First results of nulling interferometry with the Multiple-Mirror Telescope

Abstract

We have successfully used nulling interferometry at 10 wavelength to interferometrically suppress a star's radiation. This technique was first proposed by Bracewell 20 years ago to image extra-solar planets1 and is now the basis for proposed space-borne instruments to search for Earth-like extra-solar planets and their spectroscopic signatures of habitability and life2'3'4. In our experiment, the beams from two 1.8 m telescopes of the Multiple Mirror Telescope were brought into registration at a semi-transparent beamsplitter, and the images made coincident on an infrared array detector capable of taking rapid short exposure images. The atmospheric fluctuations caused the phase difference between the beams to fluctuate, changing the total flux of the star seen in the image plane. When the atmosphere caused the wavefronts to be exactly out of phase the entire stellar Airy pattern disappeared. For the unresolved star a Tauri the cancellation was such that a companion only 0.2 arcsec from the star and 25 times fainter would appear equal in intensity to the nulled star. The residual flux was spread into a wide halo suggesting the cause of this flux was imperfect cancellation of the aberrated wavefronts. To increase the precision of nulling beyond this first step several sources of error need to be addressed. We discuss the control of errors due to amplitude, polarization, chromatic differences, stellar leak, and sampling time. Improvements such as active phase tracking, adaptive optics, and cooled optics will increase the achievable gain of nulling interferometry and allow it to be used on fainter objects.

The theory of brown dwarfs and extrasolar giant planets

“…They include astrometric techniques (Horner et al, 1998), nulling interferometry (Hinz et al, 1998), spectral deconvolution (Charbonneau, Jha, and Noyes, 1998), and looking for the wobble in the light centroid for a well-chosen spectral band for which the planet and star have different absorption/emission features. Furthermore, proposed space telescopes/instruments, such as NGST (Next Generation Space Telescope; Mather, 2000), SIM (Space Interferometry Mission; Catanzarite et al, 1999), Eclipse (Trauger et al, 2000), Eddington (Penny, Favata, and Deeg, 2000), Kepler (Koch et al, 1998), FAME (Horner et al, 1998), COROT (Michel et al, 2000), MONS (Kjeldsen, Bedding, and Christensen-Dalsgaard, 2000), FIG.…”

The Darwin mission: Search for extra-solar planets

“…The first practical demonstration of nulling was undertaken in February 1998 (Hinz et al, 1998). Using the Multiple Mirror telescope on Mount Hopkins, Arizona, Angel and his team were able to cancel out the image of a star: a-Orionis.…”

Nulling interferometry and adaptive optics control system optimization

“…Then we introduce a coupling between the FPC and the AO, and study under which circumstances control laws can be derived that will improve the overall system performance. The general framework of nulling interferometry is described in the references [1] and [2] while the LBTI instrument is presented in [3] and [4]. The figure below shows a diagram of the beam path to illustrate the general concept of the LBTI.…”