Correct me if I'm wrong but there is no way to configure radEditor to open in Preview mode. I found a workaround a few years ago
function
OnClientLoad(editor, args) {
setTimeout(
function
() {
editor.set_mode(4);
}, 10);
}
but that code set focus to editor control in IE9/10, i.e. if you have a long page with an editor at the bottom then when you open the page IE scrolls it to the bottom. I wonder is there better way doing it?