For calibration purposes, it is foreseen to observe the solar disk. In this case the positioning of a filter in the optical path is required. For this operation a mechanism has been designed for each channel and has been attached to the interface flange of the entrance slits. The movement is accomplished with a limited angle torque motor and readout with a rotary variable differential transducer.
The main requirements of the filters are the following. They will cover the full optical aperture in each channel and will provide a attenuation at 1216 Å for the UV channels and attenuation at 4500-6000 Å for the WLC. They will be placed in the optical path on command and will be fail-safe in the ``out'' position. To satisfy the fail-safe requirements, an antagonist spring has been introduced with the rest position keeping the filter completely out of the light path of the slit.