Large amplitude tip/tilt estimation by geometric diversity for multiple-aperture telescopes

Abstract

A novel method nicknamed ELASTIC is proposed for the alignment of multiple-aperture telescopes, in particular segmented telescopes. It only needs the acquisition of two diversity images of an unresolved source, and is based on the computation of a modified, frequency-shifted, cross-spectrum. It provides a polychromatic large range tip/tilt estimation with the existing hardware and an inexpensive noniterative unsupervised algorithm. Its performance is studied and optimized by means of simulations. They show that with 5000 photo-electrons/sub-aperture/frame and 1024ˆ1024 pixel images, residues are within the capture range of interferometric phasing algorithms such as phase diversity. The closed-loop alignment of a 6 sub-aperture mirror provides an experimental demonstration of the effectiveness of the method.

Cophasing multiple aperture telescopes with linearized analytic phase diversity

“…Figure 5 shows the effect of LAPD correction. The departure point (iteration 0) of the cophasing sequence results from the previous coarse alignment, 19,27 where we can see small piston/tip/tilt errors from the PSF shape. The loop is closed with a pure integrator with 0.2 gain.…”

“…In order to experimentally validate LAPD, we use the BRISE bench built by Onera. 19,26 We conduct tests imaging the collimated output of a fibered single-mode fiber near 635 nm, acting like an unresolved source, through six circular apertures of a segmented mirror. Each segment is supported by three piezoelectric actuators, allowing to introduce piston, tip or tilt perturbations.…”

“…Before PD can be applied, the sub-apertures must be coarsely (geometrically) aligned. For this step, Vievard et al 19 developed the ELASTIC algorithm. The latter allows to reduce large tip/tilt aberrations over sub-apertures from several λ RMS to less than λ/8 RMS when observing an unresolved target.…”

Focal plane wavefront sensing on SUBARU/SCExAO

“…Focal plane wavefront sensors have been largely studied to correct for NCPAs, Island Effect and segment co-phasing [12][13][14][15][16][17] since they can be operated directly on the science camera, avoiding NCPAs, and can sense discontinuities in the pupil's phase. As a competitive science instrument [18][19][20][21][22][23] and experimental platform, SCExAO 24 (Subaru Coronagraphic Extreme Adaptive Optics), hosts and welcomes many new algorithms and concepts to improve the wavefront quality distributed to its different modules [25][26][27][28] , but also to prepare for the next generation of ELTs that will face similar challenges.…”

Tip/tilt Alignment for a Fizeau Interferometer via Defocus-Based Sub-Spot Separation

“…The rate of phase diversity can only achieve several Hz to several tens Hz [11,15,16] despite fast development of the computing hardware. However, impressively, a non-iterative method for large amplitude tip/tilt estimation using pseudo-defocus diversity has been proposed [17], which might be promising for real-time alignment [18]. In projects that require real-time wavefront correction, the strategy commonly employed is designing complex optical modules to spatially separate subaperture spots on the detector.…”