rgsxsrc: convolve an RGS spectrum for extended emission

This model convolves the input spectrum by an angular structure function so that it can be used with the standard RGS response matrix, which assumes a point source. This will only be of value for moderately extended (one arcmin) sources.

Before invoking this model, the xset variable RGS_XSOURCE_FILE should be set to the name of the file containing the following information:

RGS_XSOURCE_IMAGE
    <filename of image of source> 
RGS_XSOURCE_BORESIGHT
    <image boresight in format> 
RGS_XSOURCE_EXTRACTION
    <size of region in arcminutes>

Note that the angle brackets are required. An example file might look like:

RGS_XSOURCE_IMAGE
    </local/data/mymachine/myusername/MOS1.FIT> 
RGS_XSOURCE_BORESIGHT
    <05:25:02.9 -69:38:30 339.760974> 
RGS_XSOURCE_EXTRACTION
    <1.8>

The RA and Dec of the source can be determined from the image or from the headers of the source spectrum. The position angle can be found in the image headers.

The model parameter is:

par1	order (must be negative)