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Concept study of on-instrument wavefront sensors for the TMT IRIS science instrument

Abstract 142

Submitted by David LOOP


D.Loop(1), M.Fletcher(1), V.Reshetov(1), R.Wooff(1), J.Fitzsimmons(1), R.Dekany(2), R.Smith(2), D.Hale(2), D.Crampton(1), L.Simard(1), B.Ellerbroek(3), C.Boyer(3), L.Wang(3)


(1) Herzberg Institute of Astrophysics, National Research Council of Canada; (2) Caltech Optical Observatories, Caltech Institute of Technology; (3) TMT Observatory Corporation


The TMT Narrow Field Infrared Adaptive Optics System (NFIRAOS) is a facility multi-conjugate adaptive optics system that provides turbulence compensation over a moderately large field of view (2 arcmin) and feeds up to three science instruments. The reference design of NFIRAOS includes two conventional deformable mirrors conjugated at 0 km and 11.2 km, six sodium laser guide stars distributed over a 70 arcsec field of view and one or more tip/tilt/focus sensors within each instrument. NFIRAOS will sharpen the images of natural guide stars and improve the sky coverage for tip/tilt sensing with on-instrument wavefront sensors (OIWFS).

To maximize sky coverage, these on-instrument wavefront sensors will sense natural guide stars in the infrared. For the InfraRed Imaging Spectrograph (IRIS) science instrument, two tip/tilt sensors will be equipped to perform fast guiding, and one tip/tilt/focus sensor will be equipped to calibrate the focus biases in the Laser Guide Star WFS induced by the variations in the range to the sodium layer. The IRIS OIWFS will also enable computation of blind modes, comprised of a mix of focus/astigmatism and tilt anisoplanatism modes.

In this paper, we report on the conceptual design study of the IRIS OIWFS, a collaborative effort by NRC-HIA, Caltech, and TMT AO and Instrument teams. This includes work on the system design of patrol geometry, detector alternatives, sky coverage modeling, acquisition, guiding, and dithering scenarios, and interfaces with NFIRAOS, IRIS, and TMT observatory. It also includes work on probe optics, mechanics, and controls, detector arrays and controllers, instrument rotator and cable wrap, and thermal and structural design of the OIWFS enclosure. Based on this work, a baseline concept for the IRIS OIWFS is presented.

Keywords: wavefront sensors, sky coverage, natural guide star, infrared detectors, acquisition, guiding, dithering

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