  The technique of simultaneous spectral differential imaging (SSDI) is used to
image directly brown dwarfs and exoplanets around nearby stars. An analytical
PSF model is first presented with numerical simulations to estimate the PSF
noise attenuation performance. A three-wavelength infrared camera (TRIDENT)
implementing the SSDI technique is then described. Results from a survey of 35
nearby stars carried out with TRIDENT at the Canada-France-Hawaii telescope are
presented. Performance estimates show that a companion 9.5 magnitudes fainter
than a star is detectable (6 sigma) at 0.5" separation. An analysis of the
observations suggests that non-common path aberrations between TRIDENT optical
channels are the limiting factor preventing further PSF noise attenuation. A
new camera concept using a multi-wavelength detector featuring a microlens
array combined with micro-filters is presented to overcome the non-common path
aberration problem. Another imaging technique, differential angular imaging, is
also discussed to obtain a reference PSF in each optical channel while
observing a target. Finally, the accuracy of FFT-based image shifting, scaling
and rotating algorithms is studied to show that a companion 10^9 times fainter
than a star can theoretically be detected by those algorithms.
