It has been a long-standing goal in astronomy to image other solar systems. However, such a task presents a daunting instrumental challenge, as the required contrast ratios are well beyond current state-of-the art in astronomical imaging techniques.
Based on work done at the NASA Jet Propulsion Laboratory as part of the developmental effort for the Terrestrial Planet Finder mission, the group I am affiliated with has developed an innovative optical nulling interferometer. This instrument can be placed behind a standard optical telescope, and it will create a controllable null in the image which can be set to remove the star while keeping the planet.
The principle of operation is based on a modified Mach-Zender interferometer, in which the incoming wavefront is split and a phase shift of one-half wave is introduced into one arm of the interferometer (an achromatic phase shift is achieved using a pair of glass plates with carefully selected thicknesses and dispersion constants.) This path difference causes the two split wavefronts to interfere destructively at the output. A small lateral shear is introduced before the beams are recombined; this shear makes it possible to null only part of the field of view at a time, as light coming in off-axis at an angle of λ/2s will see an extra path in one arm that instead produces constructive interference (s = the shear projected through the telescope.)
Finally, the recombined, nulled output of the interferometer is directed to a standard, low-noise CCD-based imaging detector (see Figure on the left for a schematic layout). The level to which on-axis light is suppressed is a function of how flat the incoming wavefront is. A null depth of 1,000,000:1 requires flatness of ~λ/1000 or 0.6 nm (the imaging system can then detect the planet with the now-reduced contrast ratio of ~100:1.) We achieve this by placing a 1024-element deformable mirror in the beam path; each element of this mirror can be controlled with Angstrom precision.
The PICTURE Mission
I am currently spending most of my time working on the PICTURE mission, for which I am scientific lead of the MIT portion of the project (“Planet Imaging Concept Testbed Using a Rocket Experiment”; PI S. Chakrabarti of B.U.) This exciting mission aims to directly image the planet thought to be orbiting the nearby star Epsilon Eridani, by using a novel nulling interferometer to suppress the light of the primary star by a factor of 1,000,000:1. Such levels of starlight suppression require being above all, or most of, the atmosphere, and hence the instrument will be flown on a sounding rocket. It will take advantage of a favorable orbital alignment during 2007 to achieve a direct detection of an extra-solar Jovian planet. It will also represent a tremendous leap in technical capability and flight qualification in the area of high-contrast imaging. I am responsible for several critical areas of the mission including camera development and the signal-to-noise budgets. I also played a leading role in developing the mission concept, and developed the high-fidelity simulations used in the system design and proposal.
