Near Infrared Camera - Coronograph
Coronograph
To enable the coronagraphic imaging of nearby stars, each of the two identical optical trains in the instrument also contains a traditional
focal plane coronagraphic mask plate held at a fixed distance from the FPAs, so that the
coronagraph spots are always in focus at the detector plane. Each coronagraphic plate is
transmissive, and contains a series of spots of different sizes to block the light from a
bright object. Each coronagraphic plate also includes a neutral density spot to enable
centroiding on bright stars, as well as point sources at each end that can send light
through the optical train of the imager to enable internal alignment checks. Normally
these coronagraphic plates are not in the optical path for the instrument, but they are
selected by rotating into the beam a mild optical wedge that is mounted in the pupil wheel, which translates the
image plane so that the coronagraphic masks are
shifted onto the active detector area. Diffraction can also be reduced by apodization at the
pupil mask, thus the pupil wheels will be pupil with integral wedges in each case.
