Week 21 of the Early Operations system optimization period
As noted last week, the team has begun a series of dedicated on-sky engineering tests to characterize (1) the “target” optical state that the system currently converges towards and (2) the stability of the delivered image quality as measured by visit-to-visit variations of the PSF size and shape across the field of view as well as variations of the commanded optical degrees of freedom. (Accordingly, there have been no alerts generated in recent nights.) These tests have been primarily run under Active Optics System (AOS) closed loop control using the Feature Based Scheduler (FBS) to drive long sequences of visits at fixed telescope pointing, rotator angle, and filter (fixed {alt, az, rot}; almost a “transit mode”) specifically to better understand these fundamental aspects of the system in carefully controlled, idealized conditions. The team is progressing through a sequence of planned tests changing one aspect of the system in each iteration, for example, algorithmic choices for wavefront estimation, the set of basis functions used to translate measured optical aberrations from the corner wavefront sensors to the specific degrees of freedom that apply the corrections in physical hardware, and the gains used in the control loop for applying optical corrections. In some instances, the long sequences of visits have been acquired at telescope azimuth angles pointing into or away from the wind to help evaluate the impacts of the dome thermal environment, and at different rotator angles to evaluate the robustness of the corrections with respect to rotator angle.
In addition, the team has acquired a more extensive set of observations in “full array mode” with the science focal plane intentionally pistoned to a slightly out-of-focus position to provide detailed characterization of the wavefront across the entire field of view. These measurements have been collected at multiple rotator angles. The team is also gathering more observations to robustly measure the optical response to specific mirror bending mode degrees of freedom.
While the system is routinely delivering sub-arcsecond PSF FWHM during the current rounds of testing, there is evidence that the system is converging towards a slightly non-optimal optical state characterized by PSFs that are not as uniformly round as expected, and by variations of the PSF size across the field of view. In addition, there are larger visit-to-visit variations of the PSF than expected. Both observations imply that the system is capable of delivering better image quality than is currently achieved.
The team is currently exploring improvements in the wavefront estimation reliability, developing improved models for the intrinsic optical aberrations of the system that would allow more accurate calculation of optical corrections using the information from the corner wavefront sensors, and is studying higher-order mirror bending mode degrees of freedom.
The current dedicated engineering tests represent an up-front investment to better understand the intrinsic capabilities of the system and to improve the reliability of the delivered image quality. Once the team has confirmed improved and stable performance at fixed telescope pointing, the testing will shift toward observing modes that are more similar to regular survey observations. Observations of fields with deployed templates for difference image analysis that produce alerts will be made, but more intermittent during the current period of on-sky engineering emphasis. The most recent alerts were generated on the night of 9 March. Information on the status of observations and alerts is available on the Prompt Products documentation pages.
The currently installed filter set is ugriz, with the next filter swap planned for 24 March.