Convert from FPC to World (celestial) coordinates by applying an aspect solution.
Array_Type pix_dmTanPixToWorld( Array_Type pix, Array_Type asp,
Array_Type crpix, Array_Type cdelt )
This routine converts a position in the Focal Plane coordinate
(FPC) system to the World (Celestial) coordinate system.
To perform this conversion the routine needs to know the
aspect solution, knowledge of where the telescope
was pointing at the time the photon was detected.
The FPC system corresponds
to the ( DETX, DETY ) columns of a Chandra event file.
The position, in the FPC system, is given by the
two-element array pix, which has units of pixels.
The pointing position is given in the three-element
array asp, which should contain the
ra, dec, and roll values in degrees.
The crpix argument is a two-element array which gives
the pixel location for the aimpoint.
The cdelt argument is also a two-argument
array and gives the pixel size
in degrees. For the ACIS detectors you would use
crpix = [ 4096.5, 4096.5 ] cdelt = [ -0.492, 0.492 ] / 3600.0
The return value is a two-element array which gives the
World coordinates in degrees.
chips> require("pixlib")
chips> pix_init_pixlib
chips> evt = readfile( "acis_evt2.fits" )
chips> asol = readfile( "acis_asol.fits" )
chips> detpos = [ evt.detx[0], evt.dety[0] ]
chips> asp = [ asol.ra[0], asol.dec[0], asol.roll[0] ]
chips> crpix = [ 4096.5, 4096.5 ]
chips> crdelt = [ -0.492, 0.492 ] / 3600
chips> wcspos = pix_dmTanPixToWorld( detpos, asp, crpix, crdelt )
In this example we have used the pix_dmTanPixToWorld() routine
to convert the detector position of the first event
in acis_evt2.fits into WCS (RA, Dec) values using the
first element of the aspect solution.
In reality you would want to use the aspect solution that
was valid at the time the event was detected - which can
be found since both files have a time column - rather than
using an aritrary value as we did here.
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