As mentioned previously, the solar atmosphere is highly structured and dynamic, and therefore observations of the solar atmosphere require good spatial and temporal resolutions. In order to probe the solar atmosphere using spectroscopic means one must identify those ions which will be present and identify the emission lines from these ions which will provide the best diagnostic information. For a particular ion the ionisation equilibrium, which is a balance between ionisation and recombination, is a function of the temperature of the plasma (see Figure 1).
Therefore, CDS observes in the EUV where there are many emission lines of highly stripped ions of characteristic temperatures 104 - 2 x 106 K which can provide useful diagnostic information. Figure 2 illustrates the EUV wavelength region, showing the some of the most useful emission lines.
Figure 1: Ionisation equilibrium for Fe ions (derived from Arnaud and Raymond, 1992).
Figure 2: Plot of wavelength versus temperature for many of the most useful spectral lines in the EUV wavelength range (Harrison, 1993).
CDS consists of a Wolter-Schwartschild II grazing incidence telescope, with a field of view of 4 arcmin, which has a resolution of approximately 2 arcsec (Harrison and Fludra, 1995). Beyond the slit aperture, the beam is divided into two components which are fed into two different spectrometers (Harrison and Fludra, 1995). One component goes to the Normal Incidence Spectrometer (NIS), and the other goes to the Grazing Incidence Spectrometer. Since the NIS has a two dimensional detector, it can produce 2D images by rastering in one direction only, as the NIS produces images of the spectrometer slit dispersed in one direction by the wavelength (see Figure 3; Harrison and Fludra, 1995). The GIS on the other hand can only produce 2D images by rastering in both spatial dimensions. The major driving force behind having two spectrometers within the instrument is to cover a larger wavelength range. The NIS covers the wavelength range 310Å - 633Å and the GIS covers the 155Å - 787Å, with various portions covered within each (Harrison and Fludra, 1995). The studies carried out here are only concerned with the NIS as they rely on the fast rastering of 2D images, which can only be performed by the NIS. The studies will make use of some of the data obtained by the GIS, such as atomic line profiles.
Figure 3: schematic of the CDS NIS output, and how this is rastered to produce a 2D image. The resultant output is a "data cube", with two spatial dimensions and one spectral dimension.
For more information on CDS the links from the SOHO page are a good start.
