I have a tiny MCE button but I'd like to make it open a modal window(like for the spellcheck). How do I go about doing this?

    tinymce.create('tinymce.plugins.tinyplugin', {
     
            init : function(ed, url){
                ed.addButton('tinyplugin', {
                title : 'map',
                    onclick : function() {
                       // make my pop up window?
                    },
                    image: url + "/icon.png"
                });
            }
        });