APrèS-MIDI
Aperture Synthesis in the MID-Infrared (10 m) with the VLTI
B. Lopez, Ph. Mathias, P. Antonelli, N. Berruyer, Y. Bresson, O. Chesneau, M. Dugué, A. Dutrey, A. Glazenborg, U. Graser, Th. Henning, S. Lagarde, Ch. Leinert, G. Perrin, A. Roussel, E. Thiebaut
Acknowlegments : V. Coudé du Foresto, F. Delplancke, J. Gay, A. Glindemann, K. Meisenheimer, D. Mourard, R. Petrov, J. Steinacker, J.-C. Valtier
APreS-MIDI (4 VLTI beams -> 2 beams)
MIDI
Science Cases :
Active Galaxic NucleiDisc of Young Stellar ObjectsEnvelopes of evolved stars…
Le code de transfert radiatif, MC-TRANSF, exemple d’application : structure disque de la nébuleuse post-AGB ‘The Egg Nebula’.
Density law
Model parameters
• Apparent sizes of discs• Inclinations•Via models (assuming a geometry) : inner radius, optical depth, nature and typical size of dust, temperature, …• Repartition of the dust material, structures like gaps
Expected results by increasing difficulty :
APreS-MIDIOptical interface (4 beams -> 2 beams)
MIDI instrument
photometric beams or
?
Details of the main optics Top view
Miroirs segmentésD 100 mmF 700 mmDécalage 1 mm Collimateur
D 50 mmF 855 mm
Plan de renvoiD 100 mm
4 renvois plans D50 mm
Miroir à facetteD 10 mmTilt +/- 0.82°
Plane mirror
Pyramidal mirror
Segmented mirrors
Collimator
4 plane mirrors
The concept proposed : a pupil recombination (with tilt angles)
i (e-i(u+ui)(k+ki)P(u))
Considerations about the Signal to Noise Ratio
MIDI APreS-MIDIBeams 2 4
Baselines 1 6Background from 2 tel. 4 Beam splitter 1 0
Closure phases 0 3
per baseline :
SNR APreS-MIDI = 1/3 SNR MIDI
globally :
SNR APreS-MIDI = 6*(3 ?)*1/3 SNR MIDI
Additional background of about 15 % due to 5-8 mirrors added to the 30-35 mirrors of the VLTI(a lost of reflectivity is also to considere).
Background limited noise regime in the 40 pixel diameter pupil using the full N band in 0.1 second
Exposure time to increase when using filters external fringe tracking needed.
Expected performances of APreS-MIDI
Sensitivity (as for MIDI): N=4 with UTs, N=7-8 with UTs and the use of an external fringe tracking
Angular resolution: 10 mas at 10 microns
Recombination mode: pupil plane with tilt angle, up to 4 telescope beams allowed
Spectral mode: using narrow filters (R=30) with 4 beams (dispersed mode with 3 beams ?)
Field of view : corresponding to Airy disc sizes.
Expected imaging performance: one image reconstructed by aperture synthesis from about 10 hours of effective observation using 4 telescopes.
(Tuthill et al. 2000, ApJ 543, 284)
Simulated image at 10 m
Aperture synthesis at 10 m with the VLTI, expected efficiency
Simulated image at 10 m
Simulated image at 10 m 16 mas pixel size
3 telescopes reconstruced image
4 telescopes reconstruced image with MEM
4 telescopes, 6 nights
397 spectral densities,
264 phase closures, SNR=20
Status of the project
Concept appears valid
Studies in progress : coupling Zemax APreS-MIDI with Zemax MIDI; photometric beams; test and alignment devices and procedures; room avalaible in the VLTI laboratory; simulations of a serie of images for different classes of sources.
Phase A engineering study can start end 2004 (- end 2005) with good basis.
VLTI elements allowing the operation of a 4 beams instrument ?
Switchyard : OKDelay lines : OKLEONARDO for co-alignment : 2 beams, usable for 3 and 4 beamsFINITO : ‘3’ beams by now
Keck (2x10 m)80 m
(2002)
Large Binocular Telesccope (2x8 m)15 m
(2005)
ISI (3x1.5 m)72 m
(2003)
1) The VLTI has a potential that is unique for producing images, in particular in the 9-12 µm domain.2) The N Spectral band is of importance: between the optical domain and mm (ALMA)
3) Preliminary step before Mid-IR imaging in space (DARWIN, TPF)
MIDI/VLTI and other 10 µm interferometers