Multi-wavelength Imager Design & Performance

1. Multi-wavelength Imager Design & Performance R. Doyon, D. Lafrenière & C. Marois Université de Montréal S.Thibault, J-F Lavigne, M. Leclerc & M. Poirier Institut National d’Optique

2. Outline • Science Instrument Requirements • MWI concept & specifications • Optical design • Beam Splitter • Physical Optical Propagation simulations • Other observing modes for the MWI • External limitations to speckle suppression

3. TRIDENT PERFORMANCE on CFHT ~50 nm non-common path errors (simulation) Actual performance Reference star Shot + read noise limit Science Instrument Requirements 1) High speckle suppression. • Speckle attenuation tN/N, N : residual PSF noise after processing. N is the noise in the original PSF. • t  0.01 required near IWA and beyond the control radius (~1.3" radius) and near the magnitude limit (I~8) of the AO system. • Implementation must limit non-common path aberrations • TRIDENT performance without reference star:t ~ 0.3 • t ~ sDf/sf • sf : rms of common wavefront error (static component) • sDf : rms of differential wavefront error

4. Science Instrument Requirements 1) High speckle suppression. • Speckle attenuation tN/N, N : residual PSF noise after processing. N is the noise in the original PSF. • t  0.01 required near IWA and beyond the control radius (~1.3" radius) and near the magnitude limit (I~8) of the AO system. • Implementation must limit non-common path aberrations • TRIDENT performance without reference star:t ~ 0.3 • t ~ sDf/sf • sf : rms of common wavefront error (static component) • sDf : rms of differential wavefront error 2) Low-resolution spectroscopic capability to constrain both Teff and Log g. 3) Polarimetry mode for disk imaging

5. Teff and Log g diagnostics

6. (Teff, Log g) tracks (“COND” models) (400, 3.0) (800, 3.0) (400, 6.0) (800, 6.0) H-band diagnostic GL229b (T6)

7. MWI concept MWI cartoon • Hybrid concept of TRIDENT and MCDA (Marois et al 2004) • Image dissection with lenslet array eliminates non-common aberrations • PSF reformatted from micro-pupils • 4-way beam splitter yields one  per quadrant • No spectral cross-talk

8. Speckle attenuation vs cross-talk 0.5" 1" 2" 10 % 1 % 0.1 % Speckle attenuation is limited by wavelength cross-talk (Marois etal 2004, ApJL, November 1st)

9. MWI specifications • Detector: 2040x2040 Hawaii-2RG (18 um pixel) • Wavelengths: 1.52, 1.58, 1.64 and 1.70 µm • Bandwidth : 2% (R=50) • Unit magnification optical relay • f/5.6 square shaped lenslet array • Field of view (TBD) • 54 µm pitch (3 pixels), f/90 fore-optics  5.3"x5.3" • 72 µm pitch (4 pixels), f/120 fore-optics  4.0"x4.0"

10. Optical Design features (Details in INO-DN-030165-001.pdf) • f/5.6 micro-lens array (fused silica) • BaF2-SF6 collimator triplet • 15 mm “false” pupil at beam-splitter entrance • 4 different cameras (BaF2-SF6 air-space doublets). Total of 11 custom lenses for all channels. • Throughput: ~70% (beam-splitter included) • Diffraction-limited optics • Low-distortion (0.06-0.22 pixels) • Compact design (~310 mm)

11. Optical Layout (all channels)

12. Optical distortion Distortion varies slighly from one channel to another

13. 3D Layout

14. Image quality (spot size)

15. Image quality (Strehl)

16. Beam-splitter • Uses immersed dichroics (3) for maximum throughput • All pieces optically cemented • Challenging piece of optics • Requires steep (~2%) slope • Alternative designs • Separate dichroics + mirrors. • Lower throughput option to use standard 50/50 beam splitters à la TRIDENT

17. Example of a dichroic design

18. POP simulations • Goal is to quantify how spatial samples (micro-pupils) are contaminating one another and how this affects speckle attenuation performance. Also verify that the design is insensitive to non-common path aberrations. • Simulations done with Zemax & IDL • Difficult to generate well-correlated PSFs at various wavelengths in Zemax • On-going work

19. PSF at the micro-lens array PSF after the micro-lens array 95% ensquared energy within 2x2 box

20. Sensitivity to aberrations

21. Laboratory PSF (1.57 m) dissected by a micro-lens array (left) and its reconstruction (right). Lafrenière et al 2004, SPIE, 5492, in press.

22. Other modes for the MWI • Putting the beam-splitter and camera optics on a deployable mechanism would enable other observing modes. • Polarimetry • Replace beam-splitter + camera optics by polarizing beam splitters + optics to generate 4 broad band images with independent polarizing states. Main application for disks. • MWI at z, J and K. • Working at smaller  would require slower fore-optics  smaller FOV.

23. Limitations to speckle suppression • Atmospheric refraction • Without ADC, off-pupil optics have different optical footprints at various  yielding pupil shearing and PSF decorrelation. • Solution: ADC ideally located upstream of the AO system. • Refractive optics before micro-lens array (dichroic, ADC, cryostat window) • Very good optics required. • Structure in stellar spectrum within filter bandpass • These external effects cannot be corrected by the MWI or the IFU and must be quantified (simulated) in order to better specify the speckle suppression requirement of the science instrument.

24. Attenuation vs pupil shearing 1/100 pupil diameter 1/200 1/400 Marois 2004 (PhD thesis); Marois et al 2004, submitted to PASP.

25. Attenuation limit due to structure in stellar spectrum (solar spectrum) 5% 2% Filter bandpass 1% Marois 2004, (PhD thesis); Marois etal 2004, submitted to PASP.

26. Conclusions • The MWI is a very attractive option for the ExAOC science instrument. • Would do marvels on existing AO systems! • Advantages • Designed for high speckle suppression. • Relatively large FOV (5"x5"). • H-band MWI provides relatively good diagnostic of Teff and Log g. • Possible implementation of a polarimetry mode and other photometric bands. • Compact design. • Disadvantage • Limited spectroscopic capability. Geared for detection rather than characterization. • Remaining issues • Mitigate risks with beam-splitter. Further study needed. • More simulations/prototyping needed to determine speckle attenuation performance.

27. Reference Documents • INO-DN-030165-001.pdf: “MWI Conceptual Design Review” • INO-DN-030165-002.pdf: “MWI Performance Document” • INO-DN-030165-003.pdf : “MWI Management plan” • Marois 2004, PhD thesis, UdeM. • Marois et al 2004, ApJL, November 1st issue. • Marois et al 2004, PASP, submitted. • Lafrenière et al 2004, SPIE, 5492, in press.