MIRI 4-Quadrant Phase Mask Coronagraphs. Anthony Boccaletti & C. Cavarroc, P.-O. Lagage, P. Baudoz & the MIRI consortium

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1 MIRI 4-Quadrant Phase Mask Coronagraphs Anthony Boccaletti & C. Cavarroc, P.-O. Lagage, P. Baudoz & the MIRI consortium 1

2 Motivations for a mid IR coronagraph several YOUNG planets already imaged more to come with GPI / SPHERE / HiCIAO - planets in young associations (<100 Myrs, < pc) - long period / nearby stars from RV surveys & GAIA no observation at l > 5 mm prior to JWST 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) molecular species : Water bands: 6-8 mm Ammonia: mm Silicates: 10.0 mm Methane: 7.7 mm CO 2 : 15.0 mm PAH: 11.4 mm improve constraints on atmospheric modeling 2

3 MIRI spectral range Instrumental choice Seager et al 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%) 3

4 Manufacturing Issues : - thickness => l - size of transitions Several materials were tested : - ZnSe deposition - ZnSe etching - Ge etching - Diamond etching (n-1).e = l/2 7 mm MIRI 10 mm 4

5 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 5

6 Design of MIRI monochromatic coronagraphs 4 masks in focal plane ND Lyot diaph µm filter 4Q diaph mm filter 4Q diaph mm filter 4Q diaph mm filter 6

7 Additional coronagraphic modes 7

8 Performances DIFFRACTION ONLY 8

9 Performances F1140C + 4QPM Raw image is background dominated Still diffraction inside 2" in the residual image 9

10 Performances F1550C + 4QPM Just background limited at 15.50mm 10

11 Performances F0770W F0560W + Lyot bar Strong diffraction but still some bright planets 11

12 Performances 12

13 The real instrument! - 4QPMs are implemented in the FM - FM tests are starting now at RAL (UK) 13

14 Tests of MIRI-4QPM at 11.4 mm 14

15 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) 15

16 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! 16

17 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) 17

18 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 18