Speckle masking interferometry with the Large Binocular Telescope

We present a method for interferometric imaging with the Large Binocular Telescope (LET) at optical and infrared wavelengths. For example, at lambda = 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 experiments of LET speckle masking interferometry. The raw data were produced by simulating light propagation in the atmosphere, the LET pupil function, earth rotation, and photon noise. The generated data sets consist of up to 200000 LET interferograms per experiment with 200 to 2000 photoevents per interferogram. 200000 interferograms correspond to only 1.1 hours observing time for a frame rate of 50 frames/sec. In the computer simulations a Fried parameter of 40 cm was simulated which corresponds to 0.35 arcsec seeing. Diffraction-limited images were reconstructed from the various data sets by a modified 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 interferograms consisted of only 200 photoevents per interferogram. 200 photoevents per interferogram correspond to a total V magnitude similar to 14.3 for two 8 m telescopes, 20 msec exposure time per interferogram, 5 nm filter bandwidth, and 10% quantum efficiency, 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 magnitude similar to 14 of the faintest resolution element was simulated and a diffraction-limited image reconstructed successfully from only 200 000 interferograms (1.1 hour observing time). Objects of about 18th magnitude can be observed if observing time is increased and observations are made simultaneously in many spectral channels. An advantage of speckle masking is that it can be applied to objects fainter than 14th V magnitude, whereas for adaptive optics (with natural reference stars for wavefront sensing) the object or the reference star has to be brighter than about 14th magnitude. Diffraction-limited images of objects fainter than 18th magnitude can be obtained by LET speckle masking observations if partial wavefront compensation (low-order adaptive optics) is achieved by an artificial laser guide star system (Foy & Labeyrie 1985; Fugale et al. 1991; Primmerman et al. 1991).