The image of the solar corona is projected by the telescope mirrors onto the entrance slits of the spectrometer. Their width can be varied in order to optimize the spectral resolution and count rate requirements for a particular observation. Identical entrance slit mechanisms for each UV spectrometer channel accomplish slit width variation.
Each slit is formed by two components: a lower edge that is fixed and directly mounted on the interface flange, and a movable upper edge that is connnected to the moving part of a parallelogram.
The slit width is measured by linear variable differential transducers
measuring the stroke of the movable edge. This edge is translated by a
high-precision parallelogram linkage motor which is connected to a
bearing-pinion engaged with a cam driven by a stepping motor. The primary widths are 355, 213, 53, 25 and 10 
m and the slit height is 8.73 mm. The specified repeatability and readout of the width is 
1 m.
For each slit, the slit-edge, which corresponds to the smaller solar radius,
is placed at a common location. Accordingly, the near disk image edge of the
slits for all spectrometer channels are co-linear within 0.3 m in the
dispersion direction and within 2 m along the mirror focal direction.
In order to prevent possible internal stray light from entering the spectrometer,
the entrance slits are shielded by entrance slit baffles. The two baffles,
one for each UV spectrometer channel, are cylinder-shaped with internal
rectangular diaphragms for stopping inner reflections.