Weeks 16-17 of the on-sky commissioning campaign have emphasized efforts to improve the consistency of the delivered image quality for survey-mode observations. Following an extended period of weather-related closed dome activities, with only a single night of observations since 24 July, on-sky testing resumed on 6 August. However, the next three nights were also substantially impacted by winter weather conditions. More winter weather is in the forecast for next week.
Activities of the past two weeks:
- Continued characterization of the Active Optics System (AOS) has been the highest priority on-sky activity. AOS testing included further measurement and validation of the open loop Look-up Tables (LUT) for elevation, rotator angle, and filter focus offset, as well as sensitivity matrix tests to evaluate the response of the optical system to a selection of applied bending modes.
- Progress on the Science Validation (SV) surveys has been substantially slower than initially planned. Only ~650 science program visits have been acquired from 1-13 August, and fewer than 1000 science program visits have been acquired during the 3-week period since 24 July.
- In dome testing included acquiring monochromatic flats for the y band, CBP crosstalk scans for non-linear crosstalk, and a dense photon-transfer curve (PTC).
- Primary-tertiary mirror (M1M3) thermal control testing continued with further calibration of the temperature sensors and long-duration thermal equilibration tests.
- Filter swaps occurred on 7 August (u → y) and 12 August (y → g). The currently loaded filter set is griz + pinhole. The filter socket holding the pinhole socket is currently not operational.
On 10 August, Rubin Observatory deployed an updated Feature Based Scheduler (FBS) configuration for the SV surveys that modifies the survey strategy for the remaining ~6 weeks of on-sky commissioning prior to the pre-Operations engineering and maintenance period that is currently forecast for mid-September through mid-October.
There are two changes to the FBS configuration:
- For the Deep component, observations of the LSST Deep Drilling Fields (DDFs) will include intra-night translational dithers to mitigate impacts of current stray and scattered light within the finite period of the SV surveys
- The Wide component will use a reduced area footprint to concentrate remaining visits within a ~750 deg^2 region. An objective is to acquire a sufficient number of overlapping good quality visits in the next weeks to build templates and test Prompt Processing with difference imaging in a wide-area survey mode that emulates the LSST Wide-Fast-Deep survey.
In addition, the current expectation is that only the griz filters will be used for the remainder of the SV surveys due to constraints on filter availability.
These decisions are a response to multiple risks being realized during the past month:
- Winter weather conditions, the current sensitivity of the LSSTCam cryogenic refrigeration system to low ambient temperatures, and other operational issues are limiting the amount of time for on-sky testing
- The distribution of delivered image quality during the first month of SV surveys is not yet consistently meeting the LSST design specifications; Rubin Observatory will prioritize on-sky engineering during the remaining on-sky commissioning period
- Four of the five LSSTCam filter sockets are available during the next several weeks, limiting the number filters that can be used (or otherwise requiring more frequent daytime filter swaps to continue testing in all six filters)
- Rubin Observatory will prioritize daytime engineering tasks prior to the pre-Operations engineering and maintenance period
The adjustments to the SV surveys are intended to facilitate the ongoing commissioning of the LSST Science Pipelines and system verification as necessary to show construction completeness. We anticipate that the revised survey strategy will still provide a scientifically useful dataset, and are working to preserve as much early science as possible, even though the updated SV surveys cover a smaller area and with fewer bands than initially planned.
Now that these decisions have been made, Rubin Observatory will resume regular updates on the SV survey progress. Nightly observing reports will continue to be available.
The primary purpose of the on-sky commissioning period is to support the completion of the Construction Project. Rubin Observatory recognizes that changes to the SV surveys could impact the Early Science Program since several components were planned to utilize data from the SV surveys. Rubin Observatory Operations is following progress and will make a decision regarding the scope and timeline of Data Preview 2 and the start of LSST in time for resuming on-sky observations in late October following the pre-Operations engineering and maintenance period.