Fresnel phasing of segmented mirror telescopes

• Published in: Applied optics. Optical technology and biomedical optics Vol. 50; no. 33; p. 6283
• Main Authors: Chanan, Gary, Troy, Mitchell, Surdej, Isabelle, Gutt, Gary, Roberts, Jr, Lewis C
• Format: Journal Article
• Language: English
• Published: United States 20.11.2011
• ISSN: 2155-3165
• DOI: 10.1364/AO.50.006283

Shack-Hartmann (S-H) phasing of segmented telescopes is based upon a physical optics generalization of the geometrical optics Shack-Hartmann test, in which each S-H lenslet straddles an intersegment edge. For the extremely large segmented telescopes currently in the design stages, one is led naturally to very large pupil demagnifications for the S-H phasing cameras. This in turn implies rather small Fresnel numbers F for the lenslets; the nominal design for the Thirty Meter Telescope calls for F=0.6. For such small Fresnel numbers, it may be possible to eliminate the lenslets entirely, replacing them with a simple mask containing a sparse array of clear subapertures and thereby also eliminating a number of manufacturing problems and experimental complications associated with lenslets. We present laboratory results that demonstrate the validity of this approach.