I. A. The main PRIMA Objectives
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I. A. The main PRIMA Objectives |
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The scientific aim of PRIMA is triple: first to increase the sensitivity of the VLTI shifting its limiting magnitude, secondly to allow imaging of faint object with a high angular resolution and thirdly to access to a high precision astrometry mainly for planet detection. At the end of this part, we will give the publications concerning the objectives of the PRIMA facility.
Warning! The Netscape and Mozilla navigators don't recognize the greek letters, that could disturb the good reading of this page. PRIMA will allow VLTI instruments like MIDI or AMBER to observe objects up to 5 magnitudes fainter (cf fig.1) than in a single field mode (corresponding to a factor 100 onthe brightness of the star!). Thus it will enable to reach the magnitude of 19 with the UTs (cf fig.1).
Fig .1: Increasing limiting magnitude with PRIMA |
PRIMA will secondly allow reimaging such faint objects by knowing precisely the amplitude of the complex visibility and its phase.
The differential delay between the bright and faint star fringes is measured with the laser metrology system (Met or PRIMET). The bright star serves as a fringe tracking source and as a reference for the object fringe phase. This phase measurement with the visibility measurement, for a number of different baselines, enables object image reconstruction with a very high resolution (1 mas at 2.2 µm) on fainter objects than phase-closure techniques.
Fig. 2: Differential optical path length
Indeed, the optical path difference (OPD) for the bright object is:
where: B is the baseline (i.e. the distance betwe en the two telescopes), a the vector pointing to the primary object, f1 is the intrinsic phase of the bright star, A1 is caused by the atmospheric turbulences and L1 is the internal optical path difference.
In a similar way, the optical path difference for the faint object is:
where: B is the baseline (i.e. the distance betwe en the two telescopes), b the vector pointing to the secondary object, f2 is the the intrinsic phase of the faint object, A2 is caused by the atmospheric turbulences and L2 is the internal optical path difference.
Thus the difference between the two OPDs is (cf fig.2):
where
PRIMA high accuracy metrology and astrometric camera will also allow to measure relative angular positions of stars with a 10 µas accuracy. Detection of Jupiter like planets as far as 240 pc and characterization of gravitational micro-lensing events on the galactic center and on the Magellanic Clouds will then be possible (see scientific applications of PRIMA).
In a general way, in an astrometric measurement both object phases (f1 and f2) are supposed to be zero since a symmetrical intensity distribution is assumed for both objects. As in Imaging mode, DA can be considered to be zero for an observing time that is long enough. Therefore the difference of the white light fringe positions together with the measured DL gives the exact angular separation of the two objects.
Precisely, the PRIMA astrometric mode will allow observing simultaneously two fields separated by 2 to 60 arcsec, to detect and track the fringes on the brightest object, to detect the fringes on the faintest, and to measure the phase of the secondary set of fringes relative to the primary one, with an accuracy of l/1000 at 2 µm. Indeed, the differential delay is measured with a very high accuracy (5nm rms) and is related to the angle between the two observed stars. The 200m baseline would give a figure of merit of 10 µas for a 10'' separation angle, in only 30 min integration time (atmospheric anisoplanetism limit).
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More informations about the objectives of PRIMA"Scientific objectives of ESO's PRIMA Facility", F.Paresce et al., SPIE conference Interferometry for Optical Astronomy, August 2002, SPIE Vol.4838. “Phase-referenced imaging and micro-arcsecond astrometry with the VLTI”, F. Delplancke, S. Leveque, P.Kervella, A.Glindemann, L.D’Arcio, SPIE conference: "Astronomical Telescope and Instrumentation 2000", Munich (Germany), 25-31 March 2000, [4006-41] . "PRIMA: Study for a Dual Beam Instrument for the VLT Interferometer" A. Quirrenbach, V. Coudé du Foresto, G. Daigne, K.-H. Hofmann, R. Hofmann, M. Lattanzi, R. Osterbart, R. Le Poole, D. Queloz, F. Vakili, SPIE, vol. 3350, 1998. "PRIMA Technical Description and Implementation", F.Derie, F.Delplancke,A.Glindeman,S.Leveque, S.Menardi,F.Paresce,R.Wilhelm,K.Wirenstrand,Workshop "Hunting for Planets ", Lorentz center, Leiden University, 3-6 June 2002. Slides of the presentation. For the other publications, see the part: References and links.
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