Subsections

• 7.6.1 Actions
• 7.6.2 Usage and Inputs
• 7.6.3 Data Products

7.6 CONSTRUCT_REGIONS Stage

This stage is run at least once for each observation. If a source's position is revised this stage must be run again.

7.6.1 Actions

• Source names are constructed from the celestial coordinates provided¹³, and source extraction directories are made.

• PSF images are constructed for each source at each of 5 energies.

• Source extraction regions that meet the requested PSF fraction at the specified ``primary'' PSF energy are proposed. Circular mask regions that completely cover the PSF (at the ``primary'' energy) are computed.

• Plots summarizing the results of CONSTRUCT_REGIONS are presented. These plots are diagnostic tools that help you spot problems in your data, in your catalog, in your CIAO configuration, in the CIAO software, or in the AE software. Some plots may also convey scientifically interesting information about your sources.

7.6.2 Usage and Inputs

acis_extract, catalog_filename, obsname, obsdata_filename, /CONSTRUCT_REGIONS, EMAP_FILENAME=filepath, ASPECT_FN=filepath, /S_AIMPOINT, /PIPELINE_RANDOMIZATION, MASK_FRACTION=value, MASK_MULTIPLIER=value, /REGION_ONLY, /DIFFUSE

acis_extract, srclist_filename, obsname, /CONSTRUCT_REGIONS, /PLOT

EXAMPLE:
idl $\vert\&$ tee construct_regions_1875.log

acis_extract, 'all.cat', '1875', '../obs1875/spectral.evt', /CONSTRUCT_REGIONS, EMAP_FILENAME='../obs1875/obs.emap'

acis_extract, 'all.srclist', '1875', /CONSTRUCT_REGIONS, /PLOT

In the above example, IDL is started with a tee to a log file, for documentation purposes.

• The keyword ASPECT_FN must specify a CIAO aspect file covering the time range of your observation. The aspect file is provided to MARX during PSF construction so that the simulated source dithers across edges of the detector in precisely the same way as your real source. If MARX's internal dither model was used the observed and simulated PSFs would show inconsistent distortions due to detector edges. MARX accepts only a single aspect file, not a ``stack'' of files. If your observation has multiple aspect files, you must concatenate them using dmmerge, as discussed in the CIAO Data Products Guide. The aspect files must be combined in time order; the string of numbers in the file name (e.g. ``pcadf111767021N001_asol1.fits'') refers to the start time of the period for which the aspect solution is valid.

• The optional keyword /S_AIMPOINT should be supplied when the observation was taken at the S-array aim point. At present no robust method for AE to automatically determine which aim point was used is known by the authors.

• The optional keyword /PIPELINE_RANDOMIZATION should be supplied if the event data were produced with ``pipeline randomization'' (an option found in the CXC tools acis_process_events and reproject_events). The degree of blurring AE applies to PSF images depends on whether such randomization was or was not applied to the event data. See §5.2. AE can NOT determine the proper setting of this parameter from the data themselves; although acis_process_events records whatever randomization it has applied in the FITS keyword RAND_SKY, reproject_events does not (as of CIAO 3.4.1.1).

• The optional keywords MASK_FRACTION (default 0.99) and MASK_MULTIPLIER (default 1.1) indirectly control the size of a circular mask region constructed around each source (used later for construction of a background event list). Choice of these parameters is largely a matter of scientific judgment based on the analysis goals. For example the generous mask defaults may be good for studies of faint diffuse emission where you want to be certain to remove point source wings. Reduction in the mask region sizes may be necessary in very crowded fields where very little background area survives the masking process.

  Starting with a radius that encloses the extraction region, a circular region is enlarged until it encloses a PSF fraction of at least MASK_FRACTION. The mask region radius is then multiplied by MASK_MULTIPLIER. Due to the choice of starting radius one cannot produce a mask fraction less than the extraction fraction (assuming MASK_MULTIPLIER $\geq 1$). The design assumption was that the mask region should be well outside the extraction region.

• The optional keyword /REGION_ONLY should be supplied if running this stage only for the purpose of constructing extraction regions (e.g. to display on the data). This option speeds up this stage by constructing only a single PSF image (at the fiducial energy). You must eventually run this stage without /REGION_ONLY to enable computation of PSF fractions.

• The optional keyword /DIFFUSE should be supplied when diffuse sources are being extracted (§7.1.2), i.e. when catalog_filename contains a list of diffuse source region files (without the ``.reg'' part). The point source related keywords EMAP_FILENAME, /S_AIMPOINT, MASK_FRACTION, MASK_MULTIPLIER, /REGION_ONLY, and /PIPELINE_RANDOMIZATION are ignored.

7.6.3 Data Products

For each source the following files are produced:

{sourcename}/source.stats:

a FITS file containing keyword information about the source; view with dmlist source.stats header. If this file already exists it is modified, rather than destroyed and re-created (as of AE 3.106).

{sourcename}/{obsname}/extract.reg:

a ds9 region file containing source extraction and mask regions. The mask region is identified using the ds9 region property ``background''.

{sourcename}/{obsname}/source.psf:

a FITS file containing PSF images at 277, 1497, 4510, 6400, and 8600 eV, placed at the source position

{sourcename}/{obsname}/obs.stats:

a FITS file containing keyword information about this observation of the source; view with bf dmlist obs.stats header

The following interactive plots are produced:

``Area of Source Region via polyfillv'' vs ``Off-axis Angle''

a scatter plot of aperture area (estimated by IDL) as a function of off-axis angle

``PSF/Catalog Offset'' vs ``Off-axis Angle''

The quantity ``PSF/Catalog Offset'' is the offset between the requested source position passed to mkpsf and the centroid of the primary-energy PSF image returned by mkpsf. This plot was produced for two reasons: (1) to look for astrometric errors in the mkpsf software, (2) to better understand the asymmetry of the PSF, i.e. to what extent the source ``position'' is not at the ``center'' of the PSF.

``PSF Fraction'' vs ``Source #''

a plot of the requested and actual PSF fraction (at the primary energy) for each source

``$\sim$Distance Between Source Regions'' vs ``Source #''

For each source the nearest neighboring source is identified and the minimum distance between the two extraction polygons is estimated and plotted. A negative result is a crude ``crowded field flag'', i.e. a warning to the observer that the polygons of the two source are likely to overlap and should be examined by hand.

``Pairs of Crowded Sources''

a scatter plot of the source numbers for nearest-neighbor pairs with extraction regions separated by less than 2 pixels

``CROPFRAC'' vs ``Off-axis Angle''

a scatter plot of adjustment made to the PSF normalization (at 277 eV and 8 keV) to account for the cropping of the PSF images.

``PSF pixel size'' vs ``Off-axis Angle''

a scatter plot of the pixel size chosen for the PSF images of each source.

The data vectors plotted above are retained in an IDL save file named construct_regions_{obsname}.sav. They can be loaded back into IDL with the standard restore command, e.g. :

    restore, 'construct_regions_1875.sav',/VERBOSE