The slits provide various fields of view (2¢¢×2¢¢, 4¢¢×4¢¢,
2¢¢×240¢¢, 4¢¢×240¢¢, 90¢¢×240¢¢,
8¢¢×50¢¢)
and are normally oriented along the solar N-S direction.
Any position on the Sun is commonly indicated by two coordinates,
Solar X and Solar Y, also used to indicate the centres of the pointing
of any CDS observation.
Solar X is given in
arc sec from the disc centre along the East-West (E-W) direction,
while Solar Y indicates arc sec from the disc centre along the
South-North (S-N) direction. Note that 1¢¢ corresponds on the Sun
to 700 km.
The internal scan mirror and slit movement of CDS allow CDS rasters to cover a maximum area of 4' square within a single raster. The
E-W coverage is created by the scan mirror which can move in steps of 2.03" (normally assumed to be 2"). Movement can only be in one
direction from West to East.
In NIS the N-S coverage is usually provided by the long slit (normal slits are 2"x240" or 4"x240").
For GIS the N-S dimension is scanned by moving one of the small square slits which can move in steps of 1.01" (normally assumed to be
1"!).
