The 7-week on-sky commissioning campaign using ComCam concluded on the night of 11 December 2024. Roughly 16K total exposures were acquired since the first on-sky images with ComCam on 24 October, including more than 10K exposures for active optics system (AOS) commissiong (roughly one-third each of intra-focal, extra-focal, and in-focus images), more than 2K bias and dark calibration frames, and more than 2K visits for Science Pipelines commissioning. Several hundred team members directly contributed to the ComCam on-sky campaign, supporting continuous 24-hour cycles of daytime and nighttime testing at the summit, observatory operations from the Base Facility in La Serena Chile, data processing campaigns at the US Data Facility at SLAC National Laboratory, and data analysis from locations around the world. The successes of the ComCam on-sky campaign were made possible by years of preparatory work by the extended Rubin Observatory team.
The final week of the ComCam campaign included in-dome testing to commission the Collimated Beam Projector (CBP) that is used to measure the wavelength-dependent throughput for collimated light. The team demonstrated the capability to co-point the CBP and telescope to position sets of beam spots across the ComCam focal plane using multiple pinhole masks, and performed initial wavelength scans for the g and r bands.
For on-sky engineering, the AOS was tested in a variety of configurations and environmental conditions during the past week, including good and poor atmospheric seeing, sparse and crowded stellar fields, bright and dark time, and photometric and non-photometric conditions. Using the refined open-loop look-up table and pointing model, the team demonstrated the ability to maintain consistent delivered image quality following large slews in elevation and azimuth. Another test performed AOS wavefront measurements with closed loop corrections while emulating the pattern of pointings expected for LSST survey operations to characterize how the optical state evolves under nominal survey operating conditions. Additional non-AOS contributions to the delivered image quality were examined through further stuttered imaging tests at a range of telescope elevation angles.
Observations for Science Pipelines commissioning have continued alongside other on-sky engineering tests. The median time between visits for short translational dithers on the angular scale of the ComCam field of view was reduced to less than 3 seconds, enabling sustained rates of more than 90 individual 30-second visits per hour on a given target field, including occasional filter changes and rotational dithers. On the night of 7 December, a single-night record of more than 200 in-focus visits for Science Pipelines commissioning were acquired within a few hours, scheduled between other engineering tests. Observations of the Extended Chandra Deep Field South (ECDFS) LSST Deep Drilling Field during the ComCam campaign have attained integrated exposure comparable to the LSST 10-year Wide-Fast-Deep survey in the griz bands. Three target fields now have full ugrizy photometry from ComCam: ECDFS, Euclid Deep Field South (EDFS), and a low Galactic latitude field (Rubin SV 95 -25). EDFS and Rubin SV 95 -25 have integrated exposure comparable to 1-2 years of the LSST Wide-Fast-Deep survey. The ComCam measured counts from a CALSPEC spectrophotometric standard star, C26202, located in the ECDFS field, are consistent with a priori expectations based on synthetic photometry in each of the ugrizy bands, indicating that the system optical throughput including mirrors, lenses, filters, and the quantum efficiency of the sensors are reconfirmed at this level.
The on-sky testing also included dedicated investigations of stray and scattered light, rastering bright stars around the ComCam field of view, and taking images at a range of camera rotator angles, as well as using multiple filters and without a filter in position.
The process to remove ComCam from the telescope and to prepare the telescope top end assembly for installation of LSSTCam has now begun. Data processing and analysis for the ensemble of ComCam observations will continue in parallel while LSSTCam is being integrated with the telescope. Our understanding of the system performance from the ComCam campaign is still rapidly evolving. Technical details are being captured in technotes that will be shared with the Rubin science community to help prepare for the Early Science Program (RTN-011).
During the next phase, as summit work transitions from intensive data-taking to rigging for LSSTCam installation, these Community posts about commissioning will continue at a reduced frequency.