IMAGING mode

Given its main task of locating point sources and creating images of them in the observation field of view the first question is how spiros goes about locating them.

A fast and simple method results by assuming some prior knowledge of the sources expected in the field of view, namely if they are point or point like and that there are only a small finite number of them, tens but not hundreds or thousands.

On this assumption the sources can be located using the Iterative Removal of Sources (IROS) method which literally attempts to locate each source in the field of view one after the other from the strongest to the weakest.

This method works as follows:

• A fast but blurred location image is created. It shows the most probable location or region of the greatest source emission. This image may be called a ``pixel independent correlation map'' as it is constructed by scanning the imaging field of view with a ``source probe'' and calculating the flux by assuming all input counts are due to that source and any background events alone. This typically produces a ``convolved'' image with a global maximum at a good first guess for a probable source location.

• With this approximate location find the strength (probably upper limit) of the source and then the detector counts expected from it.

• Subtract source counts from the input count data to create residue count data more or less due to other sources not yet located.

• With these residue counts the procedure above is repeated creating a new location image, searching it for any new source and adding it to a growing list. Each time a new source is added to the list all sources are allowed to "float" to more precise locations and fluxes before their counts are subtracted from the input counts.

IROS repeats the iterative procedure above, locating each new source approximately, always allowing them to float to more optimal locations, creating new residue count data and stopping when nothing significant is further found. This procedure can be repeated in any number of energy bins to calculate a mean location for each source over the entire spectrum range.

For the user running spiros in its IMAGING mode the main task is to get the appropriate binned count data output from spi_obs_hist and decide how many sources to look for. The user may have an input catalog of known sources to get their values without calculating the position, or to find an extra one or two in addition, or just to make an image of them.