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

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.