On-sky demonstration of the GMT dispersed fringe phasing sensor prototype on the Magellan Telescope

Derek Kopon, Brian McLeod, Marcos A. Van Dam, Antonin Bouchez, Ken McCracken, Daniel Catropa, William Podgorski, Stuart McMuldroch, Alan Conder, Laird Close, Jared Males, Katie Morzinski, Timothy Norton

Research output: Chapter in Book/Report/Conference proceedingConference contribution

10 Scopus citations


The GMT is an aplanatic Gregorian telescope consisting of 7 primary and secondary mirror segments that must be phased to within a fraction of an imaging wavelength to allow the 25.4 meter telescope to reach its diffraction limit. When operating in Laser Tomographic Adaptive Optics (LTAO) mode, on-Axis guide stars will not be available for segment phasing. In this mode, the GMT's Acquisition, Guiding, and Wavefront Sensing system (AGWS) will deploy four pickoff probes to acquire natural guide stars in a 6-10 arcmin annular FOV for guiding, active optics, and segment phasing. The phasing sensor will be able to measure piston phase differences between the seven primary/secondary pairs of up to 50 microns with an accuracy of 50 nm using a J-band dispersed fringe sensor. To test the dispersed fringe sensor design and validate the performance models, SAO has built and commissioned a prototype phasing sensor on the Magellan Clay 6.5 meter telescope. This prototype uses an aperture mask to overlay 6 GMT-sized segment gap patterns on the Magellan 6.5 meter primary mirror reimaged pupil. The six diffraction patterns created by these subaperture pairs are then imaged with a lenslet array and dispersed with a grism. An on-board phase shifter has the ability to simulate an arbitrary phase shift within subaperture pairs. The prototype operates both on-Axis and 6 arcmin off-Axis either with AO correction from the Magellan adaptive secondary MagAO system on or off in order to replicate as closely as possible the conditions expected at the GMT.

Original languageEnglish (US)
Title of host publicationAdaptive Optics Systems V
EditorsEnrico Marchetti, Jean-Pierre Veran, Laird M. Close
ISBN (Electronic)9781510601970
StatePublished - 2016
EventAdaptive Optics Systems V - Edinburgh, United Kingdom
Duration: Jun 26 2016Jul 1 2016

Publication series

NameProceedings of SPIE - The International Society for Optical Engineering
ISSN (Print)0277-786X
ISSN (Electronic)1996-756X


OtherAdaptive Optics Systems V
Country/TerritoryUnited Kingdom


  • Active optics, adaptive optics, Giant Magellan Telescope, phasing, dispersed fringe sensor

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Condensed Matter Physics
  • Computer Science Applications
  • Applied Mathematics
  • Electrical and Electronic Engineering


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