AO4ELT5 Proceedings

Telescope Pupil Tracking using a Pyramid WFS

Veran, Jean-Pierre (National Research Council Canada), Herriot, Glen (National Research Council Canada)

During an AO observation the Lyot stop of an infrared instrument fed by the AO system must remain aligned with the image of the telescope pupil. Uncertainties and drifts of the pupil image position must be accommodated by under-sizing the Lyot stop, which, especially for AO corrected observations, reduces very significantly the science productivity of the instrument. In the case of IRIS, the first client instrument of NFIRAOS on the TMT, there is a requirement to limit the under-sizing of the IRIS Lyot stop to 1% of the pupil diameter, which therefore means to tightly stabilize the image of the telescope pupil. In this paper, we show how this can be accomplished by finely aligning the IRIS Lyot stop to NFIRAOS during calibration (sighting actuators pokes on the NFIRAOS high-altitude DM with the IRIS pupil viewing camera), and, during observation, by processing the images obtained by the NFIRAOS Truth WFS to detect drifts and feeding this drift information to the telescope control system so it can repoint the beam fed to NFIRAOS. We show that this measurement of the pupil position with the Truth WFS can be accomplished in nominal observing conditions with an RMS error of less than 0.02% of the pupil diameter in bright time, and less than 0.03% of the pupil diameter in dark time. We include in this evaluation noise sources on the Truth WFS, as well as uncertainties on the reflectivity of the primary mirror segments due to their recoating schedule. This work directly feeds into the work presented in reference [1], which describes the full tolerance stack-up required to reach the 1% under-sizing of the Lyot stop.

DOI: 10.26698/AO4ELT5.0127- Proceeding PDF

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