A&A Supplement series, Vol. 121, January 1997, 191 - 199
Received 18 January; accepted 17 March, 1996
T. Reinheimer - K.-H. Hofmann - M. Schöller - G. Weigelt
Send offprint request: T. Reinheimer
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69,
53121 Bonn, Germany
tr@specklec.mpifr-bonn.mpg.de, khh@specklec.mpifr-bonn.mpg.de,
ms@specklec.mpifr-bonn.mpg.de,
weigelt@mpifr-bonn.mpg.de
We present a method for interferometric imaging 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 experiments of LBT speckle masking
interferometry. The raw data were produced by simulating light propagation
in the atmosphere, the LBT pupil function, earth rotation, and photon noise.
The generated data sets consist of up to 200000 LBT 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 
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 
of the
faintest resolution element was simulated and a diffraction-limited image
reconstructed successfully from only 200000 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 LBT speckle
masking observations if partial wavefront compensation (low-order adaptive
optics) is achieved by an artificial laser guide star system
(Foy & Labeyrie 1985; Fugate et al.
1991; Primmerman et al. 1991).
keywords: instrumentation: interferometers -- methods: data analysis -- techniques: image processing -- techniques: interferometric -- telescopes