miri 4-quadrant phase mask coronagraphs

1Exploring Strange New Worlds, Flagstaff, May 2011

MIRI 4-Quadrant Phase MaskCoronagraphs

Anthony Boccaletti

& C. Cavarroc, P.-O. Lagage, P. Baudoz

& the MIRI consortium


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


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%)


(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


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


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


Additional coronagraphic modes


Performances

DIFFRACTION ONLY


Performances

Raw image is background dominated

Still diffraction inside 2" in the residual image

F1140C + 4QPM


Performances

Just background limitedat 15.50mm

F1550C + 4QPM


Performances

Strong diffraction but still some bright planets

F0770W + Lyot barF0560W + Lyot bar


Performances


The real instrument !

- 4QPMs are implemented in the FM

- FM tests are starting now at RAL (UK)


Tests of MIRI-4QPM at 11.4 mm


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)


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 !


Operation : peakup

• Intermediate step (Gordon & Meixner in TA document)– Star is at at ~10" from center


– Move to a position at 0.5" from center with 20mas accuracy


– 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)


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

miri 4-quadrant phase mask coronagraphs

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