This window provides access to the Virtual Observatory (VO). Using it you can search a region of sky for spectra or search for spectra associated with a specific astronomical object. Once you have done that you can then download all or some of these spectra for display and analysis in SPLAT-VO.
In the VO spectra are made available by SSAP servers (SSAP stands for Simple Spectral Access Protocol) and using this window you can also select which of these servers you want to query. That is important if you are only interested in spectra covering specific wavelengths or types of observations. These searches can be further refined using additional parameters like a range of wavelengths or times. Addtional Optional Parameters are also provided for even more refinement, but note these may not be supported by all SSAP servers.
The figure above shows the window after a query has been made. The window is divided in three main areas. The left area is the SSAP server selection area, where the current list of SSAP servers is shown and also options to select a subset of servers to be queried. The upper right area is the query parameters area, where query options can be entered. The right-lower part is the results area, showing the query results.
Server selection
When clicking on a server in the SSAP Servers list you can see all information stored about this server. A subset of servers can be selected by data source (theory, survey, pointed, custom, artificial) or by waveband. It is also possible to add a tag to servers, so only the tagged servers will be queried if the tag is selected. Servers are added to a tag by selecting them in the SSAP Servers list and then creating the tag.
To add a new SSAP server, which is currently unknown to the VO, to the current server list, click on the + button. A new window will open where the information about the new server can be filled in. Servers can also be removed from the list with Remove selected and Remove unselected buttons. The Query registry allows you to query a VO registry for a list of all the SSAP servers it has registered.
SSAP query
To lookup the coordinates of an object just type in an identifier in the Object field and either press
<Return>
or the Lookup button. Alternatively if you know the coordinates (FK5/J2000) of the region
of sky you can just enter the coordinates in the RA and Dec fields. It is also possible to query the
servers about an object by name, useful for solar system work, in that case just enter a name and leave
the RA and Dec fields empty.
The Radius value is a number of arcminutes to search about the position. The Band entries define a lower and upper limit, in meters, for the spectral bandpass (not all SSAP servers support this option, those that do may offer just an upper limit and allow you to set a wavelength to include by setting just a lower limit). The Time entries define either a time that should match the time the spectra where taken, or a range in which they should match (again not all servers support this option).
The file format, wavelength and flux calibration options can also be chosen. If the value is "None", the query will not contain these parameters.
Besides the mandatory parameters, some servers also support a set of other metadata parameters. By clicking Add Parameter a list of all supported parameters supported by the selected SSAP servers will be displayed, and one can be chosen. The chosen parameters will appear in the optional parameters list. If provided, the default values will be inserted into the value field. The checked parameters having a non empty value field will be added to the query.
Once you have identified the region to search or object press the green SEND QUERY button to contact the current list of SSAP servers. To make these queries it may be necessary for you to tell SPLAT-VO about your local web-proxy server (this is the same as that you use in your Internet browser), select the Options->Configure proxy... item to do this.
Query results
Once the SSAP query is complete lists of spectra that the servers hold will be shown in the Query results: region. There is a tab of results for each server. If the tab is empty then the server has no spectra for the selected region.
The next action is to select which spectra you want to download and view in SPLAT-VO. If you press Display all then all the spectra from all the servers will be downloaded and displayed in a single plot. Clearly this may take quite sometime if there are many spectra, so you can also download and display a subset. To do this select the rows that contain the spectra that you want to see (you can extend selections by using the control and shift keys while selecting a row and you can select spectra from more than one tab at a time) and then press the Display selected button.
Once the spectra have been downloaded they should appear together in a single plot. If this isn’t the case then some of the spectra are probably invalid and you will need to add them to a plot yourself (see the section on displaying more than one spectrum). If you want SPLAT-VO to match the coordinate systems and fluxes of the spectra then make sure that the Options->Match coordinates and/or fluxes item is still switched on in the plot window.
To view a single spectrum quickly just double click on its row. This will download the spectrum and
display it in its own plot window.
Implementation status
SSAP is a protocol being developed by the IVOA (International Virtual Observatory Alliance1).
Currently spectra returned from SSAP servers may only be in basic formats, that is simple FITS and VOTables (not the full SED data model serialisation). However, the SPLAT-VO data model includes errors as well as coordinates and data values.
Support for spectral coordinates is provided by the Starlink AST library (SUN/211) using an implementation of the FITS-WCS paper III Representations of spectral coordinates in FITS (Greisen, Valdes, Calabretta & Allen). AST also provides the flux and data value conversion utilities used by SPLAT-VO which is based on interpreting the units strings system described in FITS-WCS paper I Representation of World Coordinate in FITS by Greisen & Calabretta. Conversion between fluxes is currently restricted to flux per unit wavelength and flux per unit frequency is provided and does not require any ‘dimensional analysis’ information to be present, just correctly formed units strings.
Accelerator keys
1http://www.ivoa.net/