Lollipops for HARMONI: a sweet guide probe model

The High Angular Resolution Monolithic Optical and Near-infrared Integral field spectrograph (HARMONI) is the near infrared (NIR) integral field spectrograph for the Extremely Large Telescope (ELT). To reach the diffraction limit of the telescope and maintain an optimal image quality over long exposures, an accurate measurement of geometrical distortions in the field of view of the instrument is needed. This is achieved by means of three articulated guide probes sweeping the focal plane of the instrument. The latest iteration of the design of the guide probes of HARMONI changes the structure of the geometrical distortions of optomechanical origin, and a new formulation of the corrective model is required. In this work, we developed a 3D optomechanical model to simulate these distortions, and characterise their structure to formulate a corrective model with a reduced number of free parameters. The proposed corrective model captures the structure of the systematic pointing error introduced by these contributions based on a new series expansion named the “Lollipop expansion”. The proposed corrective model is able to remove not only the effects of the guide probe motors, but also the effects of the misalignments of the fold mirrors, with a residual well below the acceptance criterion of 10% of the total error budget.