This is the first week of operations for the Vera C. Rubin Observatory (see announcement). The Construction project is now substantially complete, and the Observatory is currently in an Early Operations period of continued system optimization prior to beginning sustained LSST observations.
Two of the primary considerations for evaluating the technical readiness to begin sustained LSST observations are the system contribution to the distribution of delivered image quality and the effective survey speed that accounts for the effective field of view and source sensitivity, rate of acquiring visits during scheduled survey operations, and the average system availability for survey observations between evening to morning twilight in sustained operations. While the commissioning-era Science Validation surveys demonstrated system capability, further optimization for reliability is needed to deliver LSST as a sub-arcsecond sky survey. The Rubin Observatory team has characterized the various factors that contribute to the delivered image quality budget, and has identified static optics (including optical focus and alignment as well as small and slowly varying gravitational and thermal distortions of the mirror figure) and dome seeing (including the thermal environment along the light path) as the two broad areas where the most gains in delivered image quality could be achieved in the next months. Improvements in the consistency of the delivered image quality will also increase the effective survey speed through better source sensitivity.
On-sky testing during the past week emphasized tests to optimize performance of the Active Optics System (AOS). Since the recent engineering downtime, new deployments for the AOS include (1) a new two-dimensional look-up table (LUT) for telescope elevation and camera physical rotator angle based on a finite element analysis model and optical model for LSSTCam, and (2) an update to double the number of pairs of “donut” out-of-focus pupil images on the corner wavefront sensors used for optical wavefront estimations. On-sky engineering included validation of the elevation and rotator angle LUT, observations to refine the filter focus offset LUT, closing the loop in “full array mode” by pistoning the full focal plane intra- and extra-focus to measure the optical wavefront across the science sensors, applying perturbations to the mirror figure while measuring “giant donuts” with the full focal plane out of focus to provide detailed characterization of the optical wavefront response, and extended stability tests with fixed optical state to evaluate stochastic variations in the optical wavefront estimation. There were multiple nights with sub-arcsecond delivered image quality during the week, allowing for efficient progress on AOS tests.
In parallel with the on-sky testing, analyses continued to characterize variations in the PSF moments across the field of view, and to characterize the thermal environment around the telescope.
This week also included the first on-sky tests for survey operations with the Feature Based Scheduler (FBS) using the configuration for the LSST survey. FBS-driven observations will be acquired on a regular basis to monitor the progress of the overall system performance in an LSST-like observing mode with representative sampling of telescope orientations, slews, and filter changes. Currently, the FBS observations are a small fraction of the on-sky work, typically 1-2 hours at the end of the night. The fraction of survey-mode night-time observing will increase as the overall system performance advances towards meeting the readiness criteria to begin the LSST.
The y-band filter was installed and the currently available filter set is grizy. The team has planned a schedule of filter swaps for the next months, alternating between ugrizy and grizy during each lunation, as expected for steady-state LSST operations.