var pWindowWidth=600;
var pWindowHeight=450;

function center(){
var cen='width='+pWindowWidth;
cen+=',';
cen+='height='+pWindowHeight;
cen+=',';

var cenw=(screen.width/2)-(pWindowWidth/2);
var cenh=(screen.height/2)-(pWindowHeight/2);
cen+='left='+cenw;
cen+=',';
cen+='top='+cenh;

window.open('coupon.php','pop',cen);
}