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Op#cal Design and Image Post-Processing for the Direct Imaging of Exoplanets Tyler J. McCabe 1 , Dr. Brian A. Hicks 2,3 , and Dr. Neil T. Zimmerman 3 1 Florida InsAtute of Technology: Department of Physics and Space Sciences 2 University of Maryland College Park: Department of Astronomy 3 Goddard Space Flight Center: Code 667 Archival Legacy InvesAgaAons of Circumstellar Environments From 1997 through 2008, the Hubble Space Telescope carried and operated a Near Infrared Camera and MulA Object Spectrometer (NICMOS) which took images of stars being acAvely formed as well as their proto-planetary disks. A[er 2010, advanced post-processing techniques were created that made use of previously known instrument PSFs. On top of that, new algorithms such as Karhunen-Loève Image ProjecAon (KLIP) allowed a be_er PSF subtracAon and fainter objects to be detected from the previously reduced data [1][2]. This process led to newly reduced NICMOS images shown in Figure 1. The ALICE team came up with a list of 29 stars that had new objects that were revealed. The Spectro-Polarimetric High-Contrast Exoplanet REsearch (SPHERE) instrument on the Very Large Telescope (VLT) was the source of the follow up data for these stars. Raw SPHERE data was obtained from the European Southern Observatory’s (ESO) website and then the instrument specific data reducAon rouAnes were run to subtract the PSF’s using angular differenAal imaging (ADI). The processed SPHERE data would show that each candidate should fall into one of two categories: a background star or a gravitaAonally bound object. A[er the star has experienced proper moAon for roughly 10 years, background stars should appear fixed in the image and gravitaAonally bound objects should be in the same relaAve posiAon to the host star. Figure 2 shows a companion candidate that appears to be a background object as it falls almost perfectly within the predicted locaAon. On the other hand, SPHERE images of HD169142 showed that two of the three candidate objects appeared to land close to their predicted locaAons for a background star. However, the third object located in the region between the two predicAons (Figure 3). Single-Mode Fiber Array To image Earth-like exoplanets, reducing sca_ered starlight within the field of view is needed to reach adequate levels of contrast. These contrasts need to be on the order of 10 -10 in order to be able to see the light from the exoplanets themselves and not have it obscured by the light of the star. A Single-mode Fiber Array (SFA) helps reach this level of off-axis contrast, which in theory outperforms what is achievable with a pinhole array used in a comparable manner[3]. Figure 4 shows the Zemax layout of the SFA and complex amplitude control of the single-mode fiber array. The single-mode fiber array adds amplitude control to the wavefront control system when paired with a piston/Ap/Alt (PTT) segmented deformable mirror (DM). The Lyot stop in the SFA setup blocks the gaps between the hexagonal segments of the DM, which helps to eliminate the intrinsic errors from a segmented mirror. In pracAce, this setup will help to remove the residual speckles ,which may be further removed by post-processing including PSF subtracAon. Radial Shear Nulling Coronagraph The Radial Shear Nulling Coronagraph (RSNC) is being explored as a complementary alternaAve to the exisAng Visible Nulling Coronagraph (VNC) as a means of improving throughput and discovery space with opAmal applicaAon toward future 10m or larger aperture space telescopes. Similar to the VNC, the RSNC uses destrucAve interference in the recombinaAon of the beams in order to null out the light of the central star to allow fainter exoplanets to be imaged [4]. The design shown in figure 5, may also use a PTT DM. A pair of hybrid beamspli_er-polarizers biases the amplitude between the two arms of the interferometer. The two opAcal paths have a 10% magnificaAon difference between them causing a difference in electric field strength. In order to balance the amplitude, rotaAng the second beamspli_er allows less of the light from the DM arm is allowed to pass through. A rotaAon about the z-axis of 19.6° with an error of 0.042° (0.05%) was found to balance out the amplitudes between the two arms. Future Work ALICE : • Further research into HD169142 to see if companion candidates are gravitaAonally bound • Determine what errors caused the companion candidates to not appear in the middle of the predicted 0.5 arcsec circles • Parallax, minor ADI reducAon errors, and orbital moAon • Adapt reducAon pipeline specifically for HD169142 SFA and RSNC : • Lab tesAng with recently constructed SFA interferometer. • ConAnue acquiring RSNC components • Demonstrate 10 -8 – 10 -9 contrast in laboratory tesAng while illustraAng the effects of off-axis transmission on the PSF similar to what is shown in Figure 6. • Further explore the capabiliAes of the RSNC and SFA in modeling and experiment to moAvate incorporaAon with future space telescopes that will include direct imaging of Earth-like exoplanets in their list of science objecAves. • The RSNC takes an incremental step toward reaching the theoreAcal performance that was assumed in generaAng the simulated image shown in Figure 7. Acknowledgements I would like to thank the NASA Goddard Space Flight Center and the UniversiAes Space Research AssociaAon for providing me with the opportunity to have this internship. This research was made possible by support from a SAT/ TDEM-13 award (PI: B. Hicks). This research has made use of data reprocessed as part of the ALICE program, which was supported by NASA through grants HST-AR-12652 (PI: R. Soummer), HST-GO-11136 (PI: D. Golimowski), HST- GO-13855 (PI: E. Choquet), HST-GO-13331 (PI: L. Pueyo), and STScI Director’s DiscreAonary Research funds, and was conducted at STScI which is operated by AURA under NASA contrast NAS5-26555. References [1] Soummer R., Pueyo L., & Larkin J. 2012. The Astrophysical Journal Le_ers, 755, L28 [2] Choquet et al. 2014, Proc. SPIE, 9143, 57 [3] Hicks B.A. et al. 2015. Journal of Astronomical Telescopes, Instruments, and Systems 1(1), 019001 [4] Lyon, R. G., Hicks, B. A., Clampin, M., & Petrone, P., III 2015, arXiv:1504.05747 IntroducAon Directly imaging exoplanets requires a combinaAon of advanced post-processing techniques as well as cuqng edge opAcal system designs to limit noise, reach a smaller inner working angle, and overcome the contrast between the star and the exoplanet. Currently, the only exoplanets that can be directly imaged are biased toward nearby young systems with planets in long period, large semi-major axis orbits. The Archival Legacy InvesAgaAons of Circumstellar Environments (ALICE) program was started to look at data from the NICMOS coronagraph along with new post-processing techniques to use known instrument point spread funcAons (PSFs) to subtract the starlight. In the future, new technologies must be developed if Earth-like exoplanets are to be directly imaged. Contrast orders close to 10 -10 can be reached through new passive wavefront control components such as a Single-mode Fiber Array (SFA) and opAcal starlight suppression systems such as the Radial Shear Nulling Coronagraph (RSNC). Figure 3: Two images of HD169142 with predicted locaAons for the candidates being background objects (white) or gravitaAonally bound (green). Le[: Image taken on 06-08-2015 Right: Image taken on 07-04-2015 Figure 2: Reduced SPHERE image showing candidates in predicted background locaAons (white) compared to the gravitaAonally bound locaAons (green) Figure 4: Current SFA layout Figure 5: Current RSNC opAcal design Figure 7: Simulated view of the Solar System as observed from 10 pc with a 10m space telescope using RSNC technology Figure from [4] Figure 6: A slice through the simulated PSFs of a star and planet with modeled off-axis transmission of the RSNC assuming a 4-ring hexagonal aperture and 10% imbalance in magnificaAon (Figure credit: Brian Hicks) Figure 1: Reduced NICMOS images showing the newly discovered companion candidates of HD106797 (le[) and HD169142 (right) along with the graphs showing the detecAon limits as a funcAon of distance from the star Credit: ALICE Team Credit: ALICE Team
