The WEAVE acquisition and guiding software: pattern recognition-based acquisition and multi-fibre guiding

Abstract

We present the architecture, implementation, and on-sky validation of the fully automated acquisition and guiding system (AG) developed for the WEAVE instrument on the William Herschel Telescope. The AG operates in two distinct modes, corresponding to the observing modes of WEAVE. For the large integral field unit (LIFU), an off-axis imaging guider is used, for which we have devised an automatic acquisition method based on pattern recognition of stellar asterisms matched against Gaia predictions. For the multi-object spectrograph (MOS) and the mini-integral field units (mIFU), a multi-fibre guider uses up to eight coherent image guide fibre bundles to derive and apply continuous corrections in azimuth, altitude, and rotation. The system performs complete astrometric calculations, including atmospheric differential refraction and instrument flexure, for each guide frame, enabling accurate target placement and stable closed-loop guiding in all configurations. To support development, commissioning, and operational validation, we have also built a high-fidelity simulation mode that reproduces the behaviour of the telescope control system and of the AG cameras, and we release the standalone camera simulator as open-source software. Using two years of routine WEAVE operations spanning commissioning and early survey phases, we present a statistically robust characterization of AG performance, demonstrating that both modes meet design requirements and are ready for sustained survey operations.

Figures (16)

• Figure 1: Top-level architecture of the WEAVE observatory control system (OCS) software and its interfaces to the wider WEAVE software environment. The AG subsystem, which is the focus of this paper, is highlighted in red. The diagram illustrates the principal communication pathways linking the AG system to other components, including the WEAVE sequencer, telescope control system (TCS), acquisition cameras, image archive, and Redis-based messaging infrastructure. These interfaces define the flow of commands, telemetry, and image data required to perform automated acquisition and closed-loop guiding during WEAVE observations. The diagram updates the architecture originally presented in fig. 1 of pico2018 to reflect the current system following integration and commissioning, and to emphasize the AG subsystem described in this work.
• Figure 2: Top-level architecture of the WEAVE AG system. Light from the focal plane is captured by the LIFU AG CCD, or transmitted by the MOS guide fibre bundles, through an optical relay system, to the MOS AG CCD. The detectors are read out by an ARC controller that transmits the data to the acquisition software (UltraDAS). Images are then processed by the AG software to determine pointing and rotation offsets, which are sent to the TCS during acquisition and guiding. Both the camera and the TCS can be replaced with their simulated counterparts. Subsystems highlighted in blue are detailed in the insets: (a) Computer-aided diagram (CAD) of the LIFU assembly, shown with the top plate removed; the fibre bundle is omitted for clarity. The distance between the centres of the LIFU AG CCD and of the LIFU (94 mm) is marked. (b) Cross-section of a coherent MOS guide fibre bundle, redrawn from Fujikura FIGH-06-300PI specifications. (c) CAD of the MOS AG sliding exchange mechanism, optical relay, and MOS AG CCD.
• Figure 3: (a) The geometric hash of a four-star asterism (shaded grey area) uses the most widely separated pair of vertices ($A,B$) to define a local coordinate system, and the remaining pair of vertices ($C,D$) to define a hash $\mathbfit{h}_{ABCD}$ (\ref{['eq:hABCD']}) that is invariant under translation, scaling and rotation of the four stars. (b) Typical finding chart presented by the LIFU AG; the background is an 8-arcmin wide DSS2 image retrieved using hips2fits. The overlaid orange circles represent Gaia stars between which the reference asterisms are calculated during a successful LIFU acquisition, the finding chart also shows, with a solid blue line, the matching asterism validated by the stars overlaid by blue circles; the nominal LIFU guide star is overlaid by a red circle -- in this case it is one of the four stars forming the matching asterism, but that does not have to be (and often is not) the case; the green rectangle shows the extent of the LIFU AG CCD at the expected position (based on the desired telescope pointing calculated by the positioner software). (c) Reduced image taken during the acquisition for WEAVE OB 8338 on 2023 November 1. The box width is 4.1 arcmin (including the overscan regions). Overlaid on the image are solid blue lines outlining a confirmed asterism -- compare with panel (b), blue circles around stars that validate the matching asterism, and a red circle around the nominal LIFU guide star.
• Figure 4: Collapsed and sky-subtracted images of comet 3I/ATLAS observed on 2025 November 29 with LIFU in low-resolution mode. Panels (a) and (b) show the images obtained with the BLUE and RED cameras, respectively.
• Figure 5: (a) Raw image taken during the acquisition for WEAVE OB 21780 on 2025 December 2. (b) The same guide fibre data, remapped in their original relative positions on to a smaller raster (by cropping the fibre images and pasting them close together, with the empty space filled with artificial noise with the same mean and standard deviation as the bias of the real camera), useful for real-time display in the control room. (c) Fibre representation tool, with the fibre images processed as described in the main text (\ref{['mosag:fibrerepr']}). In all three panels, the fibre outlines and the fibre positioner IDs are overlaid on top of the image. Also, the original colours (white on black) are inverted to improve visual clarity.