Any additional line selections are subject to the following constraints;

1. Dual line selections should have relatively small separation (~ Few Angstroms)
2. There must be no lines very nearby that could spill over given a Doppler shift, broadenings etc. i.e. relatively isolated.
3. They should have good temperature coverage, i.e. A coronal and transition region line.
4. They need a relatively high intensity in all solar regions to maximise scientific usability.
5. Crucially, they need to have an available narrowband multilayer for the given wavelength region.