AO4ELT5 Proceedings

Point Source sensitivity, Pupil alignment, Calibration and Control for TMT-NFIRAOS-IRIS

Herriot, Glen (National Research Council Canada)

We present details of end-to-end procedures, algorithms, test equipment and performance budgets for pupil alignment, during commissioning and on-sky during observing for first light adaptive optics on TMT. A surprisingly large 30% observing time penalty would arise from only 1% end-to-end pupil misalignment between the primary mirror of the Thirty Meter Telescope and the Lyot stop in IRIS, the first light client instrument for NFIRAOS. This penalty results from undersizing the Lyot stop to block background light, on a misaligned pupil, which reduces throughput and more importantly degrades PSF. On-sky, NFIRAOS will feed back image position, wavefront error and pupil shift error sig-nals to the telescope control system to adjust the alignment of TMT’s three mirrors, but this is only possible after care-ful calibration. We describe specialized test equipment that includes pupil viewing cameras and a high resolution WFS patrolling each output port to assess pupil position. The WFS stage runout would confound measurements, but is self-calibrated by reference sources and by poking DM actuators. Finally we discuss the mechanical compensation strategy consisting of shimming instruments and pointing NFIRAOS’ instrument selection fold mirror, and motorized recentering of an instrument Lyot stop on the as-built beam using its pupil viewing camera also referenced to DM ac-tuator pokes. As a result we expect to achieve a more tolerable but still significant 12.5% loss of observing efficiency from pupil misalignment.

DOI: 10.26698/AO4ELT5.0150- Proceeding PDF

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