Week 27 of the Early Operations system optimization period
Pre-LSST observations were the primary activity of the week, with a total of ~3700 pre-LSST visits, and between ~300 and ~800 pre-LSST visits acquired on each night of the week.
The current lunar bright time is the first period that the y filter has been extensively used since early March. The team noted somewhat larger optical aberrations correlated with use of the y filter relative to the other filters, and accordingly, performed Active Optics System (AOS) closed loop stability runs at fixed telescope position and full-array mode intra- and extra-focal observations to gather more diagnostic data for the y band. While analyses of those data, and other optical modeling efforts, are continuing in parallel, the team has temporarily excluded use of y band in the Feature Based Scheduler (FBS) configuration for the pre-LSST observations to minimize confounding factors when evaluating the stability of the optical performance.
Considering pre-LSST observations in the griz bands, the estimated typical per-night median instrument contribution to the delivered PSF FWHM continued to be at the level of 0.4-0.45 arcseconds.
Additional quantitative details on the delivered image quality performance during the Early Operations period can be found in the Update on the Status of Rubin Observatory through April 2026 post.
Observing plans for the next week include both pre-LSST and dedicated on-sky engineering tests.
The currently installed filter set is grizy with the next swap to remove y and install u planned for 12 May.