3.7  Other geometric effects to remember

3.7.1  Spatial offset between NIS1 and NIS2

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Both NIS1 and NIS2 use the same entrance slit to the spectrograph. However, there appears to be a N/S offset, sometimes amounting to several arcseconds, between the solar features that NIS1 sees and those that NIS2 sees. The effect seems to depend on the Solar Y position.

Figure 11: [Figure from Bill Thompson and Dave Pike] Offset between NIS 1 and NIS 2 as a function of Solar Y, from the SYNOP synoptic rasters.

The effect is more pronounced in post-recovery data and could amount to ~ 7 pixel vertical shift between NIS-1 and NIS-2 images.

Use the routine gt_nis_alignment which returns the best estimate of the offset.

3.7.2  Spatial offset between NIS anf GIS

There is also an offset between the imaging of NIS and GIS. GIS images appear approximately 13" south of their NIS equivalents. Specific (X,Y) pointings for GIS should therefore be 13" south of the values determined by the software