function popup(url, windowname, popupwidth, popupheight) {
	screen_width	= screen.width;
	screen_height = screen.height;
	hspace = (screen_width	- popupwidth ) / 2;
	vspace = (screen_height - popupheight) / 2;
	params = 'resizable=no,scrollbars=no,width=' + popupwidth + ',height=' + popupheight + ',left=' + hspace + ',top=' + vspace;
	window.open(url, windowname, params);
}