Optical test procedure for characterization and calibration of robotic fiber positioners for multiobject spectrographs

Abstract

The recent burgeoning interest in massive multiobject spectroscopy has pushed the development of massive optical fiber positioning systems. These systems rely on precise fiber placement to detect the light spectra of many stars and galaxies. One successful approach is the use of robotic fiber positioners, which allow one to automate and scale up observations. However, due to the need for high precision and accuracy, each positioner must be calibrated and verified to comply with the requirements. The calibration measurements are nontrivial, and the large number of the robotic positioners up to thousands can lead to a prohibitively long time for calibration. We describe and validate an optical calibration setup and procedure for robotic fiber positioning systems. Based on the measurements results, we have developed models describing the behavior of the positioners and we introduce new performance metrics that allow one to verify the stringent positioner specifications and furthermore help to identify and analyze design and manufacturing flaws. © The Authors. Published by SPIE under a Creative Commons Attribution 4.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

The Calibration of theta-phi Fiber Positioners Based on the Differential Evolution Algorithm

“…For example, the classic stepper model is adopted by the LAMOST and MOONS, whereas a brushless direct current (BLDC) motor is used by the SDSS-V and LAMOST second stage (Kronig et al 2020c;Guo et al 2022). The motors are controlled using an electronic board with pulse-width modulation through an open loop (LAMOST second stage; Guo et al 2022) or a closed loop (SDSS-V with Hall sensors; Kronig et al 2020b). One of the main challenges for the positioners is the tiny maximal allowable positioning error, which is telescopespecific and related to the fiber core diameter and size of the objects (Horler et al 2018;Kronig et al 2020c).…”

“…However, because of the limited available space, affordable sensors with no high accuracy exist (Kronig et al 2020c). Instead of encoders, reduction gears are used to guarantee the positioning precision as the positioning resolution is multiplied by the reduction ratio (Horler et al 2018;Kronig et al 2020bKronig et al , 2020c. The reduction ratio is the motor input divided by the actuator output.…”

Precision control of miniature SCARA robots for multi-object spectrographs

“…For our measurements, we used a similar metrology system already introduced in previous work [26,33] (Figure 4). A robot is set up with an optical fiber that is back-illuminated with a LED.…”

SDSS-V Algorithms: Fast, Collision-Free Trajectory Planning for Heavily Overlapping Robotic Fiber Positioners

“…This manifests as a nonlinear relationship between a positioner's commanded angular velocity and the true angular velocity, where a positioner's true velocity will oscillate about the expected velocity by a small amount in a repeatable way. Kronig et al (2020) provide a comprehensive analysis of all the various sources of mechanical error in an SDSS-V positioner.…”