The figure below shows one possible cause of fringes. In each detector assembly there is a bowed black polyurethane filter B anf a bandpass mesh filter F (which is wedged and anti-reflection coated), and an aperture A. An off-axis ray R may pass through the filters but bounce in the lower cavity before entering the filter cone D. While bouncing in the cavity it may experience interference. The gap of the lower cavity is about 1.54 mm.

Were this true, it might explain why extended sources exhibit this problem. Also, small difference in the gap d from detector to detector might cause the fringe function to vary slightly in its period, although there would be no discontinuities across the band. Period differences are being searched for at this time.

        R
       .
|  ___.___B |
| /  .    \ |
|/  .      \|
|__._____F__|
| . .       |
|. . .      |
|_.__ . __A_|
    \  ./
     \ /
      D