Gallery: Apertures

CIAO has several tools that can be used to automatically generate source and background aperture regions. These apertures may be based on the output from one of the source detection algorithm or by evaluation of the Chandra PSF at certain locations.

The roi tool is a flexible utility that creates source and background regions typically based on the output from one of the detect tools. It has options for dealing with overlapping source regions: excluding any overlap area, ignoring overlap, or simply grouping all overlapping sources into one. It also has options for how to create the background as either a single co-located annulus (with nearby sources excluded) or as a set of annuli around each source in the group. It also has options to adjust how the background regions are scaled and offset for the source region, options to include the edge of the detector via the field-of-view file, exclude readout streaks, and more.

In this example show just five sources detected in a merged data of NGC 6752. The other, clearly visible nearby sources have been omitted just to simplify this example.

In the first example we show the most common usage of the roi tools which is to simply exclude the area between overlapping sources.

The next roi option is to include the overlap area but identify the overlapping sources. This can be useful when you wish to include the effects of overlapping sources and need to know the fraction of the PSF from once source that is included in another source's aperture.

Knowing which individual sources overlap is useful, but it can also be useful to simply group all the overlapping source apertures together. This can be done using the roi group=group option.

In the first example we see how roi can exclude overlapping source area from all aperture regions and explained how this can be useful in some situations. However, in other situations users may wish to assign overlapping source area (and the counts they include) to one source region aperture or another. The rank_roi can be used to do this. Users can decided to assign the overlapping region area based on one of several metrics: maximum total counts, minimal total counts, largest area, smallest area, brightest pixel, or faintest pixel. In this example we assign the overlapping area to the source with the most counts using method=max.

The aperture regions created so far has been based on the output from one of the source detect tools. Regions can also be created based on the PSF. An tabulation of a circularly symmetrical approximation of the Chandra PSF is included in the Chandra CALDB and accessible using the CIAO PSF module. The psfsize_srcs script uses this interface to lookup the size of a circle needed to enclose a specified fraction of the PSF at the specified energy and generates a region file.

The psfsize_srcs script creates source regions based on a circular approximation of the PSF size. The psf_contour script creates source regions by simulating the Chandra PSF using MARX and then generating source regions using one of several contouring algorithms.

The psf_contour script handles overlapping regions differently than roi. Rather than exclude the overlapping region, the algorithm works by first reducing the overall size of overlapping sources to include a smaller fraction of the PSF. The regions are "shrunk" until they no longer overlap. This may be advantageous over simply excluding the overlapping area because it happens essentially symmetrically around the PSF; where as excluding the overlap generally happens on one side which may skew results like if computing centroids.

The psf_contour has several options for creating contours around the simulated PSF. Several of these create many sided polygons rather than simple circle or elliptical shaped apertures.

So far we have focused on the source region apertures. The roi tool also creates background region apertures that are stored in the 2nd extension of each output file; the extension in named BKGREG

dmlist ngc6752_05.exclude.reg blocks | cat

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Dataset: ngc6752_05.exclude.reg
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Block Name                          Type         Dimensions
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Block    1: PRIMARY                        Null
Block    2: SRCREG                         Table         5 cols x 1        rows
Block    3: BKGREG                         Table         5 cols x 3        rows

These can either be a single ellipse just around the individual source, or can be a collection of ellipses around each source included or excluded from the source region. Overlapping sources are excluded from the background. The size of the background is multiplicative or additive factor of the source size.

Depending on the field, the background created by roi may not have a statistically significant number of counts for some sources. While users can increase the multiplicative or additive factor but that affects all sources and may be undesirable.

The bkg_fixed_counts script can alternatively be used to create background regions that contain a minimum number of counts. A background annulus is created at each source location. The inner radius is related to the size of the PSF and the outer radius is allowed to increase until it encloses a specified number of counts, while excluding any source regions (including the source region at that particular location).