Image slicing with a Ttwist: testing and characterising a prototype image slicer for ELT-PCS

As part of our ongoing research and development activities for the Planetary Camera and Spectrograph (PCS) for ESO’s Extremely Large Telescope (ELT), we are comparing two types of integral field unit concepts—an image-slicer and a lenslet-array—for its integral field spectrograph and main science instrument. In our 2022 publication we described the design and manufacturing of the image-slicer used in this comparison, based on the concept of our twisted image slicer published in 2014; in this paper we present recent interferometric measurements of the mirrors of the image slicer, manufactured by Canon Inc. Using our 6-inch Fizeau interferometer we measured the spherical pupil mirrors, resulting in a typical peak-to-valley surface figure error of ≲ 160 nm, and an RMS error of ≲ 25 nm. Measuring the 0.3 mm×13.5 mm parabolic slicing mirrors was challenging, however, after removing low-order alignment errors our measurements indicate a surface error of RMS ≲ 70 nm. Based on these measurements we conclude that the surface quality of the mirrors is excellent and well suited for our experiment.