Week 22 of the Early Operations system optimization period
The team continued intensive on-sky engineering work and offline analyses to characterize and improve the delivered image quality. There were no pre-LSST observations, and no alerts were generated during the week. Some limited pre-LSST observations might be acquired during the next week. Activities for the current week included ongoing investigations of higher-order mirror bending modes, Active Optics System (AOS) control loop optimization, and tests of the dome ventilation and airflows around the telescope, described in turn below.
Multiple analyses from recent weeks point towards the presence of optical aberrations that are likely attributed to higher-order mirror bending mode degrees of freedom. The team is pursuing multiple approaches to study these effects. First, since some of these optical aberrations are more challenging to characterize using the corner wavefront sensors alone, the team continued gathering “full array mode” observations with the focal plane intentionally pistoned slightly out of focus to make detailed measurements of the variation of the optical wavefront across the full focal plane. Such full array mode observations have now been acquired for a range of Camera rotator angles and elevation angles. A second approach is to intentionally perturb specific degrees of freedom to measure the optical response. These two approaches can then be combined, using the optical aberrations determined from the full-array mode data, inverting the sensitivity matrix to infer the corresponding adjustments to the AOS degrees of freedom, and then applying those corrections to validate the actual system response. Initial tests using this approach to correct a subset of aberrations were conducted during the week.
The team continued AOS closed loop stability tests at fixed telescope pointing to optimize the control loops. Work this week included further investigations into the sets of basis functions used to apply corrections, as well as an experiment with hexapod adjustments to the secondary mirror (M2) position along the optical axis turned off to reduce potential degeneracies between degrees of freedom, and thereby better understand the coupling between de-focus and spherical aberrations.
During some of the AOS closed loop stability runs, the team conducted experiments to characterize dome seeing effects. In one test, the dome louvers were opened and closed each hour. For another test, the team periodically turned on a set of fans installed around the telescope. In addition, the team is analyzing telemetry from an enlarged set of sonic anemometers now installed around the ring at the top end of the telescope to study air movements within the dome and correlations with external wind velocity and delivered image quality. Collectively, these studies inform optimization of the dome ventilation and thermal controls, as well as survey strategy for the telescope pointing relative to the external wind direction.
Due to a temporary outage of one of the LSST Camera (LSSTCam) cooling systems, and subsequent recovery and testing to confirm stable vacuum pressure within the LSSTCam cryostat, no on-sky observations were acquired from 20-23 March.
The currently installed filter set is ugriz. As most of the current on-sky engineering work is using a single filter, the team elected to pass on planned filter swaps during this lunation to allow more time for other daytime engineering work at the summit. The next filter swap to exchange u for y is planned for mid-April.