Team plan and schedule every science observation to maximize science return while maintaining observatory health and safety.

JWST sunshield limits in field of regard
Figure 2: Depiction of the constraints on JWST attitude (left) and the Field of Regard (right) imposed by the sunshield and spacecraft configuration. See this video for more details.

After observations have executed on-board JWST, the science and engineering data are downlinked and processed through the Flight Operations System (FOS), then sent on to the Data Processing and Archive Services Team. Using the Science Data Processing Subsystem of the Data Management Subsystem, the Team receives JWST data, processes the science telemetry into a standard astronomical format, calibrates the science data, and submits the data to the Archive. The Team can also reprocess science data when improved calibrations become available. Very similar processing is performed for HST data, but for JWST the process is much more tightly integrated into the overall Ground System, since the FOS is part of the Science & Operations Center.

The MAST Archive is the end-point for the flow of science observations. It relies on three shared systems to operate: the archive catalog, which stores all the information about the data holdings; the authentication and authorization system, which controls who has access to which data in the archive; and the storage broker which manages all of the file storage within the archive. The Archive's ingest services work to store, catalog, and apply user access settings for new datasets entering the archive. Ingest also places appropriate data on the online cache for direct download, and creates a permanent off-site safe-store backup.  An observer may use one of the MAST interfaces (such as the Portal, or increasingly with home-grown scripts using the python astroquery library) to query the catalog for data from JWST or HST (or both), verify that they have access to the data they select, and then download.

NGC 7023 with observations overlaid
Figure 3: The footprints of existing HST images of the nebula NGC 7023 in narrow-band optical (cyan wire frames) and near-IR (yellow) passbands, as well as those planned in a JWST GTO program (red), superimposed on the Digital Sky Survey.

In reality, MAST is both the end and the beginning of the flow of science data: the end because MAST is the permanent repository of data from many missions, and the beginning because the Portal can help investigators craft new science programs by displaying, for a given target, existing or planned observations from any hosted mission (see Fig. 3). Finally, MAST is also the repository of high-level science data products (HLSPs) that are created by the community from one or more MAST missions. HLSPs are among the most valuable and highly cited data collections among all astronomical archives.

The Science Operations staff, along with many others, have been planning for JWST operations for some time now. The similarities between the HST and JWST observatory operations have been leveraged, and new features continue to be tested and rehearsed in anticipation of the JWST launch in 2021.