Fringe detection laser metrology for differential astrometric stellar interferometers

Rabien, S.; Gillessen, S.; Ziegleder, J.; Thiel, Markus; Gräter, A.; Haug, M.; Eisenhauer, F.; Perrin, G.; Brandner, W.; Straubmeier, C.

doi:10.1117/12.788099

Cited by 4 publications

(7 citation statements)

References 7 publications

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“…The fringe spacing and orientation is a function of the distance and position angle of the positions of the two fibers and thus of the celestial positions of SCI and FS. By means of the phase shifter the fringes can be sampled ('phase shifting interferometry') and the relative phase of the two beams can be recovered [19].…”

Section: Working Principlementioning

confidence: 99%

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GRAVITY: getting to the event horizon of Sgr A*

et al. 2008

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We present the second-generation VLTI instrument GRAVITY, which currently is in the preliminary design phase. GRAVITY is specifically designed to observe highly relativistic motions of matter close to the event horizon of Sgr A*, the massive black hole at center of the Milky Way. We have identified the key design features needed to achieve this goal and present the resulting instrument concept. It includes an integrated optics, 4-telescope, dual feed beam combiner operated in a cryogenic vessel; near infrared wavefront sensing adaptive optics; fringe tracking on secondary sources within the field of view of the VLTI and a novel metrology concept. Simulations show that the planned design matches the scientific needs; in particular that 10µas astrometry is feasible for a source with a magnitude of K=15 like Sgr A*, given the availability of suitable phase reference sources.

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Section: Working Principlementioning

confidence: 99%

“…The metrology system [19] will mainly be made of commercial components. The laser used for injection will operate around 1.9µm and needs a spectral stabilization since the wavelength of that laser ultimately is the ruler to which path length differences are compared.…”

Section: Metrologymentioning

confidence: 99%

GRAVITY: getting to the event horizon of Sgr A*

et al. 2008

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“…The basic working principle is the same as described for the preliminary design 4,5 . A laser beam is injected backwards at the two beam combiners, in a way that one of the two paths includes a phase shifter by which the relative delay between the two beams can be adjusted.…”

Section: Working Principlementioning

confidence: 99%

GRAVITY: metrology

Gillessen

Lippa

Eisenhauer

et al. 2012

Optical and Infrared Interferometry III

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GRAVITY is a second generation VLTI instrument, combining the light of four telescopes and two objects simultaneously. The main goal is to obtain astrometrically accurate information. Besides correctly measured stellar phases this requires the knowledge of the instrumental differential phase, which has to be measured optically during the astronomical observations. This is the purpose of a dedicated metrology system. The GRAVITY metrology covers the full optical path, from the beam combiners up to the reference points in the beam of the primary telescope mirror, minimizing the systematic uncertainties and providing a proper baseline in astrometric terms. Two laser beams with a fixed phase relation travel backward the whole optical chain, creating a fringe pattern in any plane close to a pupil. By temporal encoding the phase information can be extracted at any point by means of flux measurements with photo diodes. The reference points chosen sample the pupil at typical radii, eliminating potential systematics due differential focus. We present the final design and the performance estimate, which is in accordance with the overall requirements for GRAVITY.

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“…Since we want to measure the phase with the metrology system to 1 nm∼ λ/2000, the individual phase shifts have to be known to at least the same level of accuracy or better. The calibration routine for the phase shifter developed for this purpose makes use of similar identities as shown by Rabien et al 15 (2006), see also Rabien et al 3 (2008).…”

Section: Phase Shifter Calibrationmentioning

confidence: 99%

“…In particular, in a focal plane the small laser beam will form a larger PSF than the science light does. The preliminary design of the GRAVITY metrology follows an implementation which (Rabien et al 3 2008):…”

Section: Introductionmentioning

confidence: 99%

The fringe detection laser metrology for the GRAVITY interferometer at the VLTI

Bartko¹,

Gillessen²,

Rabien³

et al. 2010

SPIE Proceedings

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Interferometric measurements of optical path length differences of stars over large baselines can deliver extremely accurate astrometric data. The interferometer GRAVITY will simultaneously measure two objects in the field of view of the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory (ESO) and determine their angular separation to a precision of 10 µas in only 5 minutes. To perform the astrometric measurement with such a high accuracy, the differential path length through the VLTI and the instrument has to be measured (and tracked since Earth's rotation will permanently change it) by a laser metrology to an even higher level of accuracy (corresponding to 1 nm in 3 minutes). Usually, heterodyne differential path techniques are used for nanometer precision measurements, but with these methods it is difficult to track the full beam size and to follow the light path up to the primary mirror of the telescope. Here, we present the preliminary design of a differential path metrology system, developed within the GRAVITY project. It measures the instrumental differential path over the full pupil size and up to the entrance pupil location. The differential phase is measured by detecting the laser fringe pattern both on the telescopes' secondary mirrors as well as after reflection at the primary mirror. Based on our proposed design we evaluate the phase measurement accuracy based on a full budget of possible statistical and systematic errors. We show that this metrology design fulfills the high precision requirement of GRAVITY.

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Fringe detection laser metrology for differential astrometric stellar interferometers

Cited by 4 publications

References 7 publications

GRAVITY: getting to the event horizon of Sgr A*

GRAVITY: getting to the event horizon of Sgr A*

GRAVITY: metrology

The fringe detection laser metrology for the GRAVITY interferometer at the VLTI

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