Week three of the on-sky commissioning with LSSTCam was largely dedicated to collecting an early dataset for building deep coadds and testing difference image analysis with the LSST Science Pipelines. Over the weekend, more than 500 visits were acquired with the Feature Based Scheduler (FBS) on each of three consecutive nights, yielding a pixel data volume nearly 20 times larger than all of the FBS visits taken with ComCam. Images were acquired in the ugri bands for multiple fields spanning a range of stellar densities using translational and rotational pointing dithers. For several fields, the integrated depth of the test data is comparable to multiple years of the LSST Wide-Fast-Deep survey in each of the ugri bands. The team built trial templates and ran difference imaging within the Prompt Processing framework.
The Active Optics System (AOS) performed well during this campaign, and the multi-hour sequences of observations have provided useful data for evaluating the stability of the AOS and identifying areas for further optimization. LSSTCam continued to run reliably. On Thursday morning there was an anomaly in the focal plane cooling system that is under investigation.
The Rubin Observatory team also collected several calibration datasets during the week, including first wavelength scans with the Collimated Beam Projector (CBP) for the u and g filters to measure the instrumental component of the wavelength-dependent standard bandpass. The CBP placed a spot of light on each detector to provide per-detector measurements of the response as a function of wavelength. Analysis of those data is ongoing. Wavelength scans for the remaining filters are planned during the next week. The team also took first in-dome and on-sky images with a pinhole filter that is used to characterize sources of stray and scattered light. With the approaching full moon on 12 May, z and y filters have now been swapped into the LSSTCam carousel such that the currently loaded set is grizy.
A second round of AOS commissioning testing has begun, with an emphasis on the open loop system. The team refined the look-up table (LUT) for the LSSTCam rotator angle, and is starting measurements to validate the elevation and azimuth LUTs. Once the LUTs are in place, the next step will be updating the telescope pointing model. These improvements for the AOS open loop are essential for the upcoming transition from observing specific target fields (as for the recently completed data taking described above) to mapping larger areas by rapidly scanning the sky, and thereby bringing Rubin Observatory closer to full survey-mode operations.
During the extended observing sequences of the past week with stable AOS operation, the team observed correlations between the delivered image quality and the temperature of the primary-tertiary mirror (M1M3) and air temperatures inside the dome. M1M3 is equipped with systems for temperature monitoring and control. Over the next weeks, the team will be characterizing the thermal dependencies on delivered image quality and optimizing feedback loops.