Combine and Detect Pipeline
The Detect pipeline is run for each stack from the Calibrate pipeline. Source detection is performed using both a wavelet-based algorithm (wavdetect) and a voronoi-tesselation based algorithm (mkvtbkg).
Combine observation interval products into stack products
In this stage, we make full field combined data products for the stack. Images use a standard blocking—for ACIS this is 1 pixel (0.492 arcseconds), while for HRC a 2 pixel block is used (0.264 arcseconds).
- Combine field-of-view (FOV) files for stack, to determine required image size.
- A combined event file is created using dmmerge.
- Combined image files are created from the event file for each energy band.
- Combined exposure maps are generated by reprojecting and coadding the individual observation exposure maps.
- Combined background maps are generated by reprojecting and coadding the individual background images.
Calculate the limiting sensitivity
The limiting sensitivity is a measure of the flux of a point source that meets but does not exceed the maximum likelihood estimation thresholds at each location in the field to be included in the catalog. Two limiting sensitivity maps are calculated: one for sensitivities that satisfy the MARGINAL source likelihood threshold and another for sources that satisfy the more restrictive TRUE source likelihood threshold.
The background map is used to simulate counts with a Poisson mean at each location. The PSF size of each pixel is determined. Given a region the size of the PSF and the background counts in that region, the upper limit for a 3σ detection is computed. An example of a limiting sensitivity image is shown below.
Example: limiting sensitivity
Detect source candidates with mkvtbkg
The mkvtbkg tool used to create the background maps in the calibrate pipeline also creates a list of polygons around potential sources. These polygons are available in the poly3 file for each observation. These polygons are then used to find candidate compact and convex-hull sources.
For the compact-source case, nested hulls which meet a series of thresholds (such as comparing the PSF and polygon area, and th expected count level) are compared—by looking at the derivitive of the area between the polygons—to identify sources. An ellipse is generated for each such detection by calculating the convex hull around the outermost polygon, and then finding the ellipse that encloses the hull. This analysis is run per stack.
The polygons are also used to identify convex-hull candidates, where those polygons above a fixed contour level are taken, and then filtered to reject those regions that exceed a maximum density of compact-source detections. The remaining polygons are converted to a convex hull representation. The analysis is done per observation interval, and then merged to create a stack-level set of convex-hull detections.
Detect source candidates with wavdetect
Source detection candidates are identified by a wavelet transformation method, using the wavdetect tool (the version of wavdetect used for CSC is a slightly modified version of the CIAO 4.8 tool).
wavdetect is run for the detect set of bands: for HRC this is the wide band and for ACIS it is soft, medium, hard, and broad. 14 different wavelet scales are used ranging from 1.4 to 128 pixels. The significance threshold for identifying a pixel as belonging to a candidate source is set to 1.e-5.
The exposure maps and background map are both included in the source detection pipeline.
Combine detections across energy bands
The detections are combined by comparing the results across all energy bands. If the detection appears in multiple energy bands, the entry with the highest detect significance is chosen.
Output data products
The Combine and Detect pipeline produces these data products:
field of view file (fov3.fits): the field of view region for the combined stack images.
Stack event file (evt3.fits): the merged event list.