Possible cause of fringes

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 finally 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 aperture it may experience interference. The gap of the lower cavity is about 1.54 mm.

Were this true, small difference in the gap $d$ might cause the fringe function to be not exactly pure in its period, although there would not be discontinuities acroos the band. Period differences are being searched for at this time. /

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