Week six of the LSSTCam on-sky commissioning brought more clouds, cold temperatures, and some periods with sub-optimal atmospheric seeing. Still, the team found opportunities to collect useful datasets for the on-going campaign.
Active Optics System (AOS) commissioning made progress optimizing configurations for the AOS closed loop control system, including tests with various combinations of degrees of freedom enabled, verification of degeneracies between degrees of freedom, and direct calibration of the basis of non-degenerate characteristic modes. The team made further measurements of the intrinsic optical aberrations. Higher temporal frequency contributions to the delivered image quality were characterized using sequences of short-exposure visits and guider images of defocused “donut” images. In addition, the team acquired so-called “giant donuts” with the focal plane pistoned +/- 7.5 mm out of focus compared to the regular +/- 1.5 mm offsets used for the wavefront sensors. This test provides a detailed view of the pupil and will be used to tune the wavefront estimation pipeline for a range of defocused object sizes. Calibration of the optical response to the primary-tertiary (M1M3) mirror temperature using in-dome and on-sky data has also started.
Enhancements to the telescope control software have enabled observations over a wider range of elevation and azimuth angles around zenith, and validation of the AOS open loop look up table for the Camera rotator angle has allowed use of large intra-night rotational dithers.
Stray light tests have advanced from initial characterization to a suite of on-sky tests designed to investigate and mitigate reflections off specific surfaces, as well as reproducing specific patterns of stray light on multiple nights to validate ray tracing models.
The team acquired a set of ugriz observations of a moderately dense stellar field within the same night with focal plane scale translational dithers and large rotational dithers, providing an excellent dataset for characterizing the internal photometric calibration. Further observations of the COSMOS LSST Deep Drilling Field have now reached a cumulative exposure depth in the ugrizy bands similar to multiple years of the LSST Wide-Fast-Deep survey. The team is more routinely exercising data flows for trial difference image analysis (DIA) with the Prompt Processing framework at the US Data Facility (USDF).
The currently loaded filter set is ugriz, with a plan to swap u for y next week around the first quarter moon.
The team has continued to investigate the performance of the Camera focal plane cooling system under a range of environmental conditions. The team has implemented an initial set of mitigations and is developing further mitigation options.
The as-built Observatory has demonstrated the capability to acquire excellent data, and the team is working towards optimizing the efficiency and consistency of delivered performance under a range of conditions over the coming months.