Prime Focus Spectrograph (PFS) for Subaru telescope: the optimization of target convergence for Cobra positioners

The Prime Focus Spectrograph (PFS) is a new optical/near-infrared multi-fiber spectrograph designed for the prime focus of the 8.2m Subaru telescope. PFS covers 1.3-degree diameter field with 2386 fibers to complement the imaging capability of Hyper Suprime Cam (HSC). The prime focus unit of PFS called Prime Focus Instrument (PFI) provides the interface with the top structure of Subaru telescope and also accommodates the optical bench in which Cobra fiber positioners with science fibers are located. The PFI was delivered to the Subaru telescope in June 2021. Consequently, several tests and engineering runs were carried out to calibrate the PFI and also verify the performance of the PFI with the telescope. Some improvements were made including the focus adjustment of the acquisition and guiding cameras to match which of the science fibers, and the calibration of the AG camera positions and the flux calibrations. The key performance for PFI is so called the convergence rate, i.e. the successful rate of the Cobra positioners to reach the target positions within the assigned time. The convergence process includes iterations between movements of Cobra positioners and imaging of the metrology camera system for measuring the fiber positions. Through commissioning runs, we accumulated enough data to analyze the possible factors that affect the convergence rate such as the calibration accuracy for the distortion of the wide field correctors, the thresholds to stop the positioner movement, the exposure time for the metrology camera and the dome seeing, etc... With these efforts, several improvements have been implemented to enhance the performance of the positioners and determine the optimal iteration number. The overall convergence time has also been reduced substantially. We will report the analysis and improvement done in this paper.