Fringe Simulation 

The fringe pattern of an imaging Fourier transform spectrometer is predicted by Eq (4) of the Imaging Spectroscopy Tutorial.

This movie  shows the time sequence of images seen at the two complementary output ports, A and B, of a four port interferometer. The interferometer is illuminated by monochromatic light and the optical path difference is scanned. The simulation parameters are typical for the case of an IFTS on NGST:

The movie image is composed of four quadrants. The lower left quadrant represents the image at port A, and the upper right quadrant represent the image at the complemetary output, B. Also shown are the sum, A+B, and difference, A-B. The sum, A+B, is unmodulated and is used to make the pan-chromatic image, and the difference, A-B, yields the final interferogram.  The label at the lower left shows the optical path difference.

This example illustrates some of the advantages of a four-port interferometer. The difference interferogram, A-B, is unaffected by sky or telescope background variations, and the normalized interfogram (A-B)/(A+B) is unaffected by changes in the telescope plus instrument throughput.