### Virtual Observatory Reference

> Summary

> Use the Web proxy connection if your firewall does not allow your computer to connect directly to external computers. In this case, you also must use DS9's internal browser.

> Details

> When you click on one of the Virtual Observatory servers in the VO list, DS9 will attempt to connect to that server and (if the internal Web display is enabled) display its Web page. The square box to the left of the server name turns yellow while the connection is being established and then green to signal success.

> A direct connection is fast and flexible. Among other things, it allows you to perform analysis on your own local data (the VO server will retrieve the image from DS9) and also allows you to use an external browser to load images.

> Some system managers configure their firewall explicitly to prevent computers in their care from making a direct connection to an external host. Instead, they only allow external access through a Web proxy
> server (such as SOCKS). If you are using a computer behind a restricted firewall of this sort, then DS9 will not be able to connect directly to a VO server. The yellow box will not turn green and eventually DS9 will display an error message.

> In this case, you can choose to have DS9 communicate with the VO servers through your Web proxy server. DS9 will use your proxy to send its commands and retrieve its data and analysis results, rather than doing this directly. Note that the following restrictions apply:

> > The transfer of data is slower.
> > You must use the internal Web browser for loading images, etc.
> > You cannot perform analysis on local data.
> > There is a (large but finite) restriction on the number of annuli, and number of polygon points you can specify in a region, as well as the total number of regions allowed.

> If your computer and firewall have been configured to require use of a Web proxy server, you will have to tell DS9 about this server. Click the Configure Web Proxy button and type the relevant information into the boxes. (Your systems administrator will be able to tell you the details.) At this point, you should be able to connect to a VO server successfully. Please let us know if you have problems!

> > A final note: you may, of course, choose to use the Web proxy even if your computer and firewall are configured to allow direct connections. In this case, there is no need to configure the proxy server.