4.5  Other factors

The geometric area available at the slit, that depends on which slit is used.

The exposure time.

4.5.1  Intensity variations along the CDS slit

For details, see:

http://orpheus.nascom.nasa.gov/~thompson/slit_inten/slit_intensity.html

There is evidence of an intensity gradient along the slit in VDS data. Furthermore, the variation appears wavelength dependent.

It's unclear what is causing this effect. It is probably generated within the instrument optics, either within the telescope or in the two NIS gratings.

Bill Thompson analysed NISAT_S, NIMCP, and SYNOP_F data and came up with the following conclusions. The behavior along the slit falls into two classes. One class behaves like Mg IX 368, where the intensity increases as one goes from the bottom of the slit to the top. The other class behaves like O V 630 A, where the intensity descreases as one goes upward from the bottom of the slit, and then levels off. Other lines, such as He I 584 A, can be modelled as a linear combination of these two behaviors.

Figure 27: [Figures from Bill Thompson] From top to bottom: NIS-1 Mg IX-like behavior; NIS-2 Mg IX-like behavior; NIS-1 O V-like behavior.