Analysis of fringing patterns

The short-high module of IRS saw fringing patterns on its first light. The fringes are believed to be the result of interference when light passes multiple instrument layers that can reflect light, such as the filters and the anti-reflection coatings on the detector, before hitting the detector elements. Assuming a parallel layer of thickness t, the difference of the optical path is then 2tn, where n is the index of refraction of the layer. The phase difference of the interfering light is therefore 4tn/. The simplest model for the fringing pattern would be A()cos(4tn/). We do not yet have a model for A. Taking it as a constant one, the patterns generated by the filter and the anti-reflection (AR) coating separately are shown below for the short-high module.

While it would be an exactly cosine wave in wavenumber space, the patterns show constantly increasing period with wavelength, which should mimic what is seen on the spectrograph detectors. Note that for an echellogram that the high resolution modules of IRS are producing, the same wavelength range is distributed in about ten orders.

Fan Fang       Last update: 9-Aug-99