Detect whether user is access website via Apple IPAD in ASP.Net

In my website page there is a “Download” Button. When user is clicking on this button wallpaper is downloaded. Here Condition is that if a user is a ipad user then image is downloaded from “images/ipad/imagename” path. If a user is not a ipad user then image is downloaded from “images/originalImages/imagename” path. How can I detect that user viewing my website is a ipad user or windows  user using asp.net, c#.