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MIRI 4-Quadrant Phase MaskCoronagraphs
Anthony Boccaletti
& C. Cavarroc, P.-O. Lagage, P. Baudoz
& the MIRI consortium
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Motivations for a mid IR coronagraph
• more to come with GPI / SPHERE / HiCIAO
- planets in young associations (<100 Myrs, <50-100
pc)
- long period / nearby stars
from RV surveys & GAIA
• molecular species :
Water bands: 6-8 mm Methane: 7.7 mm
Ammonia: 10.65 mm CO2: 15.0 mm
Silicates: 10.0 mm PAH: 11.4 mm
• improve constraints on atmospheric modeling
• at l > 5 mm the star to planet contrast is getting
more favorable
=> JWST and especially MIRI can produce unique observations (photometry, low/med res. spectra)
• no observation at l > 5 mm prior to JWST
• several YOUNG planets already imaged
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Instrumental choice
Seager et al. 2010
MIR
I sp
ect
ral r
ange
Brightness ratio is reduced
but ….
Angular resolution is degraded
small Inner Working Angle
to take advantage of mid IR
4QPM was (2002) the only
affordable technology (Rouan
et al. 2000)
4QPM gives the same IWA
as NIRCAM but at midIR
But 4QPMs are chromatic
narrow band filters (5%)
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(n-1).e = l/2
Issues : - thickness => l- size of transitions
Manufacturing
Several materials were tested :- ZnSe deposition- ZnSe etching- Ge etching- Diamond etching
7 mm
MIRI PSF @ 10 mm
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Main coronagraphic modes
Filter 1 (10.65 µm) : NH3 line
Filter 2 (11.4 µm) : continuum
Filter 3 (15.5 µm) : continuum
Filter 4 (23 µm) : cold silicates in disk
1 filter + 1 coronagraph
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Design of MIRI
monochromatic coronagraphs
4 masks in focal plane
NDLyot diaph.
+23 µm filter
4Q diaph.+
10.65 mm filter
4Q diaph.+
15.5 mm filter
4Q diaph.+
11.4 mm filter
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Performances
Raw image is background dominated
Still diffraction inside 2" in the residual image
F1140C + 4QPM
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Performances
Just background limitedat 15.50mm
F1550C + 4QPM
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Performances
Strong diffraction but still some bright planets
F0770W + Lyot barF0560W + Lyot bar
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The real instrument !
- 4QPMs are implemented in the FM
- FM tests are starting now at RAL (UK)
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Operation : peakup
Peakup :
Center the star onto the coronograph = calculate the offset between mask and PSF
1 / Determine the centre of the coronagraph (estimation done from the background)
2/ Use dedicated filters to avoid 4QPM attenuation
- Neutral density for bright stars (mag<4.5 in N band)
- N filter for fainter stars (mag<7.5 in N band)
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Operation : peakup
2 approaches for putting the star at the right place :
• Iterative approach (detailed in Celine's paper)– Star is at ~10" from center
– Measure centroid
– Move to the center with a 20mas accuracy
– Iterate to reach 5mas
Drawback : iteration can be too long
Solution : Stability of the observatory => acceptable overheads !
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Operation : peakup
• Intermediate step (Gordon & Meixner in TA document)– Star is at at ~10" from center
– Measure centroid
– Move to a position at 0.5" from center with 20mas accuracy
– Measure centroid
– Move to the center with a 5mas accuracy
Drawback : centroid is biased by the 4QPM itself at 0.05" < r < 0.6"
Solution : move the intermediate step at 1" instead providing the error is still 20mas
Combination of the 2 scenarios (reduce nb of iterations)
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Operation : calibration
• Subtraction of the speckle pattern is mandatory. Several techniques :
• Reference star : not very accurate, large overheads, impact of centering
• Roll : not very amplitude for MIRI wavelengths but could be useful at separations >1-2"
• Build local PSF reference with LOCI algorithm. Need a sample of targets observed in a similar fashion. Less stringent wrt centering