  Extra-solar planets have not been imaged directly with existing ground or
space telescopes because they are too faint to be seen against the halo of the
nearby bright star. Most techniques being explored to suppress the halo are
achromatic, with separate correction of diffraction and wavefront errors.
Residual speckle structure may be subtracted by differencing images taken
through narrowband filters, but photon noise remains and ultimately limits
sensitivity. Here we describe two ways to take advantage of narrow bands to
reduce speckle photon flux and to obtain better control of systematic errors.
Multiple images are formed in separate color bands of 5-10% bandwidth, and
recorded by coronagraphic interferometers equipped with active control of
wavefront phase and/or amplitude. In one method, a single deformable pupil
mirror is used to actively correct both diffraction and wavefront components of
the halo. This yields good diffraction suppression for complex pupil
obscuration, with high throughput over half the focal plane. In a second
method, the coronagraphic interferometer is used as a second stage after
conventional apodization. The halo from uncontrollable residual errors in the
pupil mask or wavefront is removed by destructive interference made directly at
the detector focal plane with an "anti-halo", synthesized by spatial light
modulators in the reference arm of the interferometer. In this way very deep
suppression may be achieved by control elements with greatly relaxed, and thus
achievable, tolerances. In both examples, systematic errors are minimized
because the planet imaging cameras themselves also provide the error sensing
data.
