If you want to extract precisely the same region on the sky for all three instruments, using the above method to define the region for the first instrument is easy. However, transferring that region to the other instruments requires a different approach.
The task convregion provides an alternative method for converting
region information from right ascension and declination to detector
coordinates for circular and elliptical regions. Run with mode=2,
convregion will input an ASCII file with region information and
output an ASCII file with the necessary region information.
convregion mode=2 inputfile=convreg.txt
where in this case the input file, convreg.txt, which will extract circular and elliptical annuli, contains:
circle 207.12 26.59 4.0
!circle 207.12 26.59 2.0
ellipse 207.32 26.59 4.0 2.0 30.0
!ellipse 207.32 26.59 2.0 1.0 30.0
and the output file contains:
circle -2063.3 5786.5 4800.0
!circle -2063.3 5786.5 2400.0
ellipse 2372.8 -6303.1 4800.0 2400.0 260.153
!ellipse 2372.8 -6303.1 2400.0 1200.0 260.153
The output data must then be edited into a region expression:
&&(((DETX,DETY) IN circle(-2063,5786,4800))
&&!((DETX,DETY) IN circle(-2063,5786,2400)))
||(((DETX,DETY) IN ellipse(2373,-6303,4800,2400,260))
&&!((DETX,DETY) IN ellipse(2373,-6303,2400,1200,260)))
convregion can also both be run with direct input:
shape=CIRCLE ra=207.12 dec=26.59 radius=3
semimajor=2 semiminor=3 rotangle=30 mode=3
or using a region file as input.
It should be noted that convregion does not yet convert boxes. Specifying a region in sky coordinates and converting it to detector coordinates for the three instruments produces approximately the correct result. However, it should be remembered that the detectors do cover different regions of the sky; different chips may be off between the two MOS detectors, and the pn is somewhat offset from the MOS detectors. Thus, if it is really important to have exactly the same region for all three detectors, one has to use the chip information as well. Thus, making sure that you have exactly the same region for every detector is a messy and painful business.