3.2  NIS scattered light

Unfortunately, the NIS spectra contain a large scattered light ( `background' ) component when observed on the solar disc. The main contributors are probably the Lyman-alpha and the Lyman continuum.

The NIS 1 spectral region, where CDS has many potential diagnostic lines, is the most affected. This scattered light produces a spatially and wavelength-dependent background component in the spectra, and is most intense in the network spectra.

Typically, in the NIS 1 channel, the scattered light component is a factor of 2 higher in the network areas than in the cell centre areas. The background level can be of the order of the peak intensity of most of the NIS 1 lines in coronal hole regions, and also in the quiet sun.

The background determination provides the major uncertainty in line intensity measurements.

3.2.1  Scattered light

It is basically only important in off-limb observations. Some pre-flight information is available in Harrison et al (1995), Solar Physics, 162, 233. which gives results for measures at a wavelength of 68 Å.

See also David et al (1997), Fifth SOHO Workshop, SP-404, Page 313.

Routines to calculate the amount of scattered light and an explanation of how to apply them are available at:

 ftp://www.medoc-ias.u-psud.fr/pub1/softs/david/