XAO Data Center Site Help

This document is intended to help you discover the XAO web interface features on the XAO Data Center. Note that the site also offers VO-enabled services.

Though most of the services should work even in text-based browsers, we recommend a web browser with CSS and Javascript enabled to access the web-based services.

Service discovery

The index page lists published services available through web browsers. A "[P]" in the service listing means that the service is password protected, either because it is too rough for public consumption or because the data providers want exclusive access. In either case, you can contact the site operators to inquire about access.

In addition to web-based services, the data center also provides services accessible through IVOA standard protocols like SIAP (for images) and SCS (supporting cone searches). These can be located through a Virtual Observatory registry or common VO tools.

Search forms

Query Fields

Services not using customized interfaces have standard query forms. These consist of a series of input fields that either take

Query Modifiers

For most queries, you will have a "Table" query field near the bottom of the form. You can set sorting and limit options there. Note that depending on the query, selecting a column to sort on may slow down the answer dramatically. This is because your query may match large amounts of data, and even if only 100 items are returned, potentially millions of them may have to be sorted.

On the other hand, results overflowing the match limit are not reproducible without a sort option, i.e., you may get a different set of, say, 100 items for identical query parameters at two different times. The services warn you about this fact when they return truncated results.

In cases in which the match limits provided by the form do not suit you, you can override the match limit by editing the result link (see below). You want to substitute your value into _DBOPTIONS_LIMIT=100. The system has hardcoded match limit that you cannot override in this way, but it is unlikely that it will hurt you.

You can usually select an output format. Options here include

Next to the output format selection, there are two links. The one labeled "[Result link]" reflects your current query as a copyable URL. You could use this to send a query result to colleagues, but it is particularly useful with the VOTable output format -- you can drag and drop that link into Topcat's "Open" dialogue, for example.

The star () lets you bookmark the current form -- it is basically like the result link but doesn't actually perform the query. This should come in handy to store and communicate preset queries. Note that for split-up fields (e.g., date inputs with year, month and day) you need to enter all pieces of information for this to work.

Both the result and the bookmark links require Javascript to work.

The sidebar

The sidebar on form and result pages contains ancillary information for the resource the data comes from. In the Metadata section, only metadata titles are shown. You can view the content by clicking on ">>" and hide it again by clicking on "<<".

The "Created" metadata item shown refers to the creation of the resource descriptor, i.e., is related to the service rather than the data. The "Data updated" item gives the date of the last "import" of a piece of the data. Unless a service supports online modification, it can be used as an indicator of the version of the underlying data.


With ADQL, it is not hard to construct queries that will run for days or weeks, and even normal web-based queries can select so much data that excessive runtimes and possibly huge data volumes result. Ususally, this is not intended by the users, and thus by default relatively short timeouts are imposed on the run time of the queries on the server (of the order 10 seconds).

You can usually increase the timeout by generating a link to your query (see "Result link" above) and appending "&_TIMEOUT=100 (or whatever number of seconds suits you) to the URL. The ADQL form has an input field that does just this.

However, usually long-running queries indicate that we should be adding some index to our tables. Thus, please contact us with your queries and we will try to speed them up.

Login and logout

In the sidebar, you will usually see a login item. You should normally not need to be concerned with this, since protected services will query your credentials as needed. You may want to log out, though; if you click on the "log out <user>" link, a login mask will pop up. Leave the username blank and click OK to log out. If you click Cancel (or equivalent), you will remain logged in. This slight weirdness is inevitable when using HTTP authentication with current browsers.

There is one situation in which you will want to log in: Access to protected products from open services. You will note that in such cases, the system will deny access to the protected products without asking your credentials. If you can provide those, log in and try again.

To obtain usernames and passwords, contact zhanghailong@xao.ac.cn.

Software availability

The software driving this site is not really finished enough for a "real" release. If that does not scare you, you are welcome to a download. All our code is released under the GPLv3 (even if it doesn't say so in all places yet).


This software uses services provided by the Centre de Données astronomiques de Strasbourg, France.

Some of the services were created using software from astrometry.net.

Some of the services use q3c indexing by Sergey Koposov and Oleg Bartunov, see 2006ASPC..351..735K.


As put down in our privacy policy, we try to respect your privacy and store as little data as possible. If you want us to by default delete all log entries, bring the funding agencies and others to sense. Access data is no measure for usefulness. But who's even reading this?