Speckle masking interferometry with the Large Binocular Telescope

Abstract

We present a method for interferometric imag- ing with the Large Binocular Telescope (LBT) at optical and infrared wavelengths. For example, at = 550 nm a resolution of 6.1 mas can be obtained. The uv-coverage is excellent due to the small distance between the two 8.4 m mirrors. We show laboratory and computer ex- periments of LBT speckle masking interferometry. The raw data were produced by simulating light propagation in the atmosphere, the LBT pupil function, earth ro- tation, and photon noise. The generated data sets con- sist of up to 200 000 LBT interferograms per experi- ment with 200 to 2000 photoevents per interferogram. 200 000 interferograms correspond to only 1.1 hours ob- serving time for a frame rate of 50 frames/sec. In the com- puter simulations a Fried parameter of 40 cm was simu- lated which corresponds to 0.35 arcsec seeing. Diraction- limited images were reconstructed from the various data sets by a modied version of the speckle masking method (bispectral analysis, triple correlation method) and the iterative building block method. The reconstructed images show the dependence of the signal-to-noise ratio on photon noise and other parameters. In one of the experiments the object was a compact cluster of four stars and the inter- ferograms consisted of only 200 photoevents per interfero- gram. 200 photoevents per interferogram correspond to a total V magnitude 14.3 for two 8 m telescopes, 20 msec exposure time per interferogram, 5 nm lter bandwidth, and 10% quantum eciency of detector plus optics. In this experiment the magnitudes of the four individual stars were 15.6, 15.8, 16.4, and 17.1. In a second experiment a compact galaxy with total magnitude of 11.3 and mag- nitude 14 of the faintest resolution element was sim- ulated and a diraction-limited image reconstructed suc- cessfully from only 200 000 interferograms (1.1 hour ob- serving time). Objects of about 18th magnitude can be observed if observing time is increased and observations are made simultaneously in many spectral channels. An