Publications of the MPIfR
Optical & Infrared
Interferometry Group
T. Reinheimer, K.-H. Hofmann, M.
Schöller, and G. Weigelt
Speckle masking interferometry with the Large
Binocular Telescope
Astronomy and Astrophysics Supplement Series,
v.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
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 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 200,000 LBT
interferograms per experiment with 200 to 2000 photoevents per
interferogram. 200,000 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 ~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 LBT speckle masking observations if
partial
wavefront compensation (low-order adaptive optics) is achieved by an
artificial laser guide star system (\cite[Foy \& Labeyrie
1985]{Foy1985}; \cite[Fugate et al. 1991]{Fugate1991}; \cite[Primmerman
et al. 1991]{Primmerman1991})
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