Method

There are two methods of operation. For most observations an attitude file will be available and will give an unambiguous value for the roll angle. The program can then use this value, together with the geometry of the window support structure to reconstitute the spokes on the image. The routine uses the mean roll angle contained in the attitude file (but excludes zero rolls from the mean calculation as these are usually corrupt entries). This is MODE=C.

Occasionally, the attitude file is useless for defining the roll angle. This can be either because it is corrupt or because the observation has been completed in more than one session so that more than one roll angle is present in the data. In this case the spoke structure may be defined using the cursor, MODE=I, as below :

1. Display the XRT image of interest on an interactive graphics device with IDISPLAY.
2. Type XSPOKES from within ICL.
3. Select whether to include the region outside the circular field of view of the PSPC in the spatial file. By default the radius of the circular field is set to 0.95 degrees. This may be altered on the command line.
4. Use the cursor to select the centre of an intersection of a spoke and the central ring.
5. Use the cursor to select the centre of the intersection of the opposite spoke and the central ring.
6. The routine will now use these two points to reconstruct the XRT support structure. It will draw an outline of the structure on the image.
7. A textfile will now be produced containing the spatial definition of the support structure.