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Experimental validation of large differential piston sensing with the double-wavelength LIFT

Date Issued
2023
Author(s)
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DOI
Abstract
Adaptive optics systems for the future Extremely Large Telescopes will have to deal with large gaps in the pupil due to the spiders and/or the segmentation of one or several mirrors. These gaps are typically larger than the expected r0 at the sensing wavelength. They can thus create significant discontinuities in the wavefront, which lead to the so-called “island effect” or “petaling”: the wavefront in each segment is well corrected, but differential pistons at a multiple of the sensing wavelength appear between the segments. During the design phase of the Natural Guide star Wavefront Sensor prototype for the Giant Magellan Telescope (GMT), we have studied the Linearized Focal-plane Technique (LIFT) as a potential solution to correct the differential pistons while the adaptive optics system is running. LIFT uses a single image with a known phase offset to estimate wavefront aberrations. The reconstruction of the differential pistons from a single image is limited to the range [-lambda/2, +lambda/2], with lambda the sensing wavelength, due to the 2-pi ambiguity. However, several micrometers of capture range are needed to finely co-phase the GMT. We partially removed the ambiguity and thus increased the capture range by using two LIFT estimations at different wavelengths. At Arcetri premises we set up a test bench in order to obtain a first proof of concept for the dual-wavelength LIFT. The experimental setup includes a diffraction limited near-infrared source and a custom-made mirror with two segments. The longitudinal position of one of the halves is controlled via a motor with a precision of about 5 nm on a 10-mm range. LIFT images were taken on a defocused camera placed downstream of the segmented mirror. The dual-wavelength was achieved by alternating between two narrow-band filters. In this presentation, we show that we were able to reconstruct the differential piston either with two J-band filters or two H-band filters in the whole theoretical capture range.
Coverage
Adaptive Optics for Extremely Large Telescopes (AO4ELT7)
Conferenece
Adaptive Optics for Extremely Large Telescopes (AO4ELT7)
Conferenece place
Avignon, France
Conferenece date
25-30 June, 2023
Uri
Url
Rights
open.access
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AO4ELT_Proceedings__LIFT_DP.pdf

Description
Postprint
Size

1 MB

Format

Adobe PDF

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