Center Alignment of popup window Javascript

How to centralized popup window using Javascript. see the below code.

CODE:

1 :  var OpenWindows = new Object();

2 :  function OpenWin(sPath, w, h)

3 :   {

4 :       l = (screen.width - w)/2-5;

5 :       t = (screen.height - h)/2-20;

6 :       if (l<1) l=0;

7 :       if (t<1) t=0;

8 :       sWindowName = "mainwin_" + w + "_" + h;

9 :       if(OpenWindows[sWindowName]) {

10 :            if(!OpenWindows[sWindowName].closed) OpenWindows[sWindowName].close();

11 :       }

12 :       OpenWindows[sWindowName] = window.open(sPath, sWindowName, "left=" + l + ",top=" + t + ",width=" + w + ",height=" + h + ",scrollbars=no,menubar=no,toolbars=no,status=no");

13 :       return OpenWindows[sWindowName];

14 :  }