Active Optics System (AOS) commissioning continues as a main area of emphasis in the third week of the on-sky commissioning campaign with ComCam. The AOS system for the Simonyi Telescope includes two control loops. An open control loop uses a look-up table to make predictable adjustments for the optical degrees of freedom based on telescope elevation, temperature, and potentially other variables. A closed control loop uses wavefront sensing from out-of-focus “donut” images to make additional refinements between successive exposures. This week, closed-loop control using bending mode degrees of freedom for both the primary-tertiary (M1M3) and secondary mirror (M2) optical surfaces simultaneously was demonstrated. Using iterations of wavefront estimation and application of the computed corrections, the team observed steady convergence towards improved delivered image quality. PSF FWHM at the level of 0.7" was achieved on multiple nights. As noted in previous updates, environmental controls are not yet fully in place, and the current tests are using a subset of the AOS degrees of freedom.
Observations for Science Pipelines commissioning continued in parallel, interleaved with AOS commissioning. The ComCam filter exchanger holds three filters at a time. Over the course of the past week, the g filter was exchanged for u, and then u and for z as the moon approached full. An initial set of roughly 20-30 visits were acquired for each of the ugriz bands within an area of approximately 1 square degree located within the LSST Deep Drilling Field Extended Chandra Deep Field-South. Early tests of difference image analysis were carried out using observations in the same field in the same band across multiple nights. This dataset has also enabled early tests of the internal photometric calibration, showing encouraging results, even without a full set of calibration data products.
The telescope motion has also increased this week as testing and analysis demonstrates control of the inertial forces experienced by the mirrors during slews from one telescope position to another. Looking ahead, the team plans to test the AOS across a broader range of telescope elevation and azimuth angles to verify the open control loop and examine the closed loop performance across a wider range of conditions. This increased observing efficiency would also allow Science Pipelines commissioning observations to consider targeting a small set of fields in a preliminary survey mode.