6.2 Response to Off-axis and Extended Sources: Fringes in the Data

Channel fringes are seen on all LWS spectra of extended or off-axis point sources, as is illustrated in Figure 6.2.

Figure 6.2: Example of fringes in the spectrum of an extended source. Only the long wavelength detectors are shown here, as they are the most affected by the problem.

Extensive modelling of the response of the instrument has shown that the channel fringing is caused by Mirror 2 which was stepped, as explained in Section 2.3 and Section 5.9, causing interference between the reflecting surface of the LWS field mirror and its support structure as the diffraction pattern from the source falls off the edge of the field mirror itself.

The distance between any two fringe antinodes is predicted to be cm in frequency space. As is evident in many observed source spectra (see for example Figure 6.2), the amplitude of this parasitic fringing increases with wavelength. Further, the diameter of the Airy pattern from the telescope also increases with wavelength because of diffraction, so it is more likely that the detector will view the annular part of Mirror 2. The contour field mirror controlled the illumination of this annulus. Modelling (Section 5.9) has determined that a 120 diameter beam fell onto Mirror 2 and the annulus.

The reflectivity of the support structure material is not known well enough at these wavelengths to permit the production of an exhaustively quantitative model that would allow the removal of the channel fringes given knowledge of the spatial structure of the source. Instead a method has been devised that performs a multivariate fit for the period, amplitude and phase of the sinusoid in wavenumber space and removes it from the spectrum by division or subtraction. This has proved successful in removing the channel fringing from the continuum spectrum of most sources whilst preserving the shape and intensity of the unresolved spectral features. A special command is available in ISAP (see Chapter 8) to apply this defringing method to LWS grating spectra.
High frequency fringing also occurs on Fabry-Pérot spectra, although this is much more rarely seen. The spacing of the fringes in wavenumber is known: 0.0095 cm. As for the grating, the fringes are stronger for longer wavelengths and undetectable for short wavelengths ; their origin is probably the same as the one for the grating and related to Mirror 2. The DEFRINGE routine in LIA (see Chapter 8) allows the application of the defringing method to all three AOTs L01, L03 and L04.