Discrete representation of the HST for kernel analysis of the NICMOS images. Left panel: representation of the pupil model defined by the cold mask of the instrument. The model takes into account central obstruction and spider vanes, but neglects finer structures such as support pads. The grid pitch is 0.13 m, the outer diameter is 1.95 m, the inner diameter is 0.71 m, and the thickness of the spider vanes is 0.07 m. Right panel: corresponding uv-plane coverage.