C-90 focusing question

Group:
As a long time Celestron scope owner I got to thinking about the C-90 I once owned and how it focused. I recall I turned that big black ring around it but what happened inside? That is, how was the mirror moved (or not, maybe the secondary moved as you turned the ring?). Just curious.

It is a helical focuser. If the front part of the tube turns and lengthens it is a single helical. If it does not turn but lengthen it is a double helical type.

Walt