How to Colonize an Asteroid
The colonists will have certainly found a carbon based asteroid by the time the main sphere is excavated, and hopefully much sooner. From this they will have extracted water and the volital compounds needed to make plastics - or have found regolith material high in silicon.(Which they will have already needed to manufacture power cells). From these materials they will consturct a plug for the main shaft which will let solar energy in, but keep the harmful radiation out. This plug might be made of many layers of doped glass, or be a large plastic bag filled with water, or might take the form of a piping system - fiber optic, for example. Another option might be to NOT plug the shaft - it might be possible to hold an atmosphere inside through centrifugul force after we have the colony rotating. Much study and consideration needs to go into this subject yet.