Per-Observation Detections Region Files (reg3.fits)
The per-observation detections source region file consists of a single FITS format region file with a block containing a region for each science energy band.
FITS HDUs (CXC Data Model blocks):
The source regions for each energy band in the data blocks are the elliptical apertures for each source, defined as the ellipses that include the 90% encircled counts fraction of the PSF in each science energy band at the source location.
The per-observation detections source region FITS file is named: 〈i〉〈s〉〈obs〉_〈obi〉N〈v〉[_〈c〉]_〈r〉_reg3.fits
where 〈i〉 is the instrument designation, 〈s〉 is the data source; 〈obs〉 is the observation identification; 〈obi〉 is the observation interval identification; 〈v〉 is the data product version number; 〈c〉 is the cycle; and 〈r〉 is the region ID, formatted as "rnnnn", where nnnn is the 4-digit region number, formatted with leading zeros. The optional discriminator identified in square brackets is included only for ACIS alternating exposure (interleaved) mode observations.