Transcript Imaging with Apres-MIDI

APrèS-MIDI
Aperture Synthesis in the MID-Infrared (10 mm) 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
MIDI
APreS-MIDI
(4 VLTI beams -> 2 beams)
Science Cases :
Active Galaxic Nuclei
Disc of Young Stellar Objects
Envelopes of evolved stars
…
Le code de transfert radiatif, MC-TRANSF, exemple
d’application : structure disque de la nébuleuse post-AGB
‘The Egg Nebula’.
Model parameters
Density law
Expected results by increasing difficulty :
• 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
MIDI instrument
APreS-MIDI
Optical interface (4 beams -> 2 beams)
?
l1
4 photometric
beams or l2 ?
Details of the main optics
Top view
Pyramidal
Miroir à facettemirror
Segmented
mirrors
Miroirs
segmentés
D 100 mm
F 700 mm
Décalage 1 mm
Collimator
Collimateur
D 50 mm
F 855 mm
D 10 mm
Tilt +/- 0.82°
Plane
Plan
demirror
renvoi
D 100 mm
4 renvois plans
4 plane mirrors
D50 mm
The concept proposed : a pupil recombination (with tilt angles)
Si (e-i(u+ui)(k+dki)P(u))
Considerations about the Signal to Noise Ratio
Beams
MIDI
2
APreS-MIDI
4
Baselines
1
Background from 2 tel.
Beam splitter
1
6
4
0
per baseline :
Closure phases
3
SNR APreS-MIDI = 1/3 SNR MIDI
0
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.
Simulated image at 10 mm
(Tuthill et al. 2000,
ApJ 543, 284)
Aperture synthesis at 10 mm with the VLTI, expected efficiency
Simulated image at 10 mm
Simulated image at 10 mm
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 : OK
Delay lines : OK
LEONARDO for co-alignment : 2 beams, usable for 3 and 4 beams
FINITO : ‘3’ beams by now
MIDI/VLTI and other 10 µm interferometers
Keck (2x10 m)
80 m
(2002)
ISI (3x1.5 m)
72 m
(2003)
Large Binocular Telesccope (2x8 m)
15 m
(2005)
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)