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A Clear View of Distant Worlds

Yesterday’s post on UMBRAS and occulter technology focuses attention on the characteristics of light, some of them counter-intuitive but well demonstrated. And since we’ve also been talking recently about the nearby star Epsilon Eridani, I’ve chosen an image of that star to illustrate some of the problems with planetary detections. What you see below is via Massimo Marengo (Harvard-Smithsonian Center for Astrophysics), who has done such outstanding recent work on untangling the riddle of Epsilon Eridani’s debris disk.

Epsilon Eridani in false color

This is a false color image with red, yellow, green and blue representing different infrared wavelengths. I ran the same image last summer, when Marengo posted it on his own weblog (he had used it to illustrate his team’s work in a presentation at the American Astronomical Society meeting in San Diego).

Image: A false-color infrared image of Epsilon Eridani. Credit: Massimo Marengo (CfA).

What I want to single out here are the artifacts in the image. The red/orange cross is produced by electronic effects inside the collector, while the green and blue spikes are caused by diffraction in the optical system. In other words, this is one ravishingly beautiful image, but it’s nothing like what the eye would see if you were able to view Epsilon Eridani close up, and not just because it’s working at infrared wavelengths. The idea behind both coronograph and occulter studies is to find ways to reduce or eliminate these distortions. Diffraction, for example, happens because light is actually bent around an intervening object; using a basic telescope with an occulting disk in its focal plane (a coronagraph), you wind up with a series of concentric rings and a bright spot in the center, all of which need to be suppressed.

It’s a tricky challenge, and one being studied in many ways, from using square apertures and varying the shape of the occulting object to creating deformable mirrors that reduce scattered light. But external occulters have significant advantages, including elimination of the scattering problem, theoretically better light suppression, and the ability to use them with any properly placed telescope. Moreover, the target can be placed anywhere in the image plane. A complete list of pros and cons contrasting internal coronagraphs with external occulters can be found at the UMBRAS site.

In any event, the more I think about the recent Terrestrial Planet Finder funding woes, the more I think the situation will result in better science. Terrestrial Planet Finder seemed, not so long ago, to be firming up around an internal coronagraph design that was more costly than competing occulter possibilities and, it seems likely, not capable of the same level of performance, at least when compared to the larger occulter options. We should all be glad that external occulter designs and new missions are now back in the hunt as we try to design the technologies that will give us our first actual images of distant exoplanets.

Comments on this entry are closed.

  • qraal May 17, 2006, 17:16

    Hi Paul

    New star system at 41 light years – 3 Neptune mass planets and an asteroid belt according to the ESO…


    …the outer planet is in the habitable zone of the star.

    Over 12 parsecs is a long, long way in human terms. Even a lightspeed drive makes first science results a lifetime away. Laser-sails at 0.1-0.3 cee put it at 410 to 130 years away, with a 41 year wait for results.

    Hopefully the big scopes will be built and we can actually see the planets “up close” one day soon-ish.


  • Administrator May 17, 2006, 18:50

    Yes, and it’s quite a catch. More on this discovery tomorrow.

  • ljk July 7, 2008, 23:11

    Prototyping coronagraphs for exoplanet characterization with SPHERE

    Authors: Anthony Boccaletti, Lyu Abe, Jacques Baudrand, Jean-Baptiste Daban, Richard Douet, Geraldine Guerri, Sylvie Robbe-Dubois, Philippe Bendjoya, Kjetil Dohlen, Dimitri Mawet

    (Submitted on 4 Jul 2008)

    Abstract: The detection and characterization of extrasolar planets with SPHERE (Spectro Polarimetric High contrast Exoplanet REsearch) is challenging and in particular relies on the ability of a coronagraph to attenuate the diffracted starlight.

    SPHERE includes 3 instruments, 2 of which can be operated simultaneously in the near IR from 0.95 to 1.8 microns. This requirements is extremely critical for coronagraphy.

    This paper briefly introduces the concepts of 2 coronagraphs, the Half-Wave Plate Four Quadrant Phase Masks and the Apodized Pupil Lyot Coronagraph, prototyped within the SPHERE consortium by LESIA (Observatory of Paris) and FIZEAU (University of Nice) respectively. Then, we present the measurements of contrast and sensitivity analysis. The comparison with technical specifications allows to validate the technology for manufacturing these coronagraphs.

    Comments: 10 pages, will be published in the proceeding of the SPIE conference Volume 7015 “Adaptive Optics”, held in Marseille from 23 to 28 june 2008

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0807.0694v1 [astro-ph]

    Submission history

    From: Anthony Boccaletti [view email]

    [v1] Fri, 4 Jul 2008 08:29:24 GMT (4499kb)


  • ljk July 7, 2008, 23:12

    End to End Simulation of AO-assisted coronagraphic differential imaging: estimation of performance for SPHERE

    Authors: Anthony Boccaletti, Marcel Carbillet, Thierry Fusco, David Mouillet, Maud Langlois, Claire Moutou, Kjetil Dohlen

    (Submitted on 4 Jul 2008)

    Abstract: SPHERE (Spectro Polarimetric High contrast Exoplanet REsearch), the planet finder instrument for the VLT is designed to study relatively bright extrasolar giant planets around young or nearby stars.

    SPHERE is a set of three instruments fed by the same AO-system, two of them share the same coronagraph. This complex system has been modeled with Fourier Optics to investigate the performance of the whole instrument. In turns, this end-to-end model was useful to analyze the sensitivity to various parameters (WFE, alignment of the coronagraph, differential aberrations) and to put some specifications on the sub-systems.

    This paper presents some example of sensitivity analysis and some contrast performance of the instruments as a function of the flux for the main observing mode of SPHERE: the Dual Band Imaging (DBI), equivalent to the Spectral Differential Imaging technique.

    Comments: 11 pages, will be published in the proceeding of the SPIE conference Volume 7015 “Adaptive Optics”, held in Marseille from 23 to 28 june 2008

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0807.0697v1 [astro-ph]

    Submission history

    From: Anthony Boccaletti [view email]

    [v1] Fri, 4 Jul 2008 08:33:47 GMT (1576kb)