SIA query load dialog
The SIA query load dialogue can be opened using the SIA Query button () from the Load Window's toolbar or the Control Window's VO menu. It allows you to query one of a number of external web services for images covering a given region of the sky.
The window consists of a Registry Query Panel at the top, and a Single Positional Search Panel below. The Angular Size may be set to zero; this means that any image covering the chosen position will be selected. There is additionally an Image Format selector which allows you to restrict the result to contain only images in certain formats (the default is set to only FITS).
The result of a successful query is a table in which each row represents an image that can be downloaded from a remote site; one of the columns contains the image URL, and the other columns contain associated metadata such as image format, size, shape and so on. Different services provide different kinds of answer to these queries; some may have a static list of observed or reduced FITS files, and others may generate images on the fly. See the SIA standard, and individual registry records, for details. A useful thing to do with the resulting table might be to configure an activation action of Display Image or Send FITS Image.
TOPCAT supports two different (and incompatible) major versions of the SIA protocol, SIA version 1 and version 2. When you have selected a service to use, its protocol version number is displayed in the SIA Version indicator next to the SIA URL display. If you have entered the URL manually, you will need to make sure this is set to the appropriate value. Note that, although SIAv2 allows a number of query possibilities not offered by SIAv1, at present TOPCAT does not support this additional functionality for SIAv2 services.