Publications of the MPIfR
Optical & Infrared
Interferometry Group
1) Reinheimer, T.; Hofmann, K.-H.;
Weigelt, G.
Interferometric imaging with arrays of large
optical telescopes in the multi-speckle mode
Astronomy and Astrophysics, vol. 279, no. 1, p.
322-334
Abstract
We present a method for interferometric imaging with arrays of large
optical telescopes in the multi-speckle mode. The raw data were
produced by simulating light propagation in the atmosphere, various
pupil functions similar to the pupil function of the European Southern
Observatory (ESO) Very Large Telescope Interferometer (four 8-m
telescopes), earth rotation, and photon noise. The generated data sets
consist of up to 48,000 interferograms per experiment with 100 to
80,000 photoevents per interferogram. Since a Fried parameter r0
smaller than the telescope diameter was chosen, multi-speckle
long-baseline interferograms were obtained which consist of many
speckles with interference fringes in each speckle. This experimental
condition is called the multi-speckle mode, which is typical for
interferometric imaging with large telescopes at optical wavelengths.
From the various data sets diffraction-limited images were
reconstructed by the speckle masking method (bispectral analysis) and
the iterative building block method. Image reconstruction is possible
without the use of non-redundant masks since speckle masking is a
generalization of phase closure imaging to highly redundant arrays (or
large optical telescopes). The reconstructed images show the dependence
of the signal-to-noise ratio on photon noise and other parameters. The
proposed method can also be applied to radio interferometric data
(especially, mm- or or sub-mm-observations).
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2) Reinheimer, T., Hofmann, K.-H.,
Schöller, M., Weigelt, G.
Speckle masking interferometry with the Large
Binocular Telescope
Astronomy and Astrophysics Supplement Series, vol.
121, p.191-199 (1997))
Abstract
We present a method for interferometric imaging with the Large
Binocular Telescope (LBT) at optical and infrared wavelengths. For
example, at tex2html_wrap_inline545 = 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 tex2html_wrap_inline551 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 tex2html_wrap_inline553 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).
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