As is often the case with spectrometers, a ripple across the passband is seen to appear in many LWS spectra. These ripples, or fringes, are a modulation of gain as a function of lambda across the entire spectral range. They appear as a cosine function in wave number [cm^-1]. View some bad fringes, or or some worse fringes.

Fringes can be present up to a level of 30% of the signal. Fringes are present in the grating and FP spectra of extended source (sources larger than 10 to 50"). Fringes may be absent in general from point sources centered in the beam, while point sources located off-center can produce fringing.

The presence or absence of fringes does not correlate with source strength. However, in those sources showing fringes, the magnitude of the ripple is typically 10% to 30% of the source continuum flux.

The fringes are of remarkabley constant period and phase in all observations in which they are seen and thus are believed to be an interference effect within the instrument. Although the exact cause of the fringes is not yet known, their regular properties make them more removable from extended spectra.

The cause of the interference is not yet known, but one possibility is an internal reflection along the optical path in a gap formed by the bandpass mesh filter and the surfaces of the detector entrance cavity.
The mathematics of fringes.

Possible removal methods.