Download Stellar Intensity Interferometry: The Background John Davis Sydney Institute for Astronomy

Stellar Intensity Interferometry:
The Background
John Davis
Sydney Institute for Astronomy
School of Physics
University of Sydney
NSW, Australia
29 January 2009
Intensity Interferometry Workshop
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Personal Notes
I regret that I cannot make this presentation in person but an outline
of my involvement in stellar interferometry may be of interest:
•  I was a student at the University of Manchester when the
radio version of intensity interferometry was implemented
•  Although I wasn’t involved, I was at Jodrell Bank as a PhD
student and then as a Postdoc throughout the development
of the optical technique and the measurement of Sirius
•  In 1961 Hanbury Brown invited me to work on the Narrabri
Stellar Intensity Interferometer and no young postdoc in his
right mind would have said anything but “Yes please!”
•  I have been involved in optical stellar interferometry ever
since.
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Outline of Presentation
•  Robert Hanbury Brown’s original idea of intensity interferometry
and the role of Richard Twiss
•  The radio astronomy experiment and scintillation
•  Optical laboratory experiments
•  The measurement of Sirius by intensity interferometry
•  The Narrabri Stellar Intensity Interferometer
•  Plans for a Very Large Stellar Intensity Interferometer (VLSII)
•  The VLSII abandoned in favour of an amplitude interferometer
•  The Sydney University Stellar Interferometer (SUSI)
•  Some thoughts on the future of intensity interferometry
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The Origin of the Idea of Intensity Interferometry
•  Circa 1949, the angular sizes of the two brightest radio sources, Cygnus A
and Cassiopeia A, were unknown and some thought they were “radio
stars”. Robert Hanbury Brown (RHB) was determined to measure them
•  If these sources were galaxies their angular sizes would be of the order of
a minute of arc and easy to measure with a conventional interferometer
but, if they were stars, extremely long baselines would be needed and
RHB concluded that this was impossible with the available technology
•  RHB worried about it and had the following thought, in his own words, “If
the radiation from a discrete source in the sky is picked up at two different
places on Earth, is there anything besides the phase and amplitude of the
signals which we can compare to find the mutual coherence?”
•  He then visualised the “noise-like” signal seen by two separated observers
and realised that the noise corresponded to low-frequency fluctuations in the
intensity of the signal and convinced himself that the correlation between the
intensity fluctuations was a measure of their mutual coherence.
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The Entry of Richard Twiss
•  Although RHB had convinced himself with a simple analysis that his
idea of intensity interferometry was sound, he was not able to develop
the mathematical theory to establish the sensitivity himself
•  He sought help from a friend who put him in touch with Richard Twiss
(RQT), a gifted mathematician
•  After his initial analysis RQT announced to RHB “This idea of yours is
no good, it doesn’t work!”
•  It turned out that RQT had made a simple mistake in an integral and,
once corrected, he produced a rigorous and quantitative theory of
intensity interferometry
•  The next step was to develop a radio intensity interferometer to test
the technique and to measure Cygnus A and Cassiopeia A
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The Radio Intensity Interferometer Equipment
(Hanbury Brown, Jennison, & Das Gupta, Nature, 170, 1061, 1952)
Antenna Systems
Heterodyne receivers
tuned to 125 MHz
with Δf of 200 kHz
Square-law
detectors
Low-frequency filters
Δf from 1 to 2 kHz
Delay line
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Correlator
Intensity Interferometry Workshop
Radio link
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The Radio Intensity Interferometer Experiment
•  The output signal from the correlator is proportional to the visibility2
that would be observed with a Michelson type interferometer
•  Four baselines of different lengths and orientations were used to
determine the angular dimensions of Cygnus A and Cassiopeia A
(Hanbury Brown, Jennison & Das Gupta, Nature, 170, 1061, 1952)
•  Cygnus A was elongated with dimensions of approx. 0.5′ x 2′ and
Cassiopeia A was roughly symmetrical with a diameter of approx. 3.5′
•  It turned out that Graham Smith at Cambridge and Bernie Mills in
Australia had also measured these sources with conventional
radio interferometers and obtained similar results
(Mills, Nature, 170, 1063,1952; Smith, Nature, 170, 1065, 1952)
•  In RHB’s words they had built “a steam roller to crack a nut” because
the sources were clearly not stars – but that is another story
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Lessons from the Radio Intensity Interferometer
•  Matching paths in the arms of the interferometer was much easier
than for a conventional amplitude interferometer: the tolerance was
set by the maximum frequency of the filtered low-frequency signals
whose correlation was being measured, and not by the frequency of
the radio signal
•  It was observed that the correlation from Cassiopeia A was constant
in spite of violently scintillating signals due to the ionosphere
•  RQT found that they had overlooked, in the theoretical development,
perhaps the most astonishing and valuable feature of intensity
interferometry – it can work perfectly through a turbulent medium
•  RHB and RQT wondered at that point if intensity interferometry
could be made to work at optical wavelengths and measure the
angular diameters of stars
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The Start of Optical Intensity Interferometry
•  RHB and RQT envisaged an optical analogue of the radio intensity
interferometer
The radio intensity
interferometer
The envisioned optical
analogue
•  For the optical analogue to work the time of arrival of photons had to
be correlated at the two photocathodes for coherent incident light
•  This had never been observed and experimental proof was needed
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Optical Intensity Interferometry Laboratory Experiment
(Hanbury Brown & Twiss, Nature, 177, 27, 1956)
•  A simplified diagram of the experimental apparatus
•  A measurement was made with the photocathodes optically
superimposed and correlation was observed in close agreement
with the theoretical prediction
•  When the photocathodes were optically separated, by translating
C1 laterally with the slide, no correlation was observed
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The Controversy
•  These experimental results set the cat amongst the pigeons!
•  Experimentalists set out to check the results and concluded that
the RHB & RQT results were wrong
•  Adám, Jánossy & Varga (Acta Hungarica, 4, 301, 1955) and Brannen &
Ferguson (Nature, 178, 481, 1956) carried out experiments and did
not detect correlation – the latter went as far as stating that the
existence of correlation would call for “a major revision of some
fundamental concepts of quantum mechanics”
•  Neither group had evaluated the theoretical predictions for the
conditions of their experiments . RHB & RQT did the
calculations (Nature, 178,1447, 1956) and showed that Adám et al.
would have needed to integrate for >1011 years and Brannen &
Ferguson for >1000 years to achieve a S/N ratio of 3!
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More on the Controversy
•  The two negative experiments used a photon coincidence counting
technique and RHB & RQT showed that they were simply too
insensitive to record correlation
•  Twiss, Little & Hanbury Brown (Nature, 180, 324, 1957) repeated the
Brannen & Ferguson coincidence counting experiment with a
brilliant light source of narrow spectral bandwidth They not only
measured their predicted correlation but showed that the chance
of it being the result of a random noise fluctuation was <1 in 1015
•  Purcell (Nature, 178, 1449,1956) adopted a different approach to the
theory and his analysis of the three experiments came to the
same conclusions as RHB & RQT
•  In parallel with dealing with the controversy RHB decided to
measure the angular diameter of a main-sequence star (Sirius) to
demonstrate the astronomical potential of intensity interferometry
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The Measurement of Sirius - I
Simplified diagram of
the apparatus
The starlight collectors –
two World War II 1.56 m
diameter searchlights
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The Measurement of Sirius - II
•  The measurements were made in conditions of severe scintillation in
the winter of 1955-6 – wet, cold & muddy!
•  The maximum elevation of Sirius at transit was 20 degrees and the
observations were made at elevations between 15.5 and 20 degrees
Although the measured angular
diameter of 7.1±0.55 mas is larger
than the currently accepted value of
~6.0 mas it was a remarkable
achievment
(Hanbury Brown & Twiss, Nature, 178, 1046, 1956)
This measurement showed beyond doubt the potential of stellar intensity
interferometry and led to the Narrabri Stellar Intensity Interferometer
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Simplified diagram of the Narrabri Stellar
Intensity Interferometer (NSII)
P1 and P2 are
Photomultipliers
f1 and f2 are identical
wide-band filters
the correlator
(some details later)
Output to printer
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The Narrabri Stellar Intensity Interferometer
(NSII)
The general layout of the NSII
•  The maximum baseline, set by the track diameter of
188 m, was chosen to resolve the O5 star ζ Puppis
•  Circular track so no delay compensation as the baseline
was kept perpendicular to the star’s direction
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The Narrabri Stellar Intensity Interferometer (NSII)
Stellar Observations (1964-1972)
Reflectors
Baseline
Catenaries (Signal & power cables)
Control
Building
Reflector
Garage
188 m diameter track
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The NSII Reflectors
Diameter = 6.5 metres
252 individual hexagonal
mirrors
251 aligned on the signal
detector and 1 on a
separate star guidance
detector
•  The individual mirrors were spherical and were mounted on paraboloidal frames.
•  Given the tolerance on the radius of curvature, the shorter radius mirrors were
used at the centre and the longer radius ones towards the edge of the
paraboloids.
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The Control Desk of the NSII (circa 1967)
JD in control!
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The Evolution of Control Desks!
The Analogue Control Desk of
the NSII
(circa 1967)
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The Digital Control Desk of SUSI
(2008)
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The Correlator
•  The function of the correlator was to measure the correlation
between the fluctuations in the anode currents of the
photomultiplier detectors at the foci of the two reflectors
•  The correlator multiplied the fluctuations in the two channels
together and the correlation was a unidirectional output
superimposed on random noise
•  The r.m.s. signal to noise ratio at the output of the multiplier
was very small (<1 in 105 for a bright unresolved star)
•  The output of the correlator was extremely sensitive to DC
gain drifts, pick up, and cross-coupling. Phase switching
techniques were essential to overcome these problems as
shown on the next slide
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Block diagram of the correlator
10 second phase switch to
minimise the effect of drift
in circuits and counter
false correlation due to
pick up and coupling
between circuits
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5 kHz phase switch to
counter stability
problem of high gain
DC ampllifiers
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The NSII Correlator
Note the size!
Originally all thermionic
valves – transistors were
not an option in 1961!
The output
printer
Transistorised sequence
timer that replaced a
mechanical/microswitch
system circa 1965
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The Correlator Output
•  The integrated correlation was printed every 100 seconds and plotted
by the observer as shown in the example below for 20th May 1965 for
observations of β Crucis
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The Correlator Output
•  The integrated correlation was printed every 100 seconds and plotted
by the observer as shown in the example below for 20th May 1965 for
observations of β Crucis at baselines of 32.7 m and 94.2 m
“Dummy” run β Cru: 32.7m β Cru: 94.2m
“Dummy” run
“Dummy” Runs
Correlation
Between observations
the detectors were
exposed to the same
light levels as from the
star using incoherent
artificial sources to
monitor any drift in the
system
Number of 100 second integration cycles
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Some Key Points
•  The measured correlation depends on the gain of the system.
Calibrated with a standard source of wide band noise fed
simultaneously into both channels in place of the photomultiplier
outputs before and after each night of observations
•  The scale of the correlation depends on instrumental parameters
and the scale changes if, for example, the detectors are changed
•  Hence it is necessary to measure both short and long baselines
with the same instrument parameters – also identifies binaries
•  Large reflectors may partially resolve the source and this must
be taken into account as we did
•  The NSII had several changes in instrumental parameters and
reducing all results to a consistent scale was a tedious exercise
•  Providing sensitivity is adequate, serious consideration should be given
to using calibration sources, as is done for amplitude interferometry
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The Achievements of the NSII
•  Measured the angular diameters of 32 stars for spectral types from O5
to F8 resulting in the effective temperature scale for early-type stars
(Hanbury Brown, Davis & Allen, MNRAS, 167, 121, 1974; Code, Davis, Bless & Hanbury Brown, ApJ, 203, 417, 1976)
•  Made the first interferometric-spectroscopic study of a double-lined
spectroscopic binary (α Vir) (Herbison-Evans et al., MNRAS, 151, 161, 1971)
•  Carried out a number of exploratory experiments including:
  Detected previously unsuspected binary stars
  Measured the angular size of the emission envelope around the WolfRayet star γ2 Vel in ionised carbon lines
  Measured the effects of Cerenkov light pulses on the NSII
  Attempted to detect a corona around β Ori in polarised light
  Attempted to measure limb-darkening for Sirius
  Attempted to measure the rotational distortion of Altair
•  The signal-to-noise was insufficient to obtain astrophysically
significant results for the last three experiments but they illustrated
the potential of high angular resolution stellar interferometry
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The Hiatus at the Conclusion of the NSII Programme
•  RHB decided to close the NSII stellar programme in 1972 when further
observations would have been of low weight (due to low S/N) and
would not have added significantly to the results already obtained
•  He planned to build a 2 m telescope and use it to demonstrate the
capabilities of the new detectors that were being developed at that
time - but I persuaded him that we should build on our experience
and develop a Very Large Stellar Intensity Interferometer (VLSII)
•  We carried out a detailed study of the science that we would want to
do, including measuring the pulsations of Cepheids, and concluded
that we would need to reach a visual magnitude of >+7
•  As an aside, in parallel with this we used the NSII in a collaboration
with the CfA to detect atmospheric Cerenkov light from extensive air
showers (Grindlay et al., ApJ, 197, L9, 1975; Grindlay et al., ApJ, 201, 82, 1975)
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A Proposal for a VLSII - I
•  Based on our study of the sensitivity required for the science we
were interested in, we developed a proposal for a Very Large
Stellar Intensity Interferometer (VLSII)
•  There were no unknowns except the achievable sensitivity
•  The sensitivity parameters at our disposal were:
  The
  The
  The
  The
light collecting area (A)
quantum efficiency of the detectors (α)
radio frequency bandwidth of the signals (Δf)
number of multiplexed optical channels (N)
S/N ∝ A.α √ Δf.N
•  α and Δf were set by the detectors and A and N were
at our disposal – the latter limited by crude optics
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A Proposal for a VLSII - II
•  Initially we started with extremely optimistic predictions about sensitivity
based, in part, on predictions by RCA about future photomultipliers and
on optimistic numbers of optical channels at the foci of the reflectors
Parameter
A (m2)
Δf (MHz)
N
α (%)
Gain
mlimit
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NSII
Optimistic
VLSII
30
80
1
25
1
160
1000
10
30
72
+2.5
+7.1
N was based on a study of
polarising and dichroic
beamsplitters and was
limited by the crude optics
of the reflectors. The
specifications of the latter
were set by the maximum
funds we estimated might
be achievable.
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A Proposal for a VLSII - III
The basic configuration of the proposed VLSII
Aligned to observe a
star in the zenith
Aligned to observe a
star at ~70o elevation
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A Model of the Proposed VLSII
•  Two 10m diameter siderostats
in each arm
•  Multi-spectral channels at the
foci of fixed paraboloids
•  1 km long railway tracks
JD and RHB with the model of the VLSII
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The Model of the Proposed VLSII
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Second Thoughts on the Proposal for a VLSII
•  As we developed the design we revised our estimates of the
sensitivity to represent more realistically what we believed
could be achieved in practice within a reasonable timescale
Parameter
A (m2)
Δf (MHz)
N
α (%)
Gain
mlimit
NSII
30
80
1
25
1
Optimistic
VLSII
160
1000
10
30
72
Realistic
VLSII
160
200
6
25
21
+2.5
+7.1
+5.8
A limiting magnitude of +5.8 did not meet our needs
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Second Thoughts on the Proposal for a VLSII
•  As noted on the previous slide, a limiting magnitude of +5.8 did not
meet our needs
•  I was concerned that a modernised form of Michelson’s classical stellar
interferometer might be more sensitive and I persuaded RHB that we
should make a comparison of the two techniques
•  We carried out a detailed comparison and consulted RQT who had
been developing a small scale modern Michelson interferometer in
Italy (I had worked on an early version with RQT in England during a
sabbatical)
•  Our study showed that a modern Michelson (amplitude) interferometer
promised greater sensitivity and we decided to abandon the VLSII and
RHB left me to develop a prototype modern amplitude interferometer
•  Little did we realise how long it would take amplitude interferometry
to reach the sensitivity we were after!
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The Sydney University Stellar Interferometer 12.4 m Prototype
150 mm diameter southern siderostat & relay mirrors.
(The northern siderostat is hidden by the building)
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The Sydney University Stellar Interferometer 12.4 m Prototype
•  The prototype was used to develop and test the various sub-systems
needed for a large long-baseline amplitude interferometer
•  These included wavefront tip-tilt detection & correction, dynamic optical
path length compensation, and rapid signal sampling & processing
•  At the time, in the 1970s and early 1980s, we were pushing the
boundaries of technology and some aspects were only just possible
•  We successfully demonstrated the feasibility of our approach with a
measurement of the angular diameter of Sirius that was in good
agreement with the NSII value but achieved in a fraction of the
observing time (Davis & Tango, Nature, 323, 234, 1986)
•  Based on this success we designed and raised the funds to build
the Sydney University Stellar Interferometer (SUSI)
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The Sydney University Stellar Interferometer (SUSI)
Seen from the northern end of its 640 m North-South baseline array
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SUSI
An input station
& siderostat
Blue beam combination
system
Optical Path Length Compensator
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Red beam combination
system
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SUSI Parameters and Status
•  Baselines: 5 m, 10 m then in ~√2 steps to 640 m (5-160 m fully
operational)
•  Apertures: 20cm diameter siderostats, beam diameter 14cm
•  Spectral Range: 430 nm < λ < 950 nm
The following beam-combination systems have been used for the
scientific programme to date but are in the process of being replaced:
•  Blue beam-combination system: 430-530 nm (Δλ: 1-4 nm) Blimit ~ +2.5
  Early-type stars, early-type binaries
•  Red beam-combination system: 530-950 nm (Δλ: 5-10%) Rlimit ~ +5
  Late-type stars, binaries, Cepheids
SUSI is being upgraded for remote operation with a new beam-combination
system (PAVO) developed in a collaboration for both SUSI and CHARA
•  SUSI PAVO has 10 parallel spectral channels, spatial filtering & mlimit ~+7
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Some Highlights of the SUSI Science Programme
  Measurement of the outer scale of turbulence
  Accurate stellar angular diameter measurements (<1%)
  Combined interferometric-spectroscopic studies of binary
systems
  First direct mass determinations of masses of β CMa stars
(in spectroscopic binaries)
  Measurement of the angular diameter variations and, in
combination with spectroscopic radial velocities,
determination of distances and mean radii of Cepheids
  First interferometric spectropolarimetry
  First combined interferometric-asteroseismological study
to determine the mass of a single star
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Some Random Thoughts on II
•  Any thoughts I have are not profound, and have no doubt already
been considered by others
•  The following are some topics for consideration that I will expand
on in the next slides:
1.  What science is possible with II that cannot be done with AI?
2.  What is the sensitivity limit for a modern Intensity Interferometer
and how does it compare with current Amplitude Interferometers?
3.  Methods of calibrating measurements of correlation
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Some Random Thoughts on II - 1
First, a comment:
An often quoted advantage of II is that the problems were solved
with the NSII and that there are no unknowns
Although it has taken a long time to achieve, I believe that the
same can now be said of AI
1.  What science is possible with II that cannot be done with AI?
Some possible stellar programmes:
 
 
 
 
 
Angular diameters and limb-darkening of single stars
Binary stars, particularly double-lined spectroscopic binaries
Cepheid variables
Oblateness of rapidly rotating stars
Emission line observations
All these programmes have been, and are being addressed by current amplitude
interferometers and I do not see where II could make a significant contribution
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Some Random Thoughts on II – 1 (cont.)
It has been suggested that AI cannot operate at the short wavelengths
and long baselines needed for the hottest stars because of increasing
seeing effects. I do not believe this is true for the following reasons:
  Blimit ~ +2.5 for SUSI but this was mainly due to the detection
technique.
  I had long-term plans to upgrade the blue system and would have
easily reached Blimit +7 - even with the small SUSI beam diameter.
  Furthermore, we have shown that the outer scale of turbulence is
only a few tens of metres and seeing effects will remain constant
beyond that (Davis et al., MNRAS, 273, L53, 1995)
There are non-stellar programmes that may be possible with II but not
with AI – I have not had time to study these possibilities but they must
be carefully evaluated taking into account what is possible with current
and future amplitude interferometers
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Some Random Thoughts on II - 2
2.  What is the sensitivity limit for a modern II and how does it compare
with current AIs?
•  In spite of the earlier comment regarding “no unknowns”, there are
uncertain factors entering the calculations if the large light collectors
developed for very high energy gamma-ray observations are to be
used, including:
  Point–spread function – large, admitting sky background and adding
noise, reducing mlimit
  Shape of the tesselated surface that may give a significant spread in
path lengths and hence limit the bandwidth that can be used
  Although not directly a sensitivity factor, the pointing accuracy of the
reflectors must be good enough for II
•  For these reasons I have not attempted to carry out independent
sensitivity calculations but it would appear that with existing arrays,
the limiting magnitude would be in the range +6 to +8
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Some Random Thoughts on II – 2 (cont.)
•  A dedicated instrument such as that proposed by Erez Ribak would have
a much fainter mlimit, but would be hard to fund in the face of the
achievements of AI, unless a compelling scientific case could be made
A summary of selected long-baseline optical/IR amplitude interferometers
Acronym
SUSI
ISI
NPOI
CHARA
Location
Narrabri, Australia
Mt. Wilson, USA
Flagstaff, USA
Mt. Wilson, USA
Number
Aperture
Maximum
Wavelength
Instrument
of
Diameter
Baseline
Range
Apertures
(m)
(m)
(mm)
2
0.14
640
0.43-0.53
Blue system
0.53-0.95
0.6-0.9
Limiting
Notes
Magnitude
Status
*
B ~+2.5
Superseded
W
Red system
R ~+5
Superseded
W
PAVO
R ~ +7
C
W
2
1.65
70
10
--
?
6 (4)
0.12 (0.35)
437 (38)
0.45-0.85
--
?
6
1.0
330
0.45-2.4
W
Several other
W
0.4-0.9
VEGA
R ~ +8
instruments
W
0.62-0.9
PAVO
R ~ +10
inc. imaging
W
Only 1 baseline
W
Keck
Mauna Kea, Hawaii
2 (4)
10
80
2.2-10
--
?
VLTI
Cerro Paranal, Chile
4 (4)
8 (1.8)
130 (200)
1.0-10
MIDI (8m)
N ~+4
W
MIDI (1.8m)
N ~+0.7
W
MRO
New Mexico, USA
6 (+4)
1.4
# Spectral band not given
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0.6-2.4
AMBER (8m)
H & K ~ +7
Imaging. Fainter
W
AMBER (1.8m)
H & K ~ +5
with PRIMA tracking
W
PRIMA
K ~ +8-+11
--
~ +14 #
C
Imaging
B
* W = Working; C = Commissioning; B = Being built
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Some Random Thoughts on II – 2 (cont.)
Acronym
SUSI
ISI
NPOI
CHARA
Location
Narrabri, Australia
Mt. Wilson, USA
Flagstaff, USA
Mt. Wilson, USA
Number
Aperture
Maximum
Wavelength
of
Diameter
Baseline
Range
Instrument
Limiting
Notes
Magnitude
Status
*
Apertures
(m)
(m)
(mm)
2
0.14
640
0.43-0.53
Blue system
B ~+2.5
Superseded
W
0.53-0.95
Red system
R ~+5
Superseded
W
0.6-0.9
PAVO
R ~ +7
C
2
1.65
70
10
--
?
W
6 (4)
0.12 (0.35)
437 (38)
0.45-0.85
--
?
6
1.0
330
0.45-2.4
0.4-0.9
VEGA
R ~ +8
W
Several other
W
instruments
W
0.62-0.9
PAVO
R ~ +10
inc. imaging
W
Keck
Mauna Kea, Hawaii
2 (4)
10
80
2.2-10
--
?
Only 1 baseline
W
VLTI
Cerro Paranal, Chile
4 (4)
8 (1.8)
130 (200)
1.0-10
MRO
New Mexico, USA
6 (+4)
1.4
# Spectral band not given
340
0.6-2.4
MIDI (8m)
N ~+4
MIDI (1.8m)
N ~+0.7
W
AMBER (8m)
H & K ~ +7
Imaging. Fainter
W
AMBER (1.8m)
H & K ~ +5
with PRIMA tracking
W
PRIMA
K ~ +8-+11
--
~ +14 #
W
C
Imaging
B
* W = Working; C = Commissioning; B = Being built
•  Although the entries in the table are incomplete, inspection shows that
magnitudes of +7 and fainter are being achieved by several instruments
including imaging
•  Short wavelengths and very long baselines are not as well represented
but are feasible
29 January 2009
Intensity Interferometry Workshop
47
Some Random Thoughts on II - 3
3.  Methods of calibrating measurements of correlation
i.  Split the light with a beamsplitter to two detectors at the focus of a
reflector and measure the correlation between the signals –
corresponding to zero baseline. This would lose a factor of 2 in S/N.
ii.  Use the technique adopted for AI of alternating observations of the
target with observations of calibrators - sources of relatively small
angular size or of known angular size
iii.  Do as was done with the NSII and measure long and short baselines
and fit the expected transform. Provided no changes are made to
the instrument the zero baseline correlation value can be established
for single stars and used to detect binary systems etc.
In all cases the effects of partial resolution by the large reflectors would
need to be taken into account
29 January 2009
Intensity Interferometry Workshop
48
SUMMARY
The following are my personal conclusions - but I don’t expect everyone
to agree with them!
•  In spite of technical advances it is not obvious that II will
achieve greater sensitivity than amplitude interferometers that
exist or, in the case of the MRO, are under construction
•  AI is not necessarily limited in resolution by seeing and, if the
existing longer baselines (up to 640 m) of SUSI are brought on line,
SUSI-PAVO will be capable of measuring some of the hottest stars
•  I suspect that it is unlikely that II would contribute significantly to
stellar studies
•  The future of II as an astronomical technique is dependent on
achieving the sensitivity for programmes that it can do and that
AI cannot
29 January 2009
Intensity Interferometry Workshop
49
The End
29 January 2009
Intensity Interferometry Workshop
50