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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 1 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. 29 January 2009 Intensity Interferometry Workshop 2 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 29 January 2009 Intensity Interferometry Workshop 3 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. 29 January 2009 Intensity Interferometry Workshop 4 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 29 January 2009 Intensity Interferometry Workshop 5 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 29 January 2009 Correlator Intensity Interferometry Workshop Radio link 6 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 29 January 2009 Intensity Interferometry Workshop 7 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 29 January 2009 Intensity Interferometry Workshop 8 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 29 January 2009 Intensity Interferometry Workshop 9 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 29 January 2009 Intensity Interferometry Workshop 10 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! 29 January 2009 Intensity Interferometry Workshop 11 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 29 January 2009 Intensity Interferometry Workshop 12 The Measurement of Sirius - I Simplified diagram of the apparatus The starlight collectors – two World War II 1.56 m diameter searchlights 29 January 2009 Intensity Interferometry Workshop 13 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 29 January 2009 Intensity Interferometry Workshop 14 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 29 January 2009 Intensity Interferometry Workshop 15 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 29 January 2009 Intensity Interferometry Workshop 16 The Narrabri Stellar Intensity Interferometer (NSII) Stellar Observations (1964-1972) Reflectors Baseline Catenaries (Signal & power cables) Control Building Reflector Garage 188 m diameter track 29 January 2009 Intensity Interferometry Workshop 17 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. 29 January 2009 Intensity Interferometry Workshop 18 The Control Desk of the NSII (circa 1967) JD in control! 29 January 2009 Intensity Interferometry Workshop 19 The Evolution of Control Desks! The Analogue Control Desk of the NSII (circa 1967) 29 January 2009 The Digital Control Desk of SUSI (2008) Intensity Interferometry Workshop 20 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 29 January 2009 Intensity Interferometry Workshop 21 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 29 January 2009 5 kHz phase switch to counter stability problem of high gain DC ampllifiers Intensity Interferometry Workshop 22 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 29 January 2009 Intensity Interferometry Workshop 23 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 29 January 2009 Intensity Interferometry Workshop 24 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 29 January 2009 Intensity Interferometry Workshop 25 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 29 January 2009 Intensity Interferometry Workshop 26 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 29 January 2009 Intensity Interferometry Workshop 27 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) 29 January 2009 Intensity Interferometry Workshop 28 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 29 January 2009 Intensity Interferometry Workshop 29 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 29 January 2009 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. Intensity Interferometry Workshop 30 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 29 January 2009 Intensity Interferometry Workshop 31 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 29 January 2009 Intensity Interferometry Workshop 32 The Model of the Proposed VLSII 29 January 2009 Intensity Interferometry Workshop 33 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 29 January 2009 Intensity Interferometry Workshop 34 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! 29 January 2009 Intensity Interferometry Workshop 35 The Sydney University Stellar Interferometer 12.4 m Prototype 150 mm diameter southern siderostat & relay mirrors. (The northern siderostat is hidden by the building) 29 January 2009 Intensity Interferometry Workshop 36 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) 29 January 2009 Intensity Interferometry Workshop 37 The Sydney University Stellar Interferometer (SUSI) Seen from the northern end of its 640 m North-South baseline array 29 January 2009 Intensity Interferometry Workshop 38 SUSI An input station & siderostat Blue beam combination system Optical Path Length Compensator 29 January 2009 Red beam combination system Intensity Interferometry Workshop 39 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 29 January 2009 Intensity Interferometry Workshop 40 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 29 January 2009 Intensity Interferometry Workshop 41 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 29 January 2009 Intensity Interferometry Workshop 42 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 29 January 2009 Intensity Interferometry Workshop 43 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 29 January 2009 Intensity Interferometry Workshop 44 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 29 January 2009 Intensity Interferometry Workshop 45 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 29 January 2009 340 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 Intensity Interferometry Workshop 46 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