5.6. RSRF and effect of fringes

The Relative Spectral Response Function (RSRF) is the total throughput of the system. It arises from the convolution of the detector response as a function of wavelength, the filter response as a function of wavelength and takes such effects as geometrical considerations (e.g. slit width, source structure) into account.

Fringes are a modulation with wavelength of the light falling on the detectors. The fringes arise due to constructive/destructive interference in either in the detector material itself or in the filter material. Bands 1, 2 & 4 are only lightly affected by fringes, whereas the fringes are more pronounced in band 3. An example of fringes in the band 3D spectrum of a star can be seen in figure 6.

  
Figure 6: Example of fringe effects in band 3

An attempt is made to remove fringes during the OLP processing pipeline. However, because the fringes may shift in wavelength space depending on where the source is in the slit and whether the objects being observed is a point source or extended, fringe removal may not be perfect. This may cause problems for observations of faint sources. Faint features should be checked against the relevant Relative Spectral Responsivity File, SWS Cal-G 25_x, to ensure they are not artifacts.

Derivation of a complete set of in-orbit RSRFs is needed to obtain an optimal flux calibration accuracy. This calibration is still ongoing, and until complete the RSRFs measured in the lab have to be used for most bands. For the current version of OLP (V6.1) the Cal-G files containing the RSRF's were updated (e.g. with broad-band corrections) leading to a significantly improved calibration of the spectra. The few problems found in the new calibration files were, however, almost always present in the previous set of RSRF's, and are:

1

Band 2C - Spurious Features
Based on analysis of many sources it was concluded that features seen at 9.35, 10.05, 11.05 m are most likely due to responsivity correction. The features occur at: 9.35 m in absorption with a FWHM of 0.3 m ; 10.05 m in emission with a FWHM of 0.1 m ; and 11.05 m in emission with a FWHM of 0.1 m .

2

Band 3D - 28 m leak
Some sources exhibit a slight increase in the flux seen from about 27 m onwards. This is not real but is due to leakage from approximately 13 m , is of the order of 10% and will primarily affect objects with flux falling sharply towards long wavelengths.

3

Band 3E - Suspected Slope Change
There is a suspect slope change in the middle of this band in many observations. This is not due to a spectral feature in the source that was used to derive band 3E RSRF.

4

Band 4
This file has not been updated, due to a lack of calibration sources for band 4. It bends down at the short and long wavelength edges.