Fresnel phasing of segmented mirror telescopes

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.