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Fresnel phasing of segmented mirror telescopes.

Gary Chanan¹, Mitchell Troy, Isabelle Surdej

¹University of California, Irvine, 92697, USA. gachanan@uci.edu

|November 24, 2011

Summary

Phasing large segmented telescopes may eliminate lenslets. Replacing them with a sparse mask of subapertures simplifies manufacturing and experimental challenges for Shack-Hartmann (S-H) phasing cameras, as shown by laboratory results.

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Area of Science:

Astronomy
Optical Engineering
Optics

Background:

Segmented telescopes require precise phasing for optimal performance.
The Shack-Hartmann (S-H) test is a standard method for telescope phasing.
Current S-H phasing designs for extremely large telescopes (ELTs) involve large pupil demagnifications, leading to small Fresnel numbers.

Purpose of the Study:

To investigate an alternative to traditional Shack-Hartmann lenslets for phasing extremely large segmented telescopes.
To explore the feasibility of using a sparse mask with clear subapertures instead of lenslets.
To demonstrate the effectiveness of this lenslet-free approach through laboratory experiments.

Main Methods:

A physical optics generalization of the Shack-Hartmann test was employed.
The study considered large pupil demagnifications typical for ELTs, resulting in low Fresnel numbers (F=0.6).
Laboratory experiments were conducted using a sparse mask with clear subapertures as a replacement for lenslets.

Main Results:

The use of lenslets in Shack-Hartmann phasing for segmented telescopes can be challenging due to small Fresnel numbers in ELT designs.
Replacing lenslets with a sparse mask of clear subapertures is a viable alternative.
Laboratory results confirmed the validity and effectiveness of the lenslet-free approach.

Conclusions:

The lenslet-free Shack-Hartmann phasing approach is a promising simplification for extremely large segmented telescopes.
This method addresses manufacturing and experimental complexities associated with traditional lenslets.
The findings pave the way for more streamlined and efficient phasing of future large-aperture telescopes.