The Large Binocular Telescope an interferometer for widefield Fizeau imaging and Bracewell nulling

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The Large Binocular Telescope - an interferometer
for wide-field Fizeau imaging and Bracewell
nulling Roger Angel The University of
Arizona Steward Observatory presentation for the
Interferometry Workshop UCSC, October 15, 2000
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LBT Current Status (April 2001) Construction and
basic optical and infrared spectrographs fully
funded by consortium including Italy and
Germany Site is at 10,470 on Mt Graham, AZ -
highest, most southerly peak in continental US.
The Hertz 10 m sub-mm telescope is already
operating on site - water vapor is low outside of
summer monsoon flow Rotating enclosure now in
place on Mt Graham, AZ Both 8.4 m honeycomb
primaries have been cast, optical fabrication of
the first mirror is now being started at the
Mirror Lab Telescope mount is nearing
completion at Ansaldo Energia, Italy (maker of
the four VLT telescopes) Operation with 1st
primary and adaptive secondary expected fall
2003, full interferometric operation in 2004
The Large Binocular Telescope has two 8.4 m
telescopes co-mounted with 14.4 m center to
center spacing
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The LBT main alt-az structure in Milan, April
2001. The two 8.4 m mirror cells attach to the
sides of the rocking chair seat on the left,
the secondary support swing arms to the red
columns on the right
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s
The largest piece of glass in the world

• 8.4 m glass blank cast at the Mirror Lab
• first of two for the Large Binocular Telescope
  in Arizona
• spin-casting method pioneered at Mirror Lab,
  for f/1.14 surface
• 80 lightweighted by internal honeycomb
  structure
• to be figured this year to 14 nm rms with
  stressed lap
• cost of completed mirrors in telescope cell
  with actuators is 14M each

The rotating enclosure (silver) for the LBT
telescope on Mt Graham
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Polishing the LBT primaries
The off-axis, aspheric polishing of the LBTs
mirrors will be made with the same stressed lap
tool used to make all large mirrors in the Mirror
Lab. In operation the lap is itself continuously
distorted as an off-axis segment, to always fit
the parent no matter how far off axis it is moved
by the polishing motion. This is the method used
to make the Magellan and MMT 6.5 m mirrors.
Image from the Magellan I telescope
Representation of Magellan I 6.5 m mirror as 633
nm interferogram. It is diffraction limited at
this wavelength.
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The LBT Gregorian secondary mirrors will be
adaptive
The 0.091 m LBT adaptive secondaries will be made
of 2 mm thick glass with 672 voice-coil supports,
very similar to the adaptive secondary for the
6.5 m MMT
64 cm adaptive secondary with 336 actuators for
the MMT under test in Arizona. The mirror is not
yet silvered, and the magnet actuators are
visible through the 2 mm facesheet. The measured
response time is less than 1 msec.
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LBT Project Partners
25 Arizona The University of
Arizona (Tucson) Arizona
State University (Tempe)
Northern Arizona University (Flagstaff)
25 Italy Osservatorio
Astrofisico di Arcetri (Florence)
Osservatorio Astronomico di Bologna (Bologna)
Osservatorio Astronomico di
Roma (Rome) Other Italian
Observatories and Universities
12.5 Research Corporation The
Ohio State University
University of Notre Dame
University of Minnesota 12.5 The
Ohio State University 25 Germany
- LBT Beteiligungsgesellschaft
Max-Planck-Institut für Astronomie (Heidelberg)
Landessternwarte (Heidelberg)
Astrophysikalisches Institut
Potsdam (Potsdam)
Max-Planck-Institut für Extraterrestrische Physik
(Munich) Max-Planck-Institut
für Radioastronomie (Bonn)
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LBT characteristics

• The two 8.4 m mirrors are co-mounted on an alt-az
  yoke with a 14.4 m center-to-center separation
• during tracking, the center line is always normal
  to the target in clearest parts of 10 mm
  atmospheric window, sensitivity of ground based
  telescopes completely dominated by thermal
  emission from telescope mirrors.
• LBTs co-mounted geometry and adaptive
  secondaries lead to very low projected emissivity
  of 7
• This arises because diffraction limited beams are
  delivered to the beam combiner cryostat after
  total of only 3 warm reflections (primary,
  adaptive secondary, tertiary).
• Low thermal background
• as a result, the beam combination is very direct,
  with no need for trombone path compensators
• broad fringes - 1/2 fringe width (peak-to-null)
  at 11 mm 0.08 arcsec
• the fringe pattern on the sky rotates as the
  target transits, when observing conditions are
  optimum
• Atmospheric wavefront correction made with
  adaptive Gregorian secondary mirrors
• no additional optics needed
• 672 actuators per mirror allow for very high
  order wavefront correction

LBT Science The Scientific goals are very broad,
reflecting the range of interests of the
participating institutions. One unique strength
will be the telescopes ability to image a deep
field with greater resolution that any other. A
key project is to obtain a very deep image of a
30 arcsec field, taking many nights of
integration. The telescopes unique capability
for deep nulling and high sensitivity in the
thermal infrared will be exploited in programs to
study extrasolar planetary systems.
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Optical diagram of the The LBT interferometer
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Comparison of Keck and LBT interferometers
Like the VLTI, the KI uses separately planted,
widely spaced telescope elements. It lends
itself to Michelson interferometry and Bracewell
nulling of distant stars
The LBT is unique in that its large elements are
comounted, and always perpendicular to the
source. It is ideally suited for wide field
Fizeau imaging interferometry and Bracewell
nulling of nearby stars
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Comparison of Fizeau and Michelson methods
Fizeau and Michelson interferometry are
fundamentally different. In a Fizeau
interferometer, the focal planes are brought into
coincidence as shown, by division of wavefront.
The interference is detected fringes in the focal
plane images. Provided the exit pupil mirrors the
entrance pupil geometry, every star in the field
appears as an Airy pattern appropriate to the
individual aperture, crossed by Youngs fringes
with spacing set by the baseline
(center-to-center). The central fringe in each
star image corresponds to zero path length, and
is achromatic. By contrast, a Michelson
interferometer combines the beams by overlapping
the pupils at a semi-transparent beamsplitter
(division of amplitude). There are no
interference fringes formed in the focal plane,
interference appears as a modulation in star
intensity according to optical path difference.
Zero path length corresponds to a single
achromatic fringe on the sky. For normal
photometric bands of width l/Dl 0.1, the field
of view for interferometry is very narrow, with a
width of only 10 fringes. The fundamental
advantage of Fizeau interferometry for
optical/infrared interferometers is its much
larger field of view for wide bandwidths. In the
LBT it is implemented with far fewer warm
mirrors, and hence much lower thermal
background.
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Simulated LBT image of IO
LBT Fizeau snapshot. The beam profile is the 8.4
m Airy pattern convolved with vertical Youngs
fringes.
Reconstruction from 3 snapshots at 60? angles (K.
Hege). The resolution is equivalent to a 22.8 m
filled aperture telescope. Note that Io is only
1 arcsec in diameter, the full field for Fizeau
imaging is about 30 arcsec.
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Fizeau interferometry tests at the old MMT
Fizeau interferometry has been tested at the old
MMT, and will be a major thrust at the LBT. For
stable fringes the random path length
fluctuations between the two light paths caused
by atmospheric turbulence, must be corrected.
Since the shift is the same for all stars in the
field, we choose the brightest one and measure
its fringe position in real time. A servo system
is used to stabilize phase by rapid piston motion
of a mirror in the beam combiner. Fringes
stabilized in this way at the old MMT are
illustrated. For the large apertures of the
20/20 telescope, the individual apertures must be
corrected to their diffraction limits. Adaptive
optics secondaries will be used to do this.
Fringes recorded at 2.2 mm at the old MMT,
illustrating the wide field of view for Fizeau
interferometry. In this test, phase was locked
on the star g And A. The images is of g And B
and C, a 0.7 arcsec binary located 10 arcsec from
g And A. The fringe spacing is 0.2 arcsec
Stabilized Fizeau fringes recorded at 2 mm at the
old MMT. Two 1.8 m telescopes separated by 4 m
were used.
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Dependence of interferometer resolution and
sensitivity on baseline and aperture The KI has
6 times the baseline and 6 times the resolution
of the LBT. However, resolution comes at the
cost of sensitivity and imaging capability. Here
B is the background brightness, b is the
interferometer baseline and D the single dish
diameter. Dependence of noise/signal in
different cases is given. This does not depend
on baseline for unconfused point sources, but for
imaging a diffuse source the noise increases in
proportion to the square of the baseline.
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Bracewell Nulling Interferometry The green
wave, representing the electromagnetic wave at
from the same star at the second mirror, when
flipped up by antiquated overhead technology,
inverts and overlaps the red to cancel The
close LBT spacing results in a deep null at 10
microns over the 1 mas disc of a sun at 10 pc,
while still resulting in full constructive
interference for a planetary companion separated
by only 0.1 arcsec or 1 AU. The LBT will be able
to image and study the 10 micron spectrum of
planets at this distance found by radial velocity
studies
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Bracewell nulling interferometry Thermal
images of circumstellar dust obtained by nulling
interferometry at the old MMT (Hinz et al,
Nature 1998 395, 251)

The value of nulling interferometry is
illustrated by these constructive and destructive
interference images, shown directly as recorded
on a 10 mm imaging array. The spacing of 5 m
between the co-mounted telescopes results in an
angular separation from null to maximum of 0.2
arcsec a Tau is unresolved star, and is strongly
canceled. Because no adaptive correction of the
individual wavefronts was possible, there is a
slight residue. a Ori is surrounded by a dust
cloud whose thermal emission is seen directly
when the star is nulled out.
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Nulling Interferometer for the LBT (NIL)
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LBT and Keck will test fundamentally different
methods of nulling

• Keck method - 180 degree rotation of one pupil
• requires 2 passes through beamsplitter
• automatic amplitude balance and 180 degree
  achromatic phase shift
• fields combined with 180? rotation
• exploits spatial filter to improve wavefront
• give 2 constructive and 2 destructive outputs
  (more if applied to TPF)
• LBT method - achromatic retardation of one pupil
• one pass of beamsplitter only, but requires
  achromatic phase retarder and explicit amplitude
  balance
• ZnS ZnSe achromat designed by Burge for full
  TPF range
• true imaging field around nulled star will work
  for 2-d TPF concepts requiring phase shift ? 180?
• outside achromatic range, can take advantage of
  non destructive, common path output to monitor
  phase difference
• will use continuous fine control of adaptive
  optics to maintain exact null, in detail across
  combined pupil, field around nulled star imaged
• The LBT has many characteristics in common with
  TPF
• LBT and Keck will explore and develop quite
  different and complementary nulling technologies
  applicable to TPF

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Satisfying the null condition in detail across
the combined pupil
The plots show phase and transmitted intensity
for the semi-transparent beamsplitter to be used
for LBT nulling. The phase difference is very
close to pi in the 10 micron band, for strong
destructive interference of the star. At 2.2
microns, though, the phase difference at the to
interferometer outputs is 90 degrees. The
outputs are both gray, and very sensitive to
small path length deviations from the null
condition. By controlling path to keep the to
gray outputs equal, we ensure that the null
condition is maintained at all times in the 10
micron band. In fact, pupil images will be
formed at the K band outputs, and a signal
derived to correct the adaptive optics for a good
null in detail across the pupil