OHP preprint 122
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C. Moutou1, A. Boccaletti2 & A. Labeyrie1

1 Observatoire de Haute-Provence, 04870 St Michel, France,
email: moutou@obs-hp.fr, email: Labeyrie@obs-hp.fr
2 DESPA, Observatoire de Meudon, 5 Pl. J. Janssen 92195 Meudon, France,

Faint features near bright quasi-stellar sources, whether stellar planets or quasar nebulosity, can be imaged with remarquable sensitivity using recently developed cleaning techniques to remove straylight from the central object. In addition to classical coronagraphic masking, phase masks, adaptive apodization and dark-speckle imaging provide additionnal levels of cleaning. This paper describes the concept of the prototype dark-speckle imager, briefly presents its performance, and shows the feasibility of the high-contrast coronagraphy with NGST. Scientific objectives as challenging as exoplanet imaging are pursued.


Images with extra-solar planets appearing as "pale dots" near their parent star can be produced by the Next Generation Space Telescope if it is equipped with a "dark-speckle" camera (Labeyrie 1995, Gezari et al. 1997). It exploits the self-destruction of stellar straylight, happening randomly at certain locations in the field, to improve the detectability of a planet peak when present at one of these locations. The camera also has adaptive optics to concentrate stellar light, as much as possible, in the central Airy peak, and adaptive coronagraphs to further attenuate the surrounding diffraction rings and speckles. Refined techniques of apodization and image analysis can darken the star's average halo level by several orders of magnitudes and thus they offer new hopes of detecting extra-solar planets and other faint features near stars, QSO's, etc... if installed on board of the Next Generation of Space Telescopes.

Astrophysical issues

The scientific objectives achievable with the proposed instrument cover a broad field, from stellar environments to extragalactic sources.

Feasibility of exoplanet images and spectra

The Dark Speckle Imager

Adaptive coronagraphy

Present performance of the dark-speckle coronagraph prototype

The exact performance of the proposed instrument can be conveniently verified in the laboratory, using the same kind of artificial star and simulation techniques which have proved successful for speckle interferometry and adaptive optics. This is an essential step in the development of a space instrument.

The present performance of the dark speckle coronagraph is rather promising, since it was able to detect easily in the laboratory a companion at a separation 0.5" with magnitude difference larger than 6 (Boccaletti et al., in preparation). This is the preliminary result of the last simulations performed at ONERA (Office National d'Etudes et de Recherches Aérospatiales) with a turbulence generator followed by adative optics. It was obtained in half an hour integration.

The dark-speckle coronagraph has also been tested on the sky, at the Observatoire de Haute Provence 152cm telescope, in october 1997. The stellar beam from the telescope entered the adaptive optics bench BOA, of the ONERA. The target star [[eta]] Psc was compared to a reference star and the image is the subtraction of both taken in similar atmospheric conditions. The highest magnitude difference between both companions was 4, at a separation as small as 0.5".

Figure 4: Coronagraphic images of an artificial binary star obtained on the turbulence generator at ONERA. a: cleaned image of the binary; b: cleaned image of the reference single star; c: unmasked reference; d: normalised subtraction of both top maps. The faint companion is evidenced at a separation of 0.51" (arrow); the magnitude difference estimate is 6.4.

Our results were affected by the presence of static optical defects mostly due to the pupil stop and to the adaptive system, which limits the speckle smoothing. The fixed residual speckles remain the dominant source of noise in these coronagraphic data and should be removed from future systems.


A wide range of astrophysical topics can be addressed with advanced coronagraphic techniques on board NGST. Exoplanet, brown dwarves, circumstellar envelopes, as well as extragalactic objects are observable. The central (stellar or galactic) light cancellation is achieved in three steps: advanced coronagraphy with the Achromatic Interfero-Coronagraph or the Phase-mask occultation; adaptive apodisation; and a final cleaning step using the dark-speckle algorithm. A chromatic correction for full bandwith detection or an integral field spectrograph would allow either high-contrast imaging or spectral analysis of the detected sources.

In the longer term, these principles of coronagraphic nulling can be extended to the future diluted telescopes, i.e. interferometric arrays of free-flying telescopes spanning one to tens of kilometers. A so-called " densified pupil " imaging mode will provide a recombined focal image where the field will be restricted by a window effect (Labeyrie 1996), but which can be exploited with the same instrument proposed here for NGST, with an obvious gain in angular resolution.

Acknowledgments: This article borrows from the team work achieved for preparing an HST proposal. We thank particularly F. Malbet, F. Vakili, P. Nisenson, K. Dohlen, R. Ragazzoni, D. Gezari, D. Kohler, R. Stachnik, F. and C.Roddier, M. Northcott, W. Danchi, M. Harwitt, for their contributions.


Angel, J.R.P., 1994, Nature, 368, 203.

Bacon R., Adam G., Baranne A et al., 1995, A&AS 113, 347

Boccaletti A., 1998, to appear in IXe Rencontres de Blois.

Boccaletti A., Ragazzoni R., Labeyrie A., 1998, to appear in A&A.

Boccaletti A., Moutou C., Labeyrie A., Kohler D. & Vakili F., 1998, to appear in A&A.

Gay J., Rabbia Y., C.R. Acad.Sci.Paris, 322, série IIb, p.265-271, 1996.

Gezari D. et al. 1997, Columbia Univ. and Ball Aerospace Systems Division, proposal in response to NRA 98-GSFC-1.

Labeyrie A., 1994, A&A 284, 689.

Labeyrie A., 1995, A&A 298, 544.

Labeyrie A., 1996, A&AS 118, 517.

Lyot, B. MNRAS 88, 580, 1939.

Malbet F., Yu J.W. and Shao M., 1995 , PASP 107, 386.

Peacock T., Verhoeve P., Rando N., et al., 1998, A&AS 127, 497

Rabbia Y., Baudoz P. & Gay J., these proceedings.

Roddier ,F. & Roddier, C., 1997, PASP 109, 815.

Wynne C.G., 1979, Opt. Comm. 28, 21.

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