This window provides basic access to the Virtual Observatory. Using it you can query the current list of SSAP servers (Simple Spectral Access Protocol) for lists of the spectra that they hold about a circular region on the sky or about a particular object. You can then select which of these you are interested in and download these into SPLAT-VO.
The figure above shows the window after a query to all the currently known servers has been made.
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).
Once you have identified the region to search or object press the Go 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.
The list of SSAP servers available can be queried by selecting the Options->Configure SSAP servers... item. This allows you to choose a subset of the known servers, find out more about them, and to query a VO registry for any new servers.
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 Alliance) and is not yet a full recommendation so the services offered here should be considered a simple example of what will eventually become available.
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.
The SSAP query is limited to just a cone region on the sky or single object name and only some optional query parameters are not supported, namely the wavelength and flux calibration options. These are located in the Options menu.
Accelerator keys
SPLAT-VO -- A VO-enabled Spectral Analysis Tool