java script popup close and then postback main form

I have javascript to open a popup. when this form closes I want the calling form to postback

     function OpenChild(strImgType) {

        var WinSettings = "center:yes;resizable:no;dialogHeight:300px"
   
        var MyArgs = window.showModalDialog("../PopupForm.aspx?Product_ID=" + prodid + "&ImageType=" + strImgType, MyArgs, WinSettings)

    }