The 23-week on-sky commissioning campaign with LSSTCam concluded on the night 21 September 2025. More than 80K exposures were acquired with LSSTCam since 15 April, including ~20K flat, ~9K bias, and ~5K dark exposures for in-dome calibrations, ~22K on-sky engineering images primarily for Active Optics System (AOS) commissioning, and ~22K images intended for Science Pipelines commissioning. The total volume of pixel data acquired during the on-sky campaign with LSSTCam is 100 times larger than that of the on-sky campaign with the engineering Commissioning Camera (LSSTCam) in late 2024. A dedicated, international team of hundreds of people directly supported the LSSTCam on-sky commissioning campaign, providing logistical support and sustaining continuous 24-hour cycles of daytime engineering, LSSTCam monitoring, nighttime observing at Cerro PachĂłn, data processing campaigns at the US Data Facility at SLAC National Laboratory, as well as data analysis from locations around the world.
Engineering tests during weeks 22 and 23 of the campaign continued efforts to characterize contributions to the delivered image quality budget. For AOS commissioning, the team tested two new approaches for wavefront estimation that could potentially improve the closed loop convergence. First, the team successfully deployed and demonstrated on-sky operation of a wavefront estimation pipeline that utilizes a machine learning model to estimate optical corrections. Second, the team performed an analysis of giant “donut” images (with larger defocus than the corner wavefront sensors) with applied perturbations to the mirror figure to directly estimate the optical response, independent of an optics model or Zernike basis for wavefront aberrations. The team also continued stability tests, as well as validation of a refined open-loop Look-up Table (LUT) informed by laser tracker metrology measurements. The updated LUT was deployed for survey-mode observations during the last nights of the campaign. Analysis of these data is ongoing.
The team continued studies of the primary-tertiary mirror (M1M3) temperature relative to ambient as well as thermal gradients across the M1M3, and began initial tests of the dome environment thermalization using a first operational dome louver out of 34 that will eventually be available.
In-dome testing included dynamic testing with increased telescope motion settings.
On September 15, the Feature Based Scheduler (FBS) configuration for the Science Validation (SV) surveys was updated to target only sky regions and bands with deployed templates for difference image analysis, including the ECDFS and ELAIS-S1 Deep Drilling Fields (DDFs) as well as an ecliptic region of ~230 deg^2, so as to maximize opportunities for testing Prompt Processing with survey-mode observations during the limited remaining time of the on-sky campaign. During September, alerts were generated for more than 400 visits from the SV surveys.
Rubin Observatory is preparing a brief report for the SV survey progress webpage to provide an inventory of all visits during the on-sky campaign acquired for Science Pipelines commissioning. As expected for engineering data, the delivered data quality is more heterogeneous than expected for the LSST survey. No data quality selection criteria have been applied to the initial inventory. The report will include summary visualizations of the achieved sky coverage, band coverage, and preliminary estimates for the distribution of delivered image quality for both the SV surveys and targeted observations of specific fields. A visit database will be included with the report.
On-sky testing with the Simonyi Survey Telescope is paused for the next several weeks for an engineering and pre-survey maintenance period. The observing team continues to operate the Auxiliary Telescope (AuxTel). Engineering work at the summit facility commenced this week to replace a hexapod strut for the secondary mirror (M2), to prepare LSSTCam for maintenance, and to finalize preparations to install a first of six panels for the light-wind screen.
In parallel, the distributed team continues to process and analyze data from the on-sky campaign, compile summative analyses on the as-built system performance, and debrief on the lessons learned from commissioning.
During the past year of on-sky commissioning with both LSSTComCam and LSSTCam, the Rubin Observatory Construction and Operations teams executed all aspects of operations, from engineering and maintenance, to planning the observing campaigns, to the processing, validation, and release of on-sky data products to the science community, and demonstrated that the as-built Observatory is capable of meeting the LSST performance specifications.
The Operations team is ready to assume responsibility for the Observatory on 25 October 2025, and the goal is to begin the Legacy Survey of Space and Time (LSST) by the end of this year, though some challenges and tasks remain that must be addressed first. While there has been substantial progress to characterize the system performance, the system is not yet consistently meeting LSST design requirements for PSF size (FWHM) and ellipticity with wide-area survey-mode observations. The highest priority in early Operations will be to continue optimizing for reliability of performance before starting the LSST. The team will use the data obtained, analyses already completed and in progress, and discussions both within the team and with the science community during the September-October engineering and maintenance period to plan activities for November and December. Rubin Observatory will continue to provide regular narrative updates on engineering work and technical progress through the start of steady-state survey-mode observing.