Full-Field Polygon Region File (poly3.fits)
The polygon file consists of a single FITS format region file for each observation tracing extended emission in the observation using polygons. These polygons are not guaranteed to be smooth (as shown in the example below).
The full-field polygonal region FITS file is named: 〈i〉〈s〉〈obs〉_〈obi〉N〈v〉[_〈c〉]_〈b〉_poly3.fits
where 〈i〉 is the instrument designation, 〈s〉 is the data source, 〈obs〉 is the observation identification, 〈obi〉 is the observat ion interval identification, 〈v〉 is the data product version number, 〈c〉 is the cycle, and 〈b〉 is the energy band designation. The optional discriminator identified in square brackets is included only for ACIS alternating exposure (interleaved) mode observations.
Example
An example poly3 file from release 2.0 is:
unix% dmlist acisf09335_000N021_b_poly3.fits.gz blocks
--------------------------------------------------------------------------------
Dataset: acisf09335_000N021_b_poly3.fits.gz
--------------------------------------------------------------------------------
Block Name Type Dimensions
--------------------------------------------------------------------------------
Block 1: PRIMARY Null
Block 2: POLY Table 11 cols x 216 rows
which has columns:
unix% dmlist acisf09335_000N021_b_poly3.fits.gz cols -------------------------------------------------------------------------------- Columns for Table Block POLY -------------------------------------------------------------------------------- ColNo Name Unit Type Range 1 pos(X,Y)[348] physical Real8(348) -Inf:+Inf Physical coordinate 2 Shape String[10] Shape of the regions 3 Component Int4 - Region number 4 Level Int4 - Contour level associated with the region 5 Parent Int4 - Component id of the parent contour 6 Contour_ID Int4 - Component id of the parent contour 7 Area Real8 -Inf:+Inf Geometric area inside the polygon 8 Counts Real8 -Inf:+Inf Number of counts inside this polygon 9 X_Counts Real8 -Inf:+Inf Number of counts inside ONLY this polygon 10 Moment Real8 -Inf:+Inf Second moment of count event 11 ccd_id Int4 - CCD that the polygon was computed on (polygons
The location of the polygon with the largest area can be found using dmstat (the row number is given after the @: characters):
unix% dmstat acisf09335_000N021_b_poly3.fits.gz"[cols area]" sig- | grep max
max: 4505.4756904 @: 97
This row can then be selected and used to filter the full-field per-obsid event list using dmcopy:
unix% dmcopy acisf09335_000N021_evt3.fits"[sky=region(acisf09335_000N021_b_poly3.fits.gz[#row=97]),energy=500:7000][bin sky=::1][opt null=-999]" poly.img
This command also applied an energy filter (to only retain those events with an energy between 0.5 and 7.0 keV), and uses the value -999 for those pixels outside the polygon.
Displaying events within a poly3 contour
![[The SAOImage DS9 image displayer is used to show the events that fall within the polygon. There is a central extended source (a galaxy cluster that has also been detected by the South Pole Telescope due to the Sunyaev-Zeldovich affect) but the region is larger than this, and the bounary is not smooth, with a lot of protrusions.]](imgs/acis9335.poly3.png)
Displaying events within a poly3 contour
The resulting image can be viewed in ds9 (using magenta to indicate those pixels outside of the polygon, a background color of grey for outside the image, and selecting a zoom factor appropriate for this particular image):
unix% ds9 -background grey -nan magenta -zoom 2 poly.img
Related Science Documents
- ahelp: dmimghull
- CIAO Gallery: Region—Convex Hull