This is Week 7 of Early Operations. Summer rain and hail storms at Cerro Pachón forced two nights of closed-dome testing during which the team completed a long photon-transfer curve (PTC) measurement as well as in-dome stray-light tests using the collimated beam projector (CBP).
As described in the update last week, Rubin Observatory has begun demonstrating capability to maintain an estimated system contribution to the delivered image quality at the level of 0.45" or better over multiple hours in a pre-LSST wide-area survey mode with some constraints on the observing strategy implemented with Feature Based Scheduler (FBS). The current overall strategy is to systematically release those constraints on the pre-LSST FBS configuration as refinements to the Active Optics System (AOS) demonstrate stable control, while simultaneously maintaining stable thermal control within the dome, until achieving reliable image quality with a candidate FBS configuration acceptable to begin the LSST survey.
Several new deployments to improve the reliability of delivered image quality were tested this week. First, the team validated a refined calculation for the optimum global focus, accounting for micron-scale sensor height variations across the focal plane and intrinsic aberrations of the as-built optics to better translate measurements from the corner wavefront sensors to improve the focus across the mosaic of science sensors.
Towards the end of the week, the team tested a refined active optics system (AOS) open-loop look-up table (LUT) for elevation angle and Camera rotator angle that utilizes additional rigid-body degrees of freedom controlled by the Camera hexapods. With this refined LUT in place, the next validation step was updating the pre-LSST FBS configuration to remove constraints on elevation angle range and Camera rotator angle range, while continuing to avoid intra-night filter changes. That testing is ongoing. The team is working on a next version of the elevation and rotator LUT that would additionally incorporate bending mode degrees of freedom for the primary-tertiary (M1M3) and secondary (M2) mirrors.
In parallel with the work on the refined elevation and Camera rotator LUT, the team began testing a refined pointing model that better accounts for the Camera rotator angle. The team also tested an observing mode that uses the hexapods to refine the rigid-body orientation of the optics between visits, but does not adjust the hexapods within individual exposures, which showed encouraging initial results for reducing pointing drift during exposures. All of these updates are anticipated to be included in the next version of the pointing model.
Converging on the filter focus offset LUT has continued to be a stubborn challenge. Over multiple iterations of testing, the team has measured apparent differences in the relative focus offset between filters that sometimes change slowly within a night and vary from night to night. Similar effects have been observed during AOS open-loop stability tests. The physical origin of these slow focus variations, and identifying system telemetry that is correlated with the variations, are under active investigation.
The multi-week study of procedures for evening dome ventilation and thermal control led to the adoption of a standard start-of-night procedure using both the dome aperture and louvers.
As a further step towards normal nighttime operations, during the on-sky campaigns of the past year, the team has demonstrated sufficient reliability of summit electrical power utilities and dome control systems, including the dome aperture shutter, to extend routine open-dome nighttime observations from morning -18 deg (astronomical) twilight to morning -12 deg (nautical) twilight, leading to a typical gain of ~30 minutes effective observing time per night. In addition, administrative constraints on the telescope azimuth range during the last hours of the night that had been in place during the on-sky commissioning period have now been lifted.
A filter swap from y to u occurred on 9 December such that the current filter set going into dark time is ugriz.