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