The NIS is composed of two normal incidence
gratings that disperse the light into the NIS detector,
known as the viewfinder detector subsystem, or VDS.
The gratings are slightly tilted, in order to produce two
wave-bands
(NIS 1: 308 - 379 Å and NIS 2: 513 - 633 Å;) on the same
detector, one on top of the other (see Figure 3.6.1).
The spectral resolution is @ 0.35 Å for NIS 1
and @ 0.5 Å for NIS 2.
Second order lines have been observed only in NIS 2.
The NIS can provide monochromatic images of the solar field of view.
This is accomplished with the rastering
of a region, moving a mirror and producing contiguous
images of one of the long slits.
The rastering is performed from west to east (being the normal
alignment of CDS with the slits in the solar N-S direction).
Telemetry constraints usually make it necessary to
pre-select a number of NIS wavelength windows to be extracted
on-board from the spectra, before the data are
telemetered to the ground.
