Cryogenic beam-combiner for very low background, 2-20 micron interferometry on the 22.8 m Large Binocular Telescope

D. W. McCarthy, E. Sabatke, R. Sarlot, P. Hinz, J. Burge

Research output: Contribution to journalConference articlepeer-review

6 Scopus citations


The 22.8 m Large Binocular Telescope Interferometer will be a uniquely powerful tool for imaging and nulling interferometry at thermal infrared wavelengths (2-20 μm) because of the LBT's unusual combination of low emissivity, high spatial resolution, broad (u,v)-plane coverage, and high photometric sensitivity. The Gregorian adaptive secondary mirrors permit beam combination after only three warm reflections. They also control the relative pathlength, wavefront tip/tilt, and focus of the two telescope beams, thus greatly simplifying the complexity of the beam-combiner. The resulting four-mirror beam-combiner reimages the original focal plane and also images the telescope pupil onto a cold stop to limit thermal background. At first-light in 2004, an all-reflective, cooled beam-combiner can provide a 2 arcmin diameter field for Fizeau-style imaging as well as the low thermal background and achromaticity required for nulling interferometry. In designing the optics of such a beam-combiner, we can maximize the field of view at the combined focus by balancing the competing effects of differential phase, tilt, distortion, focus, and pupil matching. To achieve a `peak Strehl' of 0.9 at a wavelength of 4.8 μm across a 1.0 arcmin field radius, strong constraints are placed on differential image overlap (approximately 0.03 arcsec), single beam distortion (0.1%), and pupil matching (0.1%). This cryogenic beam-combiner can feed a variety of interchangeable cameras and spectrographs. Tip/tilt and pathlength (phase) sensors near the final focus within each science instrument will control the adaptive secondaries to maintain precise alignment and provide the highest possible Strehl ratio.

Original languageEnglish (US)
Pages (from-to)II/-
JournalProceedings of SPIE - The International Society for Optical Engineering
StatePublished - 2000
EventInterferometry in Optical Astronomy - Munich, Ger
Duration: Mar 27 2000Mar 29 2000

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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